SURFACE
THE
AIR TEMPERATURE
AND
ITS CHANGES
OVER
PAST 150 YEARS
P. D. Jones,
• M. New,• D. E. Parker,
2
S.Martin,
3 andI. G. Rigor
4
Abstract. We review the surface air temperature
recordof the past 150years,consideringthe homogeneity of the basicdata and the standarderrors of estimation of the averagehemisphericand global estimates.
We presentglobal fieldsof surfacetemperaturechange
over the two 20-year periods of greatestwarming this
century,1925-1944 and 1978-1997. Over theseperiods,
global temperaturesrose by 0.37ø and 0.32øC,respectively. The twentieth-centurywarming has been accompaniedby a decreasein thoseareasof the world affected
by exceptionallycool temperaturesand to a lesserextent
by increasesin areas affected by exceptionallywarm
temperatures.In recent decadesthere have been much
greater increasesin night minimum temperaturesthan
in day maximumtemperatures,so that over 1950-1993
the diurnal temperature range has decreasedby 0.08øC
per decade.We discussthe recentdivergenceof surface
1.
INTRODUCTION
and satellite temperature measurementsof the lower
troposphereand considerthe last 150 years in the context of the last millennium.We then provide a globally
complete absolutesurfaceair temperature climatology
on a 1ø x 1ø grid. This is primarily based on data for
1961-1990. Extensive interpolation had to be undertaken over both polar regionsand in a few other regions
where basic data are scarce, but we believe the climatol-
ogy is the most consistentand reliable of absolutesurface air temperature conditions over the world. The
climatologyindicatesthat the annual average surface
temperatureof the world is 14.0øC(14.6øCin the Northern Hemisphere (NH) and 13.4øCfor the Southern
Hemisphere).The annualcycleof globalmean temperatures follows that of the land-dominated NH, with a
maximumin July of 15.9øCand a minimum in Januaryof
12.2øC.
areas and gradual changesin observingpracticesover
the ocean. Section 3 reviews several methods
that have
The surfaceair temperaturedatabasehasbeen extensivelyreviewed on severalearlier occasions,most notably by Wigleyet al. [1985,1986]andEllsaesser
et al. [1986]
and by the IntergovernmentalPanel on Climate Change
been used to aggregatethe basicdata to a regular grid,
the basicaim of which is to reduce the effectsof spatial
and temporal variationsin the spatial density of available data. Future improvementsto the network are
(IPCC) [Follandet al., 1990,1992;Nichollset al., 1996].
This work extendstheseby usingslightimprovementsto
the basicsurfacedatabaseand updatesthe recordsto the
end of 1998. Most importantly,it also providesa comprehensiveanalysisof surfaceair temperaturesin absolute rather than anomalyunits,by the developmentof a
1øby 1øgrid box climatologyfor all land, ocean,and sea
ice areasbasedon the 1961-1990 period.
discussed.
The
structure
of this review
is as follows.
Section
2
discusses
the quality of the raw land and marine temperaturedata, addressingthe long-termhomogeneityof
the basicdata and consideringthosesubtlechangesthat
might influence the data, such as urbanization of land
•ClimaticResearchUnit, Universityof EastAnglia,Norwich, England.
Before
consideration
of what
the data tell us about
the post-1850 instrumental era, section 4 assessesthe
accuracyof the estimatesof hemisphericand global
temperature anomalies, particularly during the nineteenth century, when coveragewas poorer. Section 5
then analyzesthe surfacetemperature record. It compares patterns of warming over the 1978-1997 period
with thoseof the earlier 1925-1944 period, which experienceda comparablerate of warming.It also considers
trends in the areas affected by extreme temperatures,
recent trends in Arctic temperatures and in maximum
and minimum temperatures, the recent divergence of
surface temperature trends and satellite retrievals of
lower tropospherictemperaturetrends,and the last 150
years in a longer near-millennial context.
The
discussion
in section
5 is limited
to what
the
2HadleyCentre,Meteorological
Office,Bracknell,
England.
3School
of Oceanography,
University
of Washington,
Seattle. surfacerecord tells us about past temperaturechanges.
of the reasonsfor the changes(both
4AppliedPhysicsLaboratory,Universityof Washington, Extensivediscussion
Seattle.
variationsin atmosphericcirculationand past external
Reviewsof Geophysics,
37, 2 / May 1999
Copyright1999 by the AmericanGeophysicalUnion.
pages 173-199
8755-1209/99/1999
RG900002 $15.00
Papernumber1999RG900002
e173e
174 ß Joneset al.: SURFACE AIR TEMPERATURECHANGES
37, 2 / REVIEWS OF GEOPHYSICS
forcing)may be foundin the IPCC assessments
[Folland smallerthan individualdaily or evenhourlyoccurrences.
et al., 1990, 1992; Nicholls et al., 1996] and elsewhere Effectsup to 10øC,for individualdays,havebeenquoted
[e.g., van Loon and Williams, 1976; Karl et al., 1989, for some cities [see, e.g., Oke, 1974]. To assessurban1993a; Trenberth, 1990; Parker et al., 1994; and Hurtell, ization influenceson monthly and longer timescalesin
1995]. In section 6 the developmentof the absolute certain regions (Australia [Torok, 1996]; Sweden
climatology is described,and section 7 presents our [MobergandAlexandersson,
1997]), specificrural station
conclusions.
data setshavebeen developedto allow estimationof the
magnitude of possible residual urban warming influencesin the Jones[1994] data set. Resultsshowthat if
2.
HOMOGENEITY
OF THE BASIC DATA
there are residualinfluences,they are an order of magnitude smaller than the nearly 0.6øCwarming evident
However mathematically cunning or appealing an during the twentieth century, confirmingearlier work
analysistechniquemaybe, any resultsor conclusions
are [Joneset al., 1989, 1990;Karl and Jones,1989].
dependentupon the quality of the basicdata. In climatology, quality of the data is essential.A time seriesof 2.2. Marine Component
monthly temperature averagesis termed homogeneous
Historical temperaturedata over marine regionsare
[Conradand Pollak, 1962] if the variationsexhibitedby largelyderivedfrom in situ measurementsof seasurface
the series are solely the result of the vagariesof the temperature(SST) and marine air temperature(MAT)
weather and climate. Many factorscan influencehomo- taken by shipsand buoys.Inhomogeneityproblemswith
geneity,which we next discussby divisionof the surface bothvariablesrelate principallyto changesin instrumendata into land and marine components.
tation, exposure,and measurementtechnique.For SST
data the changein the samplingmethod from uninsulated buckets to engine intakes causesa rise in SST
2.1. LandComponent
The most importantcausesof inhomogeneityare (1) valuesof 0.3ø-0.7øC[Jamesand Fox, 1972]. Engine inchangesin instrumentation,exposure,and measurement take measurementsdominate after the early 1940swith
technique;(2) changesin stationlocation(both position uninsulated bucket measurements the main method beand elevation);(3) changesin observationtimesand the fore. For MAT data, the averageheight of ships'decks
methods used to calculatemonthly averages;and (4) above the ocean surface has increased during the
changesin the environmentof the station, particularly present century, but the more seriousproblem is the
with referenceto urbanizationthat affectsthe represen- contaminationof daytimeMAT throughsolarheatingof
tativenessof the temperaturerecords.All of thesehave the ships'infrastructure,rendering only the nighttime
been discussedat length in the literature [see, e.g., MAT (NMAT) data of value [Parkeret al., 1994,1995a].
Mitchell, 1953;Bradleyand Jones,1985;Parker, 1994].A
Follandand Parker [1995] developeda model to estinumber of subjectiveand relatively objective criteria mate the amount of cooling of the seawaterthat occurs
[e.g.,Joneset al., 1985, 1986a,b, c; Easterlingand Peter- in bucketsof various types, dependingupon the time
son, 1995; Alexanderssonand Moberg, 1997] exist for between samplingand measurementand the ambient
testingmonthly data. Petersonet al. [1998a] provide a weather conditions.Adjustmentsto the bucket SST valcomprehensivereview of these and numerous other ues for all yearsbefore 1941 are estimatedon the basis
techniques.
of assumptions
aboutthe samplingtime and the typesof
Here we use the land station data set developedby bucketin use at different times.The adjustments,which
Jones[1994]. All 2000+ station time seriesused have are greaterin winter, are tuned usingan assumptionthat
been assessed
for homogeneityby subjectiveinterstation SST annual cycle amplitudeshave not experienceda
comparisonsperformed on a local basis.Many stations longer-term trend [Folland and Parker, 1995]. Canvas
were adjustedand someomitted becauseof anomalous buckets,which evaporativelycool more than wooden
warming trends and/or numerous nonclimatic jumps buckets, were in dominant use after the late nineteenth
(completedetailsare givenbyJoneset al. [1985,1986c]). century,havinggraduallyreplacedthe woodenbuckets
All homogeneityassessmenttechniqueswould per- that would havebeen usedinitially in the mid nineteenth
form poorly if all station time serieswere affected sim- century.Somebucketsare still in usetoday,but sincethe
ilarly by commonfactors(e.g. thoserelatedto urbaniza- early 1940s they have been much better insulated, altion). Urban development around a meteorological though it is believedthat uninsulatedbucketswere still
station means that measuredtemperaturesare slightly used on some ships until the 1960s (C. K. Folland,
warmer than they would otherwisehavebeen. The effect personalcommunication,1995.).
In the combination
of marine data with land-based
is a real (rather than spuriousas in the other causesof
inhomogeneities)changein climate.It is consideredan surfacetemperatures,SST anomaliesare used in prefinhomogeneity,however,as the measuredtemperatures erence to NMAT because they are consideredmore
are no longer representativeof a larger area. The ur- reliable [Trenberthet al., 1992]. The basicassumptionis
banization influence we are interested in is the effect on
that on monthlyand longer timescalesan anomalyvalue
the monthly time series. This effect is considerably of SST will be approximatelyequal to that of the air
37, 2 / REVIEWSOF GEOPHYSICS
Joneset al.: SURFACEAIR TEMPERATURECHANGES ß 175
immediately(typically 10 m) above the ocean surface
(NMAT). The rationale is that SST observationsare
more reliable [Parkeret al., 1995a]throughoutthe world.
Fewer individualSST than MAT readingsper unit area
would be required to give averagesof the samereliability, owing to the strongerday-to-dayautocorrelationin
SST comparedwith MAT values[seeParker,1984],even
The FDM is the newestapproachand works on the
first difference(FD) time series,the differencein temperature between successivevalues in a station time
series, e.g., Jan(t) minus Jan(t - 1). FD values are
averaged together for all stationswithin each 5ø x 5ø
grid box as with the CAM technique. To revert to a
similar analysisas CAM, the cumulative sum of each
if the MAT observations had been as reliable as those of
grid box seriesis calculated.The resultingseriesthereSST. In practice,the number of usable MAT observa- fore have similar potential changesin variance due to
tions is only about half that of SST.
changingstation numbersbetween and within boxesas
The independenceof the land and marine compo- in CAM. This aspectwill alsobe presentin RSM series,
nents of the surface temperature record means that but to a lesser extent.
hemisphericand regionalseriescan be used to test the
In all three techniques,large-scalehemisphericand
veracityof the other component.The componentshave global seriesare computedby a weighted averageof all
been shownto agreewith each other on the hemispheric the grid boxes with data. For CAM and FDM the
basisby Parkeret al. [1994] and, usingislandand coastal weightsare the cosineof the central latitude of the grid
regionsin the southwestern
Pacific,byFollandand Salin- box, while for RSM the box size was chosen to be of
get [1995] and Folland et al. [1997].
approximatelyequal area. Petersonet al. [1998b] show
that the differences
3.
• AGGREGATION
OF THE
RAW
DATA
between the methodg for trends over
the 1880-1990 period are only a few hundredthsof a
degree Celsiusper 100 years,the differencesbeing due
more to differencesin techniquesfor calculationof a
3.1. LandComponent
linear trend than to differences
Severaldifferent methodshave been usedto interpolate the station data to a regular grid. Petersonet al.
[1998b] recognize three current and different techniques:(1) the referencestationmethod(RSM) usedby
Hansen and Lebedeff [1987], (2) the climate anomaly
method (CAM) usedby Jones[1994], and (3) the first
difference method (FDM) used by Peterson et al.
[199861.
The CAM technique differs from the other two in
requiringthat eachstationbe reducedto anomaliesfrom
monthly meanscalculatedfor a commonperiod. Jones
[1994]uses1961-1990,requiringeachstationto have at
least 20 years of valuesfrom the base period, and estimates mean values,where possibleusingnearby series,
for stationsthat have long recordsbut that do not have
20 years data during 1961-1990. Station temperature
anomaly values are then averagedtogether for all stations within each 5ø x 5ø grid box. Resulting grid box
time seriesdiffer in the number of stationsbetweengrid
boxesand through time. Individual grid box time series
may therefore have differencesin varianceswith time
due to changesin station numbers.The implicationsof
ods. The
this are discussed in sections 4 and 5.
In the RSM technique,the world is divided into a
number of areas (Hansenand Lebedeff[1987] use 80),
and a key long stationis selected,where possible,in each
area. Within each area, successivelyshorter station
records are adjusted so that their averagesequal the
compositeof all stationsalready incorporatedover the
overlap with the composite.The method enables all
recordsto be usedprovidedthat they have a reasonable
length of overlap (e.g., 10 years) with the composite.
Differences in variance of the area average serieswill
also be present owing to changesin time of station
numbers.
interannual
three
methods
variances.
between
also differ
CAM
the three meth-
somewhat
and FDM
in their
have similar vari-
ances,but the RSM has a significantlysmallervariance
[Petersonet al., 1998b].
A fourth approach, optimal interpolation, has been
proposed by l/innikov et al. [1990]. The approach is
basedon pioneeringRussianwork on the averagingof
meteorologicalfields (recentlypublishedin Englishby
Kagan [1997]). Regional and hemisphericaveragesand
their errors are computeddirectlyand it is not necessary
to calculategrid box values.The most recent exampleof
its use is by Smith et al. [1994] in the calculation of
averageglobal sea surfacetemperature.
3.2. Marine Component
For marine regionsthe raw data must be aggregated
in a different
manner.
Each
SST
or NMAT
value
is
accompaniedby location data. Climatologicalmonthly
fields for each 1ø square and for each pentad (5-day
period) and day of the year were calculatedon the basis
of all in situ data from the 1961-1990 period. For SST a
backgroundclimatologybased partly on satellite data,
adjustedto be unbiasedrelative to the in situ data, was
used to guide interpolation of data voids in each individual month in 1961-1990 [Parker et al., 1995a, b].
These complete monthly fields were then smoothed
slightlyin an attempt to reduce errors in regionswhen
there are few observationsbefore averagingover the 30
years.Harmonic interpolationwasthen usedto generate
the pentad climatology,whichwas then linearly interpolatedto the dailytimescale.Grid box(5ø x 5ø)anomalies
for eachmonth are now producedby trimmed averaging
of all 1ø anomalies(from their respectivedaily average)
for each of the pentads that make up the month. The
trimmingremovesthe effectsof outliers.A final checkis
176 ß Joneset al.: SURFACEAIR TEMPERATURECHANGES
then made for unreasonableoutliers,on a monthlybasis,
by assessing
whether 5ø grid box anomaliesdeviatedby
more than 2.25øCfrom the averageof the eight neighbors in space(of which four had to be nonmissingto
make the test) or time (of whichboth the previousand
next months had to exist to perform the test). If the
value failed the test, it was replaced by the neighbor
average. The 2.25 threshold was chosen empirically:
higher thresholdsadmitted bull's eyes;lower thresholds
causeddeletion of possiblycorrectvalues.Full detailsof
the methodare givenby Parkeret al. [1995a,b]. The grid
box anomaliesare based only on in situ measurements
from shipsand buoys;no satellite data are used.
3.3. Combinationof Landand Marine Components
The two data setsused are Jones[1994] for the land
areasand Parkeret al. [1995a]for the oceans.Both have
been producedon the same5ø x 5øgrid boxbasis.In the
combineddata set, the two data setsare merged in the
simplestfashion.Anomalyvaluesare taken, where available, from each component.For grid boxeswhere both
components are available, the combined value is a
weightedone, with the weightsdeterminedby the fractions of land
and ocean
in the box. Because
the land
componentfor oceanicislandsis potentiallymore reliable than surroundingSST values, the land fraction is
assumed to be at least 25% for each of the island and
coastalboxes.A correspondingcondition is applied to
the ocean fraction
in boxes where this is less than 25%.
The method is describedin more detail by Parkeret al.
[1994, 1995a].
37, 2 / REVIEWSOF GEOPHYSICS
would be slightlylarger using,for example,the period
since t881.
Warming evident in Table 1 is slightly greater for
both periodsfor the SouthernHemisphere(SH), but
there are greater seasonaldifferencesin the NH compared with the SH. The global seriesis calculatedas the
average of the two hemisphericseries.Nicholls et al.
[1996]calculatethe globalseriesasthe weightedsumof
all grid boxes.The latter method can bias the global
averageto the NH value when there are marked differencesin hemisphericcoverage,but the effectis relatively
small [Wigleyet al., 1997].
In the Northern Hemisphere, Figure 1 showsthat
year-to-yearvariabilityis clearlygreaterin winter than in
summer. The greater variability in all seasonsin both
hemispheresduring the yearsbefore 1881 is due principallyto the sparserdata availableduringtheseyearsand
representsgreateruncertaintyin the estimates(see also
section4). This feature is much more marked in the
Northern Hemisphere, especiallyover the land regions
[e.g.,Jones,1994].Both hemispheresclearlyexhibittwo
periodsof warming:1920-1940, especiallyin the Northern Hemisphere,and sincethe mid-1970sin both hemispheres.In the globalannualseries(Figure 2), 10 of the
12 warmestyearsoccurbetween 1987 and 1998. The two
yearsnot occurringin thisperiodare 1944and 1983.The
warmest year of the entire series on a global basis
occurredin 1998, 0.57øCabovethe 1961-1990 average.
The next warmest years were 1997 (0.43øC), 1995
(0.39øC), and 1990 (0.35øC). More discussionof the
database
will be undertaken
in section 5.
3.5. FutureImprovementsto the Database
3.4. Hemisphericand Global AnomalyTime Series
Figure 1 showsthe two hemispherictime seriesby
seasonand year. The global annual series exhibits a
warming (estimatedby a linear trend calculationusing
least squares)of 0.57øCover the 1861-1997period. On
these large spatial scales,the seriesshown agree with
more recent analyses[e.g.,Petersonet al., 1998b],attestingto someextentthat the homogeneity
exercise
of the
previoussectionhas been successful.Different station
At present only --•1000 station records are used to
monitor air temperatures over land regions [Jones,
1994].This representsan apparentreductionby nearlya
factor of 2 from the number availableduringthe 19511980 period, but this reductionis principallyin areasof
good or reasonable station coverage. The reduction
stemsfrom the greater availabilityof past data in some
countries(e.g., United States, Canada, former Soviet
Union (fUSSR), China and Australia),data that are not
temperature compilationsnever use exactly the same
basicland stationand marine data, but the vastmajority
(>98%) of the basicdata is in common.
Table 1 givesmonthly trend values for each hemisphereand the globecomputedover the 1861-1997and
1901-1997 periods.Trends for all months,seasons,and
years in both periods are significantat better than the
99% level. Estimation of the changesover the last 137
yearsusinglinear trendsis just one method that can be
used.All monthly,seasonaland annualseriesare poorly
approximatedby linear trends,particularlyin the Northern Hemisphere (NH). Alternatives,using the differencesbetween the first and last 10- or 20-year periods,
however, give similar results.The estimated trends are
alsopartly dependentuponthe startingand endingyears
and the period used(see alsoKarl et al. [1994]). Trends
available
in real time.
The reductionin land data availabilitydoesnot seriouslyimpair hemispherictemperature estimates,but it
doesreducethe quality of the grid box data set and the
quality of subregionalseries,particularlyin the tropics
[Jones,1995a]. Efforts are currentlyunder way to improve the land componentof the data set. Nearly 1000
siteshave been designatedas the Global Climate ObservingSystem(GCOS) SurfaceNetwork (GSN). Most
World MeteorologicalOrganization(WMO) members
have agreed to attempt to maintain these stationsand,
where possible,the sitesaroundthem well into the next
century.The selectionaimed to utilize the best stations
while achievingand improving the spatial coveragein
recentyears,but accedingto memberswho havebudget
constraints[Petersonet al., 1997]. Once the network is
37, 2 / REVIEWS OF GEOPHYSICS
Jones et al.' SURFACE AIR TEMPERATURE CHANGES ß 177
'
i
LU
'r
Z
o
z
i
i
•
i
i
i
,
i
,
i
i
I
'
I
I
I
'
i
i
i
,
i
,
,
,
i
I
!
I
i
I
,
I
i
I
i
i
,
I
,
178 ß Joneset al.' SURFACEAIR TEMPERATURECHANGES
TABLE 1. Temperature ChangeExplained by the Linear
Trend
Over
Two
Periods
1861-1997
the nineteenth century. Once available, the new data
shouldsignificantlyimprove analysesover much of the
Pacific before World
1901-1997
NH
SH
Globe
NH
SH
Globe
Jan.
0.64
0.57
0.61
0.60
0.59
0.60
Feb.
March
0.70
0.66
0.55
0.56
0.63
0.61
0.75
0.75
0.60
0.64
0.68
0.69
April
May
0.52
0.50
0.58
0.66
0.55
0.58
0.65
0.61
0.62
0.72
0.64
0.66
June
0.31
0.73
0.52
0.53
0.68
0.60
July
Aug.
Sept.
0.29
0.40
0.41
0.59
0.58
0.56
0.44
0.49
0.48
0.48
0.49
0.49
0.68
0.67
0.66
0.58
0.58
0.57
Oct.
Nov.
Dec.
Year
0.63
0.74
0.69
0.54
0.62
0.60
0.60
0.60
0.63
0.67
0.65
0.57
0.51
0.59
0.74
0.60
0.67
0.68
0.60
0.65
0.59
0.63
0.67
0.62
All temperature changevaluesare in degreesCelsius.
37, 2 / REVIEWSOF GEOPHYSICS
War II and in the Southern
Hemi-
sphere.
4.
ERRORS
This sectiondiscusses
the accuracyof the estimation
of hemisphericand global averageanomalies.The error
of estimation
is due to two factors: measurement
error
and, most importantly, changesin the availability and
densityof the raw data, i.e., samplingerrors.
4.1.
Measurement
Errors
Both the basic land and marine
data are assessed for
errors each month. Extreme land values exceeding3
standard deviations
from the 1961-1990
statistics are all
checkedand spatialfields of normalizedvaluesare asfully operational,it will improvethe quality of the data, sessedfor consistency.
Marine valuesmust passnearparticularly for recent years,but it will succeedonly in neighbor checks, and 1ø area pentad anomalies are
partially restoringthe data coverageavailableduringthe trimmed values using winsorization [Afifi and Azen,
1951-1980 period.
1979]to temperthe effectof outliers.Our applicationof
Improvementsare alsobeingundertakenovermarine winsorizationwas to replace all valuesabovethe fourth
areas. Buoy data are becoming increasinglyavailable, quartile by the fourth quartile and all valuesbelow the
particularlyin the SouthernHemisphereand the tropi- first quartile by the first quartile, before averaging.Alcal Pacific.Efforts are underwayin the United States(at ternative thresholds such as the thirtieth and seventieth
the National Climatic Data Center (NCDC), through percentile would have been possible.Despite these
the Comprehensive Ocean-Atmosphere Data Set checks,it is likely that erroneousvalues enter both the
project)and in the United Kingdom(the Hadley Centre land and marine analyses,though any effect is considof the MeteorologicalOffice) to digitize much of the ered to be essentiallyrandom [Trenberthet al., 1992].It
ships'log material that is knownto be availablein some is also probable, although less likely, that correct but
regions,principallyfor the two World War periodsand extremevaluesmay be discarded.
0.5
0.0
-0.5
0.5
-
Figure 2. Hemisphericand global temperature
averageson the annual timescale(1856-1998),
0.0
relative
-0.15
0.15 -
0.0
-0.15
Global
18•60
' 18•80
' 19•00
' 19•20 19•40 19•60 19•80 2000
to 1961-1990.
37, 2 / REVIEWSOF GEOPHYSICS
Joneset al.' SURFACEAIR TEMPERATURE
CHANGES ß 179
MAM
D•F
120E
180
120W
60W
0
60E
120E
120W
180
120E
60W
0
60E
120E
0
60E
120E
SON
JJA
OON
120E
180
120W
60W
0
60E
120E
120E
180
120W
60W
Annual
90N
60N
30N
EQ
30S
60S
•20E
•e0
•20W
60W
0
•0E
•20E
Temperaturetrends (øCper 20 yr) over 192•44
-3
-2
-1
-0.5
0
0.5
I
2
3
Plate la. Trendof temperature
ona seasonal
andannualbasisfor the20-yearperiod1925-1944.Boxeswith
significant
lineartrendsat the 95% level(allowingfor autocorrelation)
are outlinedby heavyblacklines.At
least2 (8) month'sdatawererequiredto definea season(year),andat leasthalf the seasons
or yearswere
required to calculatea trend.
180 ß Joneset al.' SURFACEAIR TEMPERATURE
CHANGES
37, 2 / REVIEWSOF GEOPHYSICS
DJF
[10N
120E
180
120W
60W
0
60E
120E
120E
180
120W
60W
JJA
0
60E
120E
SON
90N
.•••••.••,•..•---•,•.
•
o o ..:..
_,••
•
,
--T
..
120E
160
120W
60W
0
60E
120E
120E
180
120W
60W
0
60E
Annual
90N
60N
EQ
60S
90S
120E
18O
120W
60W
0
60E
Temperaturetrends (øCper 20 yr) over 1978-97
b
-3
Plate lb.
-2
-1
-0,5
0
0.5
I
2
3
Sameas Plate la but for the 20-yearperiod 1978-1997.
-.
120E
120E
37 2 / REVIEWS OF GEOPHYSICS
Joneset al.' SURFACE AIR TEMPERATURE CHANGES ß 181
(a)
90N
60N
30N
EQ
30S
60S
90S
0
30E
60E
90E
120E
150E
180
150W
120W
90W
60W
30W
0
0
30E
60E
90E
120E
150E
180
150W
120W
90W
60W
30W
0
Figure3. Spatialpatterns
of (a) s/2and(b) ?,calculated
fromannualdataontheinterannual
timescale.
In
Figure 3b, dark shadingindicatesvaluesof <0.8.
4.2. SamplingErrors
The availabilityand densityof data usedin the compilationof the surfacetemperaturefield are everchanging. For mostof the post-1850period, apart from the
two World War periodsand yearssinceabout1980,the
changesshould have led to an improvementin the
accuracyof the large-scaleaverageanomaliesthrough
the useof morecompletedatafields.Mapsshowingdata
availabilityby decadefor the last 120yearsare givenby
Parker et al. [1994]. Samplingerrors were initially assessedby using"frozengrids"[Jones,1994],i.e., computation of the hemisphericanomaliesusingonly the grid
boxesavailablein earlier decades.Although theseanalysesprovedthat samplingerrorswere relativelysmall
and that therewasno long-termbiasin the estimationof
temperaturesusingthe sparsegridsof the nineteenth
century,they failed to take into accountthe effectsof
regionsalwaysmissingand changesin the densityof the
networkwith time in someareas.The effectsof sampling
errors in the estimation of hemispherictemperature
trendshas alsobeen addressedby Karl et al. [1994].
Samplingerrorsare now assessed
in a more thorough
way,usingeitherbasicprinciples[Joneset al., 1997a]or
optimal averaging[Smithet al., 1994]. Standarderrors
(SEs) of regional and hemisphericestimatesdepend
upon two factors:the locationsand the standarderrors
of the grid boxeswith data.Joneset al. [1997a]calculate
the SE of each grid box using
SE2 - s/27(1
- 7)/[1 + (n - 1)7]
(])
wheres•2istheaverage
variance
of allstations
in thebox,
7 is the averageinterrecordcorrelationbetween these
sites,and n 'is the number of site recordsin the box. Over
marine boxesthe numberof stations(n) is taken to be
the number of individual SST measurementsdivided by
5. The parameters
s•2 and7 maybe considered
asthe
temporalvarianceand a functionof the spatialvariance
within each grid box, respectively.Standarderror estimates are also made for grid boxeswithout data using
interpolation
of thes•2 and7 fields,withn setto zero.
The method takes into accountthe changingnumber
182 ß Jones et al.' SURFACE AIR TEMPERATURE CHANGES
37 2 / REVIEWS OF GEOPHYSICS
(a) Global mean decadal temperature anomaly
0.4
0.2
0.0
-0.2
-0.4
-0.6
1860
1880
1900
1920
1940
1960
1980
Year
(b) Northern hemisphere decadal temperature anomaly
I
I
I
I
I
0.6
0.4
0.2
Figure 4.
One and two standarderrors
(SE) on the hemisphericand globaltemperatureson the decadaltimescale.The
shadedareas highlight +__
1 SE.
o.o
-0.2
-0.4
-0.6
_
_
_
I
,
,
•
1860
I
,
1880
I
,
•
•
1900
I
,
•
•
1920
I
•
,
•
1940
I
•
•
•
1960
I
•
,
1980
Year
(c) Southern hemisphere decadal temperature anomaly
'
'
i
'
'
'
i
'
'
'
I
'
'
i
I
i
0.0
-0.2:
-0.6
1860
1880
1900
1920
1940
1960
1980
Year
of stationswith time in the individualgrid box seriesbut
doesnot correctfor it. Time seriesof individualgrid box
serieswill exhibit variance changesthat may be unrealisticand simplya resultof changingstationdensity.This
factor
is discussed more in section 5.
grid box size, and thus never approacheszero when (1)
would imply an SE value of zero. Joneset al. [1997a]
showthat an alternate assumptionabout the standard
error(SE = s/2,forn = 0, instead
of s•27)
givesslightly
largervalues(by ---0.01-0.02)than thosepresentedhere.
In midlatitude continental regions where there is
The implicationsof the formula are that in any global
it is more important to havetemperature
greatvariabilityof temperature
with time,s/2will be SE assessment
higher than over lessvariable regionssuch as coastal estimatesfrom continentalregions,as opposedto oceareas, the oceansand tropics. In these continentalre- anic regions,of the same size, becauseof the former
gions, ?, which has a maximum value of 1, is lower regions' greater variability. If, for example, a limited
(---0.8-0.9) thanoveroceanicandtropicalregions(>0.9). network of siteswere to be deployedto measureglobal
Figure 3 illustratesthis with the fields for s/2and ?, temperature,a muchgreaterpercentageof themwouldbe
calculated for annual data on the interannual timescale.
locatedoverlandthanthe land/oceanfractionwouldimply.
Figure3 shows
thatthefieldsofs•2 and?arerelatively
The value of ? is alwaysgreater than 0.7, for the 5ø x 5ø
37, 2 / REVIEWS OF GEOPHYSICS
Joneset al.: SURFACE AIR TEMPERATURE CHANGES ß 183
smoothon the annualtimescale.This alsoapplieson the
monthly timescale,where ? falls to minimum values of
about 0.7 in some regionsin the winter season.Joneset
al. [1997a] used thesesmoothfeaturesto enablevalues
of these variables to be estimated for all grid boxes
without temperaturedata. SE valuesfor suchgrid boxes
can then be estimatedfrom Eq. (1)•with n = 0. The
reliability of these assessments
was tested using 1000year controlrunsof generalcirculationmodels(GCMs),
for which completeglobalfieldsof surfacetemperature
data were available[Joneset al., 1997a].
With SE estimatesfor each grid box, which will vary
in time due to data availability,we can calculate the
averagelarge-scaleSE in a similar manner to the calculation of the large-scaleaveragetemperatureanomalies,
Ng / Ng
SE2- • SE•
2cos(lati) • cos(lati)
i=1
(2)
only and do not include additional uncertainties that
relate to the correction
of SST bucket observations
or to
any residual urbanization effects over some land areas.
5.
ANALYSES
OF THE
SURFACE
TEMPERATURE
RECORD
The record has been extensivelyanalyzed over the
last 10-15 yearsin a seriesof reviewpapersor chapters
in nationaland internationalreviews,includingthosefor
the IPCC [e.g., Wigleyet al., 1985, 1986, 1997;Ellsaesser
et al., 1986;Folland et al., 1990, 1992;Jonesand Briffa,
1992; Jones, 1995a; Nicholls et al., 1996]. The hemisphericand global seriesand their trends have already
beendiscussed
in section3.4. In thissectionwe compare
the two periods of twentieth century warming, 19251944 and 1978-1997, and discuss trends in the areas
i=1
affectedby extremewarmth, trends in Arctic temperatures, trends in maximum and minimum temperatures,
where Ng is the total number of grid boxesin the region
the apparent disagreement between recent warming
under study.If all the grid boxeswere independentof
measuredat the surfaceand by satellitesfor the lower
eachotherthenthe large-scale
SE2 wouldbe SEe ditroposphere,and the last 150 yearsin the contextof the
vided by the number of grid boxes.However, the effec- millennium.
tive numberof independentpoints(Neff) is considerably
lessthan the total number of grid boxesand difficult to
of the Two Twentieth-Century
estimate.Neff hasbeen discussed
by earlier authors[e.g., 5.1. Comparison
Warming
Periods
Livezeyand Chen, 1983;Briffa and Jones,1993;Madden
Figures 1, 2, and 4 clearly indicate that most of the
et al., 1993 and Jonesand Briffa, 1996] and referred to
warming during this century occurred in two distinct
also as the number of spatial degreesof freedom. Neff
periods. The periods differ between the hemispheres
valuesin Joneset al. [1997a]were estimatedusingthe
and amongthe seasons,but the periodsfrom about 1920
approachsuggested
by Maddenet al. [1993].
to 1945 and since 1975 stand out. Plate 1 shows the trend
The SE of the global, hemisphericor large-scaleavof temperature changeby seasonfor the two 20-year
erage is therefore
periods,1925-1944(Plate la) and 1978-1997 (Plate lb).
2
Smgloba
I = sm2/Neff
(3) The annual globaltemperatureincreasesduring the two
periods were 0.37 and 0.32øC, respectively,with the
followingan analogyfrom earlier work by Smith et al. secondperiod on average0.27øCwarmer than the first.
[1994].Typicalvaluesof Neff dependon timescale,makIn both periods,the regionswith the greatestwarming
ing comparisonsbetween different studies difficult. and coolingoccurover the northern continentsand,with
Joneset al. [1997a], for annual data on the interannual the exceptionof a few isolated grid boxes, during the
timescale, obtain values of ---20 for both surface obser- December-January-February
(DJF), March-April-May
vations and similar fields from the control
runs of
(MAM), and September-October-November(SON)
GCMs. Values increase to --•40 for seasonal series on
seasons.The warming is generallygreatestin magnitude
this timescale,with slightlyhighervaluesin the northern during the DJF and MAM seasonsover the high latisummer comparedto the winter season.The timescale tudesof the northerncontinents,althoughtheseregions
dependenceof all the formulaemeansthat the SE values are not alwaysthosewhere local statisticalsignificanceis
must be calculated
on the timescale of interest. SE values
on the decadaltimescaleare onlymarginallysmallerthan
achieved.
Small
trends
over
some
oceanic
areas
are
significantbecauseof low variabilityin year-to-yeartemperature values. The 1978-1997 period has a much
Figure 4 showsthe global and hemispherictempera- larger area with statisticallysignificantwarming comture values with appropriate standard errors on the pared with 1925-1944.The pattern of recentwarmingis
decadaltimescale.Given the greaterpercentageof miss- strongestover northern Asia, especiallyeasternSiberia.
ing regionsin the SouthernHemisphere, the SE values It is also evident over much of the Pacific basin, western
are highestthere. SE valuesdecreasewith time during partsof the United States,westernEurope, southeastern
the courseof the last 150 years,reachingtypicalvalues Brazil, and parts of southern Africa. The 1925-1944
for the globeduringthe last 50 yearsof 0.048øC(0.054) warming was strongest over northern North America
on the decadal(interannual)timescale.It must be re- and is clearlyevidentover the northernAtlantic, partsof
memberedthat these estimatesare for samplingerrors the westernPacificand centralAsia. The improvements
those calculated on the interannual
timescale.
184 ß Joneset al.: SURFACE AIR TEMPERATURECHANGES
37, 2 / REVIEWS OF GEOPHYSICS
in coveragebetweenthe two periodsare apparent,and
some shippingroutes are surroundedby data voids in
1925-1944 (see alsoParkeret al. [1994]).
5.3. Trendsin Arctic Temperatures
There has been recent controversyregardingArctic
temperaturetrends.We discussin this sectionthe three
principalanalyses.Chapmanand Walsh[1993]examined
5.2. Trendsin the AreasAffectedby Extreme
the overall seasonal behavior
Warmth
of Arctic
land station tem-
peratures between 1961 and 1990. Their results show
The previous section showsthat 20-year warmings that in winter and spring,warming dominatedwith valwere greatestover continentaland high latitude regions uesof about0.25 and 0.5øC/decaderespectively.In sumwhere temperatures vary most on interannual time- mer, the trend was near zero, and in autumn the trend
scales:extreme colorsin Plate 1 are not apparent over either was neutral or showeda slightcooling.Considerthe tropicsand the oceans.The 95% significancelevels ing the longerrecordback to the mid nineteenthcentury
on Plate 1, however,showthat warming,particularlyfor [Jones,1995c], only the spring seasonhad its warmest
the mostrecent 20 years,wasjust assignificantin the less yearsof the entire period recently,i.e., in the 1980sand
interannually and decadallyvarying regions compared 1990s.
with the highly variable regions.Recently,Joneset al.
For the temperature trends over the pack ice, there
[1999] has transformedthe surfacetemperature data- are two investigations[Kahl et al., 1993; Martin et al.,
basefrom one in anomalieswith respectto 1961-1990 to 1997], which yield contradictoryresults. We refer to
percentilesfrom the sametime period. This enablesthe these as KCZ and MMD, respectively.For the western
rarity of extreme monthly temperaturesto be directly Arctic (70ø-80øN, 90ø-180øW) and the time period
comparedbetween grid boxes.The transformationwas 1950-1990, KCZ used a combination of surface air
performed usinggamma distributions,which treat each temperature data determined from dropsonde data
grid box impartially, accountingfor the effects of both taken by the U.S. Ptarmigan flights for 1950-1961 and
North Pole
differences in interannual variability acrossthe world, radiosonde data from the fUSSR-manned
and differences in skewness.
(NP) drifting ice stationsfor 1954-1990 (data are disPlate 2 compares the anomaly and the percentile cussedin more detail in section6.4). From the combimethod for displayingannual temperaturesfor the year nation of these two data sets, KCZ find a significant
1998(the year 1997is shownbyJoneset al. [1999,Figure cooling trend of 1.2øC/decadein autumn and 1.1øC/
7]). The percentilemap indicatesunusualtemperatures decade in winter, which contradicts the land-based studover many tropical and oceanicregionsthat would not ies. Walsh [1993, p. 300], however,observes"that only
warrant a secondglance in the anomaly form. Similar one [NP] track extends as far south as 75øN in the
maps could be constructedusing the normal distribu- Western Arctic domain..." and that many of the droption, but seasonaland annual temperatures are often sondeprofileswere obtained from the climatologically
negativelyskewedover many continentaland high lati- warmer marginal ice zone, "... where all these (droptude regions.The gammadistributionalsotakesaccount sonde)measurements
were made in the first 12 years...
of this.
of the study period." This implies that their observed
Figure 5 showsthe percentageof the analyzedarea of coolingtrends may be generatedby the geographically
the globe with annual temperatures greater than the warm bias of the dropsonde measurements.Another
ninetieth and lessthan the tenth percentile since 1900. sourceof potential biasis the instrumenttime lag as the
An increasein the percentageof the analyzedareaswith dropsondefalls through the Arctic inversion,and the
warm extremesis evident,but by far the greatestchange fact that the NP radiosondesurface temperaturesare
is a reductionin the percentageof the analyzedareawith always taken from multiyear ice or ice islands,while
cold extremes. Some caution should, however, be exer- some of the dropsondesurface temperaturescould be
cisedwhen interpretingthese results.Large changesin from open leads or thin ice.
Given the impact that the KCZ paper had on the
coveragehave occurredover the twentieth century,so
some of the changesmay be due to areas entering the sciencecommunity,MMD searchedfor trendsin the NP
analysisduringthistime. Coveragechangesare minimal, 2-m air temperature data for 1961-1990, where these
however, since 1951. Second,grid box values based on temperatureswere taken at 3-hour intervals inside a
fewer stations or few individual SST observations, a Stevensonscreen, then resampled to 6-hour intervals.
more commonsituationat the beginningof the century, Their area of interestis an irregularregionin the central
are more likely to be extreme in this percentile sense Arctic, excludingthe climatologicalwarmer regions of
(see earlier discussionin section4.2 and Joneset al. the Beaufort Sea southof 77øN and the vicinityof Fram
[1997a]). Again, changesin the densityof observations Strait. In their trend analysis, MMD used both the
per grid box are minimal since1951.In future analysesit measuredand anomalyfields,where the anomalieswere
shouldbe possibleto correct for this secondproblem, the difference between the daily NP temperaturesand
producinggrid box time seriesthat do not havechanges the griddedmean daily field definedfrom optimal interin variance due to changesin station or SST numbers, polation of the buoy, NP, and coastalstation temperausing some of the formulae developedby Joneset al. tures for 1979-1993 [Martin and Munoz, 1997]. The
purpose in using the anomaly fields was to remove
[1997a].
37, 2 / REVIEWS OF GEOPHYSICS
Jones et al.' SURFACE AIR TEMPERATURE CHANGES ß 185
Surface
Temperature
Anomalies
(øC,w.r.t.1961-90)
1998
90N
60N
30N
0
30S
ß
60S
90S
180
120W
-5
-3
60W
-1
-0.5
0
-0.2
60E
0
0.2
120E
0.5
Ocean: Parker et al (Climatic Change199•)
1
180
3
5
Land: Jones(J.Climate 1994)
Surface Temperature Anomaly Percentiles(w.r.t. 1961-90)
1998
•
(Anomalies fitted to Gamma Distributions)
I
90N'
I
I
•_.••
I
•
I
_•_,, .___ __ I
60N --•'-"X• '-'• '-"•'•x._/-30N
"
0
30S
x..
60S
90S
•0
0
120W
2
10
60W
20
30
0
40
50
60E
60
70
120E
80
90
180
98
100
Plate 2. Surfacetemperaturesfor 1998,relativeto the 1961-1990average,expressed
(a) as anomaliesand
(b) as percentiles.The percentileswere definedby fitting gammadistributionsto the 1961-1990 annual
deviationsrelativeto the 1961-1990average,for all 5ø x 5øboxeswith at least21 yearsof annualdata in this
period.
186 ß Joneset al.: SURFACEAIR TEMPERATURECHANGES
1910
1920
1930
1940
1950
1960
40
1970
37, 2 / REVIEWSOF GEOPHYSICS
1980
1990
Figure 5. Percentageof the monitored area
of the globe, for eachyear from 1900 to 1998,
with annual surface temperaturesabove the
ninetieth percentileand below the tenth percentile.The percentileswere definedby fitting
gamma distributions,based on the 1961-1990
period, usingall 5ø x 5ø boxeswith at least 21
years of data.
'
3O
2O
10
1910
1920
1930
1940
1950
1960
1970
1980
1990
geographicalinhomogeneitiesfrom the temperatures. by includingmore data (54% of the globalland areas)
For both fields, MMD found statisticallysignificant and extendingthe time seriesto 1993. (Until very rewarming to occur in May (0.8øC/decade)and June cently,mean monthlymaximumand minimumtemper(0.4øC/decade),and on a seasonalbasis, they found atureswere not routinely exchangedbetweencountries.
significantwarmingin the summeranomalyfield (0.2øC/ This is one data set it is hoped will be dramatically
decade).Although the other seasonaltrendswere not improvedthroughGCOS (see section3.5).) This study
significantat the 95% level, MMD alsofound warming confirmedthe earlier study,showingthat for nonurban
in winter (0.35øC/decade)and spring (0.2øC/decade), stationsover 1950-1993,minimumtemperaturesroseby
and a coolingin autumn (-0.2øC/decade).Their num- 0.18øC/decade,
maximaroseby 0.08 per decade,and the
bers are consistentwith land stations(Chapman and DTR decreasedby 0.08øCper decade.The new study
Walsh[1993] and the data used in 5ø grid box analyses loweredthe trend of minimumtemperatures(0.18 comhere).
paredwith 0.21) somewhatby incorporatingmuchmore
tropicalstationdata. It alsodecreasedthe possibilityof
5.4.
Trends in Maximum and Minimum
urbanizationeffectsbeing the causeby usingonly nonTemperatures
urban stationsin the analysis.
Up to now all the surfacetemperatureanalysesprePlate 3 showsthe trendsfor each5øby 5øgrid box on
sentedin this paper have usedmonthlymean tempera- an annual basis for the 1950-1993 period. Minimum
tures. This situation arisesprincipally from the wide- temperatures decreasein only a few areas, while the
spread availability of this variable. Over the last few DTR decreasesin most regionsexceptArctic Canada
decades,greaterunderstandingof the causesof someof and some Pacific Islands.The new studymade use of
the changeshas been gained by consideringrelated manyregionalassessments
reportedin a specialissueof
changesin other variables,suchas circulationand pre- the journalAtmospheric
Research[seeKukla et al., 1995].
cipitation [e.g., Parker et al., 1994; Jonesand Hulme,
1996]. Other temperature variables to receive similar 5.5. Comparisons
of RecentSurfaceWarmingWith
widespread interest are the monthly mean maximum Satellite Estimates
and minimum temperaturesand their difference, the
Figures l, 2, and 3, and Plates lb and 3 all indicate
diurnal temperaturerange (DTR). These temperature that surfacetemperatureshave risen this century and
extremeserieshavemeaningonly over land regions,and particularlyover the last 20 years or so. The increaseis
there are only enoughdata spatiallyto warrant analyses seen in most areas and is evident over land and marine
sinceabout 1951. Longer data setsshouldexistback to regions(see especiallyPlate lb). Recently,somedoubt
the late nineteenthcenturybut are availabledigitallyfor hasbeen caston the surfacerecordby comparisons
with
only a few countries(e.g., United Statesand fUSSR).
satellite measuresof the average temperature of the
Karl et al. [1993b]were the first to extensivelyanalyze lower troposphere.The satelliterecord usesmicrowave
trends in these variables,using gridded data covering soundingunit (MSU) instrumentson the NOAA series
37% of the global land area. They found that for the of polar orbiters (see Christyet al. [1998] for the most
1951-1990 period, minimum temperatureswarmed at 3 recentreviewof the lower tropospheresatelliterecord).
times the rate of maximum temperatures.DTR de- Comparisonsare usuallymade with the MSU2R record
creasedover this period by 0.14øC/decade,while mean (whichmeasuresthe weightedverticaltemperatureprotemperaturesincreasedby 0.14øC/decade.Their study file of the tropospherefrom the surfaceup to 250 hPa,
confirmedearlier studiesfor the United States[Karl et centeredon -750 hPa) but the original MSU2 record
al., 1984].Easterlinget al. [1997]haveextendedthisstudy (surfaceup'to 200 hPa, centeredon -500 hPa) is also
37, 2 / REVIEWS OF GEOPHYSICS
Jones et al' SURFACE AIR TEMPERATURE CHANGES ß 187
Plate 3. Trends(in degreesCelsiusper 100years)for each5ø x 5øgridboxfor (a) maximum,(b) minimum,
and (c) diurnal temperaturerange for nonurban stations.Redrawn with permissionfrom Easterlinget al.
[1997]. Copyright 1997 American Associationfor the Advancementof Science.
188 ß Joneset al.: SURFACE AIR TEMPERATURE CHANGES
37, 2 / REVIEWS OF GEOPHYSICS
....
•'.•
, , ..,.
.
-
0-3.0
_
C
Plate 3.
(continued)
relevant [see Huh'ell and Trenberth,1996, 1997, 1998]. a trend close to -0.04øC/decade. However, over the
The latter is known to have partial lower stratospheric longer period from 1965 to 1996, the radiosonderecord
influences[see Christyet al., 1998].
agreeswith the surface (0.15øC/decadewarming). RaThe doubt comes from the fact that over the 1979diosonderecordsare known to contain many problems
1996 period the surfacewarmed relative to MSU2R by of long-term homogeneity[Parkeret al., 1997, and ref0.19øCper decade[Joneset al., 1997b].Althoughneither erencestherein],but it doesseemstrangeto concentrate
the surfacewarming,the slightMSU2.R cooling,nor the on the 1979-1996 agreementsbetweenradiosondesand
difference is statisticallysignificant,doubts have been MSU2R and forgel the longer 32-year agreementbecaston the veracityof the surfacerecord since1979 and, tween the radiosondes and the surface. Fourth, and
by inference,the pre-1979period aswell. The doubtsare often ignoredin the comparisons,
is the long-termagreeoften expressedin the non-peer-reviewed,"grey litera- ment, both over this centuryand since1979,betweenthe
ture," in newspaperarticles,and on Web pages.
!and and marine componentsof the surfacerecord (see
Several intercomparisonshave been undertaken to also section2.2).
Several reasonshave been postulatedfor the differunravel reasonsfor the differences[see, e.g. Christyet
al., 1998;Hurrell and Trenberth,1996, 1997, 1998;Joneset ences.The MSU2R record is an amalgamationof many
al., 1997b].Severalissuesare undisputedin thesestud- different satellite series, each requiring calibration
ies. First, 20 years is too short a period over which to againsteachother aswell ascorrectionsfor orbital decay
calculate trends and draw meaningful conclusions.Sec- of the satellite and possibledrift in the satellitecrossing
ond, the two records are not the same, and their trends times. Changesrelative to the surfacerecordwere most
need not be equivalent.The recordsmeasuretempera- noticeable during 1981-1982 and again during 1991
tures at different levels.At the surface,recent warming (both times of satellite changes);these may be at least
is greater in minimum temperatures,and the shallow partly instrumentaland havebeen highlightedby Hurrell
nocturnal boundary layer over land areas is often de- and Trenberth[1996, 1997, 1998]andJoneset al. [1997b].
coupled from the lower troposphere. More realistic There are no evident reasonsfor heterogeneityin the
comparisons with lower tropospheric temperatures land or marine surface record at these dates. The
might be made with maximum temperatures,but these MSU2R-radiosonde agreement is generally good, alare less spatially extensive.Also, becausethe MSU2R though a similar comprehensivecomparisonsimilar to
record is short, it may be possiblefor its trend to differ the surface-MSU2R assessment
of Joneset al. [1997b]
substantiallyfrom that for the surfacewithout the two has yet to be carried out. Comparisonsare usuallycarbeing physicallyincompatible.
ried out as zonal and hemisphericmeans.Hurrell and
Third, the MSU2R trend agreesalmost exactlywith Trenberth[1998] showthat local colocatedcomparisons
radiosondedata [Parkere! al., 1997]aggregatedtogether showpoor agreementover tropicalcontinents.They also
to mimic the equivalentverticalprofile of MSU2R, with highlight the original MSU2 record, which shows a
37, 2 / REVIEWSOF GEOPHYSICS
Joneset al.: SURFACEAIR TEMPERATURECHANGES ß 189
warmingof 0.05øC/decade
over the 1979-1993 period. If
the MSU2R is in error, the implication is that the
radiosonde
record must also contain
1200
1400
1600
1800
2000
1200
1400
1600
1800
2000
errors. The reason
favoredby Christyet al. [1998] is that the differencesare
real and representdifferencesin warming rates at different levelsin the lower troposphere.
Recently, another potential inhomogeneity in the
MSU2R recordhasbeenidentifiedby Wentzand Schabel
[1998]. It relates to the orbital decay of each satellite,
which is driven by variations in solar activity causing
variationsin orbital drag.They calculatethat on a global
0.0
-0.5
basis, the effect should result in a correction of the
MSU2R data by +0.12øC/decade.The effect does not
changeother MSU channels,includingthe raw MSU2
data. J. R. Christy (personal communication,1998)
claimsthat the effect is alreadyimplicitly accountedfor
duringthe numerousintersatellitecalibrationsnecessary
to producethe continuousseriessince1979.Santeret al.
[1999], in an extensivestudyof troposphericand stratospheric temperature trends from radiosondes, MSU
data, and model-basedreanalysessince 1958, indicate
that all have potential biasesand inhomogeneitiessuch
that the wholeissueis unlikelyto be resolvedin the near
future. Only comprehensivecolocated comparisonsof
radiosondesand the MSU2R and greaterunderstanding
and correction
of the biases in radiosondes will resolve
the issue.In manyregionsthe necessarymetadata,giving
information on which sondes have been flown, are not
alwaysavailable.
5.6.
The Last 150 Years in the Context of the Last
Millennium
The globalsurfacetemperaturehas clearlyrisen over
the last 150 years (Figure 2 and Table 1), but what
significancedoesthis havewhen comparedwith changes
over longer periodsof time? The last millennium is the
period for whichwe know mostaboutthe preinstrumental past, although it must be remembered that such
knowledgeis considerablypoorer than for the post-1850
period. Information aboutthe pastcomesfrom a variety
of proxy climatic sources:early instrumental records
back to the late seventeenthcentury,historicalrecords,
tree ring densitiesandwidths,ice core and coral records
(see Joneset al. [1998], Mann et al. [1998], and many
referencestherein for more details). Uncertaintiesare
considerablygreater because information is available
only from limited areas where these written or natural
archives survive and, more importantly, because the
proxyindicatorsare only imperfectrecordsof past temperature change(see, e.g., Bradley[1985], Bradleyand
Jones[1993],and the papersin volumessuchas thoseof
Bradleyand Jones[1995] andJoneset al. [1996]).
Over the last few years,a number of compilationsof
proxy evidencehave been assembledfollowingthe pioneeringwork of BradleyandJones[1993].In Figure 6 we
show three
recent
reconstructions
of Northern
Hemi-
Figure 6. Northern Hemisphere temperature reconstructions from paleoclimaticsources.The three seriesare Mann et
al. [1998,1999](thick),Briffaet al. [1998](medium)andJones
et al. [1998] (thin). All three annuallyresolvedreconstructions
havebeen smoothedwith a 50-yearGaussianfilter. The fourth
(thickest) line is the short annual instrumentalrecord also
smoothedin a similar manner.All seriesare plotted as departures from the 1961-1990 average.
[Mann et al., 1998, 1999] and two definitionsof summer
[Briffaet al., 1998;Joneset al., 1998]). The shortinstrumental record on an annual basis is superimposed.
Agreement with the annual instrumentalrecord is poorest during the nineteenth century,partly becauseof the
different seasons(summer in two of the series)used.
The instrumental record also rises considerablyin the
last 2 decades,and this cannotbe seenin the multiproxy
seriesbecausethey end before the early 1980s,as some
of the proxy recordswere collectedduring theseyears.
The most strikingfeature of the multiproxyaverages
is the warming over the twentieth century, for both its
magnitude and duration. The twentieth century is the
warmestof the millenniumand the warmingduringit is
unprecedented(see alsodiscussion
by Mann et al. [1998,
1999]andJonesetal. [1998]).The four recentyears1990,
1995, 1997, and 1998, the warmest in the instrumental
series,are the warmest since 1400 and probably since
1000. The end of the recent E1 Nifio event (suchevents
tend to warmer temperaturesglobally)and the greater
likelihood of La Nifia (which tends to lead to cooler
temperatures)as opposedto E1 Nifio conditionsduring
1999and 2000 meansthat 1998will likely be the warmest
year of the millennium.The coolestyear of the last 1000
years, based on these proxy records,was 1601. If standard errors could be assignedto this assessment,they
would be considerablygreater than errors given during
the instrumentalperiod [Joneset al., 1997a]. The standard errors givenby Mann et al. [1998, 1999] may seem
relatively small, but they only relate to their regressions
and include neither the large additional uncertaintiesin
someof the proxyrecordsbefore the nineteenthcentury,
nor the standard
errors of the instrumental
record.
Numerous paleoclimaticstudieshave consideredthe
spheretemperaturefor part of the last millennium.The
reconstructionsare all of different seasons(annual last millennium and two periods, the Little Ice Age
190 ß Jones et al.: SURFACE AIR TEMPERATURE CHANGES
[Grove,1988;Bradleyand Jones,1993] and the Medieval
Warm Epoch [Hughesand Diaz, 1994], are often discussed.The latter two studiesquestionperceivedwisdom that these periods were universallycolder and
warmer, respectively,than the present.The questioning
arisesfrom the considerableadvancesin paleoclimatologythat havebeen made over the past 15-20 years.The
series used in Figure 6 contain from ten to several
hundred Northern Hemisphere proxy climatic series,
considerablymore seriesthan previouslyanalyzed.The
coldest century of the period since 1400, the seventeenth, was on averageonly 0.5ø-0.8øCbelow the 19611990 base. The
differences
between
these studies and
someearlier work is that they are basedon considerable
amountsof proxy data rather than individual seriesor
schematicrepresentations
of perceivedwisdom[Folland
et al., 1990, Figure 7.1].
6.
ANOMALY
VERSUS
ABSOLUTE
TEMPERATURES
Throughout this review, temperature serieshave alwaysbeen quotedin degreesCelsiuswith respectto the
"normal" period 1961-1990.Earlier analysesused19511970 [Joneset al., 1986a,b] or 1951-1980 [Parkeret al.,
1994]. The only systematicdifferencesbetween these
analysesare simplychangesin referencelevel,dueto the
different normal periods. There will, however,be random differencesdependingon the normal period, due to
missingdata values affecting the computation of the
referenceperiod means(see also Wigleyet al. [1997]).
Anomaly valuesovercomemost of the problemswith
absolutetemperaturessuchas differencesbetween stations in elevation, in observation times, the methods
used to calculate monthly mean temperatures, and
screentypes.Any changeswith time in these aspects
should be corrected in the initial homogeneityassessment of a station temperature time series. Anomaly
valuesshouldbe all that are required in any analysisof
the basic5ø x 5ø grid box data set.
The first absolutetemperature climatologywas producedby Crutcherand Meserve[1970]for the NH and by
Taljaardet al. [1969]for the SH. The 5øgrid point values
from theseearlyworksare availabledigitally.Data avaiIabilitymusthavebeen poorerthen than now.Also, these
earlier analysesmust be basedon few yearsof record in
data sparseregionssuchas Antarctica and parts of the
tropics,makingit difficultfor eachregionto be basedon
a consistentcommonperiod.
Recent work, however,has enabled absolutetemperatures for the 1961-1990 period to be much better
defined, using considerablyenhanceddata sets.In this
sectionwe produce a globallycompleteanalysisof the
absolute value of surface temperature for a common
period, 1961-1990.Our absoluteclimatologyis made up
of four components:land areas excludingAntarctica,
oceanicareasfrom 60øNto 60øS,Antarcticapolewardof
60øS,and northern oceanic areas including the Arctic,
37, 2 / REVIEWS OF GEOPHYSICS
north of 60øN. The four componentsare discussedseparately.
6.1. LandAreasExcludingAntarctica
For these regions we make use of the 1961-1990
climatologydevelopedfrom station data by New et al.
[1999].In this analysis,12,092stationestimatesof mean
temperature for 1961-1990 are used to constructa 0.5ø
latitude by 0.5ø longitude climatology for global land
areas excludingAntarctica. The temperature averages
were givenreliabilitycodes,expressedasa percentageof
the number of years available during the 1961-1990
period. In regions of dense coverage, stations with
higher percentageswere used preferentially.For most
stations,mean temperaturewas calculatedas the average of maximumand minimum temperature,thusavoiding potential differencesdue to the method of calculating mean temperature(see discussion
in section2.1).
The station data were interpolated as a function of
latitude, longitude,and elevationusingthin plate splines
[e.g., Hutchinson,1995]. The use of elevation enabled
the splinesto define spatiallyvaryingtemperaturelapse
rates in different mountainousareas,providedthat the
basic data were adequate (see New et al. [1999] for
further discussionof this point). Station data were insufficientto achievespatiallyvaryinglapseratesin Antarcticaand Greenland(seelater in sections6.3 and 6.4).
The elevationdata usedoriginatedfrom a 5-min digital
elevationmodel (DEM), availablefrom the National
GeophysicalData Center (NGDC) (see Acknowledgments) which had been degradedto 0.5ø cellsby averagingthe thirty-six5-min pixelsin each cell. The accuracy of the interpolationshasbeen assessed
usingcrossvalidationand by comparisonwith earlier climatologies
(see referencesof New et al. [1999]). We believe this
climatologyrepresentsan important advanceover previouswork in that it is strictlyconstrainedto the period
1961-1990, elevationis explicitlyincorporated,and errors are evaluated on a regional basis.
For combinationwith the other three components,
the 0.5ø x 0.5øresolutionwasdegradedto 1ø x 1ø.In this
degradation,four 0.5ø x 0.5ø cellswere averagedto give
one 1øcell. The DEM was also degradedto providea 1ø
elevation
6.2.
field for all land areas.
Oceanic
Areas From 60øN to 60øS
In the anomaly data set we used SST anomaliesover
marine regionsbecauseof their superioraccuracycomparedwith MAT data. For the climatology,however,it is
preferable to use MAT, as MAT shouldmatch better
with land temperature on islandsand from coastlines
than an SST climatology.
MAT climatologies,however,will be affectedby solar
heating on board ship (see section2.2). To overcome
this problem, we combineda new SST climatologyfor
1961-1990 [Parkeret al., 1995b]with an air-seatemperature difference climatology,also for 1961-1990. The
SST climatologymakesuse of all availablein situ and
37, 2 / REVIEWSOF GEOPHYSICS
Joneset al.: SURFACE AIR TEMPERATURECHANGES ß 191
satellite SST data during the period, new marginal sea
ice zone SST estimates,and new Laplacian-basedadjustment schemes[Reynolds,1988] in areas where there
were insufficientdatato producea true climatology.The
air-seatemperaturedifferencedata usedsimilarinfilling
techniquesbut made useof only NMAT and in situ SST
values
from
1 hour before
local sunset to 1 hour
after
local sunrise,to minimize the effect of solar heating on
ships'MAT. In the developmentof the air-seatemperature difference climatology, we assume that values
based on nighttime-onlyvalues are the same as those
basedon all 24 hoursof the day. Both climatologiesare
availableon a 1ø x 1ø resolution.Our air temperature
version
is the sum of the air-sea
resultsin a linear increasein temperaturebetweenthe
ice sheetmargin and 60øS.In reality, the field is likely to
exhibita sharpincreasein air temperatureat the margin
of the continental ice sheet, although this step will be
smoothedout to someextentby using30-year averages.
Plate 5 illustratesthe Antarctic climatologyfor the
four midseasonmonths. Even at the relatively coarse
resolution, the influence of the sea ice can be seen,
principallyby its absenceduring January. The strong
gradientsacrossthe Antarctic Peninsuladuring summer
and autumn
are evident
as well as the coastal-sea
ice
extent effectsin the Weddell and Ross Seasand Prydz
Bay.
and the SST climatol-
ogy.
6.4.
Arctic Areas North of 60øN
Plate 4 illustratesthis climatologycombinedwith the
In this region, data for land areascome from the New
land data for the four midseasonmonthsfor the region et al. [1999] climatology.There are considerablymore
60øN to 60øS. At this contour resolution, most of the stationsin this region than in the Antarctic, although
large spatialchangesare due to latitude, elevation,and there is only one high-elevationstationin the interior of
land-oceancontrasts.The latter are markedly stronger Greenland(and eventhisis for a relativelyshortperiod
in the winter hemispherebut are still evident in the of record).As with Antarctica,elevationover Greenland
transition seasons in some areas.
couldbe incorporatedonly as a covariate.Elsewherein
the Arctic, elevationwasas an independentvariableand
6.3.
Antarctica Poleward of 60øS
followedNew et al. [1999].
For this region, averagetemperaturesfor 35 stations
The most difficult part of this region is the Arctic
were collectedfrom similar sourcesto those given by Ocean and the extensivesea ice regionsin the northern
New et al. [1999]. Only 15 were near completefor the Atlantic and PacificOceans.The systematiccollectionof
1961-1990 period, the remaining 20 being either from temperaturedata within the packice of the Arctic basin
siteswith recordsfor part of 1961-1990 or from loca- beganwith Nansen'sdrift acrossthe Arctic in the Fram
tions with data only in the 1950s. Additional sources during 1893-1896. During this drift, temperatureswere
includeda number of Antarctic data publicationsfrom recordedat 6-hour intervals.Sverdrup[1933] describesa
countriesoperatingthe stations(e.g.,Schwerdtfeger
et al. similar time seriesthat was taken during the 1919-1926
[1959],JapanMeteorological
Agency[1995],and sources Maud drift, where the temperaturerecordextendsfrom
listedbyJones[1995b]).In a few years'time it shouldbe August 1922 to September1924. Sverdrup[1933] compossibleto increasethe number of sites available by bined these two sets of drift observations with land
using the then extended information from automatic stationobservationsand publishedthe first monthlycliweatherstations(AWSs). A numberof new AWS sites matologyfor the basin.
are located in regions considerable distances from
In 1938, the fUSSR began their seriesof North Pole
manned stations[Steamset al., 1993]. As yet, though, stationice camps,whichwere oceanographicand meterecord lengthsare too short.
orologicalstationsestablishedby aircraft on either mulThe monthlystationtemperaturenormalswere inter- tiyearice floesor ice islandsin the centralArctic [Colony
polatedusingthe samesplinesoftwareas the other land et al., 1992].The seriesbeganwith NP-1, whichgathered
areas. Only four of the 35 stationsare located at eleva- data from May 1938 to March 1939. Following World
tions in excessof 1500 m, the remainderbeing at eleva- War II, NP-2 gatheredduringApril 1950 to April 1951;
tions below 300 m. This was too sparsea network for a then in 1954 the establishment of NP-3 marked the
full three-dimensional interpolation. Elevation was beginning of the continuousoccupationof the basin,
therefore used as a covariaterather than a truly inde- where the fUSSR maintained 1-4 stationsuntil April
pendent variable, enabling the definition of monthly 1991, when NP-31 was recovered. The NP 2-m air temcontinent-widerelationshipswith elevation.Theselapse peratureswere taken at 3-hour intervalsinsidea Stevenratesvariedmonthlyfrom 8 (summer)to 11 (winter) øC son screen.They have recentlybeen resampledto every
km-•, largerthanwouldbeexpected.
Thisismostlikely
6 hours and are available
due to the strongcovarianceof elevationand position,
i.e., elevationincreasesand temperaturedecreases
polewards.The interpolationassigned
moreweightto elevation than position.The interpolationover land was extendedoveradjacentoceanareas(seaice areasfor part
of the year) to 60øS,where the MAT climatologyprovided the northern boundary.The use of sucha control
tional Snowand Ice Data Center (NSIDC). During the
NP period the United States and Canada also carried
out a seriesof shorter-termoccupationsof ice floesand
on CD-ROM
from
the Na-
ice islands; these records are available from the NCDC.
The next major set of temperatureobservationsbegan on January 19, 1979, as the part of the U.S. contribution to the First Global AtmosphericResearchPro-
192 ß Joneset al.' SURFACEAiR TEMPERATURECHANGES
37, 2 / REVIEWSOF GEOPHYSICS
gram (GARP) Global Experiment (FGGE), and
consistedof an array of air-dropped,satellite-tracked
buoys [Thomdike and Colony, 1980]. The purpose of
thesebuoyswas to record both the large-scaleice deformation by determiningthe buoy position,and the pressure field driving this deformation.A temperature sensor was included only to aid in the pressure sensor
correlationdecay length of 1300 km, to produce a 12hour, 200-km griddedtemperaturefield for the basinfor
the 1979-1994 period.
Buildinguponthiswork,Rigoretal. [1999]studiedthe
monthly mean dependence of the correlation length
scales.Although they find that during autumn, winter,
and spring,their correlationlengthscalesapproximately
calibration. The instruments were mounted inside a venequal the Martin and Munoz value, during summerthe
tilated 0.6-m-diametersphericalhull. This programwas correlationlength scalesare smaller than during other
known as the Arctic Ocean Buoy Program until 1991, seasons,and much smaller between the coastal land and
when to reflect the growing contributionsfrom other ice temperaturesthan betweenthe differentice tempernations,the program name was changedto the Interna- atures, i.e., the buoy or NP temperatures.They used
tional Arctic Buoy Program(IABP). Since 1979 a large these length scaleestimatesto derive an improvedopnumber and variety of buoyshave been deployedby timal interpolationdata set calledIABP/POLES, which
parachute,
aircraftlandings,
ship,andsubmarine.
Begin- provides12-hourtemperatureson a 100-km rectangular
ning in 1985the programbeganthe installationof buoys grid, for 1979-1997. In this analysisthe summerbuoy
having ventilated and shielded thermistors at 1-2 m data were alsoadjustedto matchthe NP statistics.Rigor
(R. L. Colony,private communication,1998). As illus- et al. find that the IABP/POLES data set has higher
trated on the IABP Web site (see acknowledgments),
a correlationsand lower biasescomparedwith the NP and
large variety of designswere deployed,with both inter- land station observations than the POLES data, and
nal and external thermistors.Today buoyswith external providesbetter temperatureestimatesespeciallyduring
and radiation
shielded thermistors
mounted
at 2-m
summer in the marginal ice zones. The IABP/POLES
data set is available from the IABP Web site.
height are deployedas much as possible.
As Martin and Munoz [1997] describe, there is a
We use this new analysisfor the Arctic Ocean for the
general problem with the buoy thermistorsduring sum- years1979-1997 and combineit with the land data north
mer, which became apparent when the buoy summer of 60øN from New et al. [1999] and the marine air
meanswere comparedwith the NP means.For the air temperaturesnorth of 60øNin the northernPacificand
temperaturemeasurementsabovethick sea ice, a natu- Atlantic Oceans.The Arctic Ocean analysesare thereral calibrationperiod occursin summer.The reasonfor fore based on a slightly more recent period, but the
this is asfollows:at the beginningof summer,becauseof qualityof the 1979-1997 data for the regionjustifiesthis.
the natural sea ice desalinationcycle, the upper ice Any temperature change between these two periods
surfaceis virtually saltfree. Then as summerprogresses, over this region is thoughtto be minimal (see section
freshwatermelt ponds form on the ice surface,so that 5.3). Plate 6 illustratesthe derived climatologyfor the
the air temperature measurementsabove the ice are Arctic for the four midseasonmonthsof January,April,
provided with a natural calibration point that is very July, and October. The coolestparts of the Arctic are
close to 0øC. From the Fram and Maud data, Sverdrup over the land regionsof northern Canada and eastern
[1933] first describedthis isothermalperiod. From anal- Siberia. Over the central Arctic in winter, temperatures
ysis of those NP stationsnorth of 82øN, Martin et al. are coldest acrossthe region joining the coldest land
[1997] found for the period June 23 to July 31 a mean regions.In the summerseason,the Arctic Ocean is near
temperatureof -0.2øC with a standarddeviationof 0.9øC. 0øC almost everywhere,as may be expectedfrom the
Martin and Munoz [1997] found that most of the previous discussion.The influence of the warm North
individualbuoysfor the sameregionand for the summer Atlantic watersis evidentin all the seasonalplotsexcept
periodhadm•eantemperatures
thatwere 1ø-3øCabove
summer.
freezing, or much warmer than the means of the NP
temperatures.We attribute this temperatureincreaseto 6.5. Combinationof the FourComponents
Into the
solar heating of the thermistors.Out of 261 buoysde- 1961-1990 Climatologyof SurfaceAir Temperatures
Combination of the analysesfor the variousdomains
ployedduringthe period 1979-1993, only29 had a mean
temperature that fell within one standarddeviation of was achieved in the most straightforwardmanner. For
the NP summermean, and only four buoyslay within the zone 60øN to 60øS,the climatologicalvalue for each
half a standarddeviation.Of the remainder, only four 1ø squareis selectedfrom either the land or the marine
had a cold bias, so that 229 buoys had a warm bias. analysis.If both analyseswere available,the averageof
Informal investigationsby two of us (S.M. and I.R.) the two was taken. The two separateanalysesfor the
showthat the problemwith summeroverheatingcontin- Arctic and Antarctic regionswere then combined.The
ues throughthe 1997 summer.In spite of this problem, climatologyavailable on a 1ø x 1ø basis,together with
Martin and Munoz [1997] used a variety of filtering the surface DEM used, can be found at the Climatic
techniquesto remove these summeroffsets,then used ResearchUnit Web site (see acknowledgments).
It is now a trivial task to calculate the mean hemian optimal interpolationschemefor the NP, buoysand
coastalland station temperatures,assuminga constant sphericand global temperaturesfor the 1961-1990pe-
37, 2 / REVIEWSOF GEOPHYSICS
Joneset al' SURFACEAIR TEMPERATURECHANGES ß 193
60N
30N _
z
c
0
30S
60S
ß
180W
60N
t
f
!
f
•
f
135W
90W
45W
0
45E
90E
135E
180E
30N _
o
3os.
ß
60S
180W
60N
I
!
I
I
I
!
I
135w
90w
45w
0
45E
90E
135E
180E
135w
90w
45w
0
45E
90E
135E
180E
30N
0
30S
60S
180W
60N
..•
30N
"
0
o
o
t
0
30S
60S
180W
135W
90W
-40
-30
0
45W
-20
-10
-5
0
...
45E
5
10
90E
15
20
25
135E
180E
30
:......
Plate 4. Climatologicalvalues of averageair temperature for the 1961-1990 period for the midseason
monthsof January,April, July, and October for the region 60øNto 60øS.
194 ß Joneset al.' SURFACEAIR TEMPERATURECHANGES
37, 2 / REVIEWSOF GEOPHYSICS
JANUARY
APRIL
0
%
08[
JULY
OCTOBER
0
o
O3
I"/1
m
O3
t
08 I.
-80
-50
-30
08 I.
-20
-15
-10
-5
-2
0
2
5
10
15
20
Plate 5. Climatologicalvaluesof averageair temperaturefor the 1961-1990period for midseasonmonths
of January,April, July, and Octoberfor the Antarctic regionsouthof 60øS.
37, 2 / REVIEWSOF GEOPHYSICS
Joneset al.' SURFACE AIR TEMPERATURE CHANGES ß 195
JANUARY
APRIL
10
1:0
o
ill
Jb.,... '•
ß
OCTOBER
JULY
10
1!•o
•
ITI
o•o
C
['
!
-80
-50
-30
,
!
-20
-15
-10
-5
-2
0
2
5
10
15
20
Plate 6. Climatologicalvaluesof averageair temperaturefor the 1961-1990 period for midseasonmonths
of January,April, July, and October for the Arctic region north of 60øN.
196 ß Joneset al' SURFACEAIR TEMPERATURECHANGES
25
37, 2 / REVIEWSOF GEOPHYSICS
i
i
i
i
2O
_
GLO-
GLO
_
_
_
NH
_
_ NH
_
_
_
_
_
_
_
_
_
_
_
_
_
_
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
riod. The averageglobal annual temperatureis 14.0øC,
with the NH warmerthan the SH (14.6øCcomparedwith
13.4øC).In the CrutcherandMeserve[1970]and Taljaard
et al. [1969] atlas climatologiesthe global average is
14.1øC(NH, 14.9øC;SH, 13.3øC).Given the warmth of
the 1961-1990 period compared with earlier periods
(--•1931-1965)usedby theseworkers,the differencesare
lessthan 0.5øC.We would expectour climatologyto be
about
Figure 7. Seasonalcycleof hemispheric
and globalmean temperaturesin absolute
degreesCelsiusbased on the 1961-1990
period.
0.2øC warmer
than the earlier
ones if climatic
changeswere the only causesof the differences.
New et al. [1999] have estimatedcross-validationerrors for their climatologyover large regionalland areas.
Typical errors are in the range 0.5ø-1.0øC,the larger
valuesoccurringover either data sparseregionsor areas
with complex terrain. Because there are potentially
greater errors in our climatology over the Southern
Ocean and Antarctica, due to sparseand incomplete
(with respectto 1961-1990) data and the slightlydifferent Arctic Oceanbaseperiod,we havenot attemptedto
estimatestandarderrors for the climatology.Comparisonwith the earlier climatologiessuggests
that the value
of 14øC is within
0.5øC of the true value.
Figure 7 showsthe annual cycleof temperaturesin
the two hemispheresand the globe. The magnitudeof
the hemisphericseasonalcycleis 13.1øCin the NH and
5.7øC in the SH. In the NH
the warmest
and coldest
months are the expectedJuly and January.In the SH,
Januaryis warmest,but August rather than July is the
coldest.
NOV
DEC
from the 1961-1990 period. Our main conclusions
are as
follows:
1. Annual global surfacetemperatureswarmed by
0.57øCover the period 1861-1997 and by 0.62øCover
1901-1997.Over both periodsthe warmingwas slightly
greater in the SH than in the NH.
2. The measured temperature trends on a hemisphericbasisare not very dependentupon the method
usedto combinethe basicland and oceanicdata, provided that all the basicdata have been adequatelyadjusted for variousinhomogeneities.
3. The warmestyearsof the record all occur in the
1990s.The four warmestyears,in descendingorder, are
1998 (0.57øC above the 1961-1990 average), 1997
(0.43øCaboveaverage),1995 (0.39øCaboveaverage)
and 1990 (0.35øCaboveaverage).
4. The errors of estimation,due to sampling,are
dependentupon the timescaleof interest.On the interannualtimescale,recentindividualglobalmean, annual
mean temperature anomalies have standard errors of.
_0.05øC.
5. Patternsof warmingover the two majorwarming
periodsin this century(1925-1944 and 1978-1997) indicate that the warming has been greatest over the
northern
continents
and in the DJF and MAM
seasons.
The warmingover the earlier periodwasslightlygreater
(0.37øCcomparedwith 0.32øC)on a globalbasis.The
recent warming has been accompaniedby increasesin
areas affected by significantlywarm temperaturesand
reductionsin the areas affected by significantlycool
temperatures.
7.
SUMMARY
6. A recent reanalysisof Arctic temperaturedata
from the packice areasindicatesa slightwarmingon an
We have revieweda surfaceair temperaturedata set annual basiswith statisticallysignificantwarmingover
and produceda climatologyof surfacetemperatureson the 1961-1990 period in May and June.
a 1ø x 1øbasisbased, as best as can be achieved,on data
7. The recent increasein mean temperatureis the
37, 2 / REVIEWS OF GEOPHYSICS
Joneset al.: SURFACE AIR TEMPERATURE CHANGES ß 197
result of strongerwarming in nighttime comparedwith
daytime.Over the 1950-1993 period, nighttime (minimum) temperatureshavewarmedby 0.18øCper decade
and daytime(maximum)temperatureshavewarmedby
0.08øCper decade.
8. Recent surfacetemperatureincreasessince1979
have not been observedby satellite retrievalsof lower
tropospherictemperatures.Much has been written and
said on this issue,highlightingthe agreementbetween
satellite and radiosondemeasurementsof lower tropospherictemperaturesover the same period. However,
surfaceand lower tropospherictemperaturechangescan
and do differ, and the available databasesare not yet
good enoughto estimateconfidentlythe magnitudeof
possibledifferencesover this near-20-yearperiod. The
agreementbetweensurfaceand lowertropospherictemperature trendsfrom radiosondesover the longer 19651997 period is generallyconvenientlyforgottenin many
commentaries.
9. Proxy evidence for the preinstrumentalperiod
providesour only estimatesof temperature capable of
puttingthe periodsince1851into a longer-termcontext.
The most recent proxy compilationsindicate that the
twentieth century was the warmest of the millennium
and that the warmingduringit wasunprecedentedsince
1400. The coldest century of the millennium was the
seventeenth,
with a temperatureaverageonly0.5ø-0.8øC
below the 1961-1990average.
10. The 1ø x 1ø climatology,developedfor surface
air temperature,indicatesan averageglobal temperature of 14.0øC(14.6øCin the NH and 13.4øCin the SH).
ACKNOWLEDGMENIS.
The
authors
thank
Alan
Rob-
ock for suggesting
this review,and ChrisFolland, Mike Hulme,
Tim Osborn,the technicalreviewersof Reviewsof Geophysics
for suggestions
that improvedit. BrionyHorton is thankedfor
producingFigure6 and assistance
with Figure7. This work has
been supportedby a number of researchprograms.P.D.J.
acknowledges
the supportof the U.S. Department of Energy
(grant DE-FG02-98ER62601). M.N. has been supportedby
the U.K. Natural Environment Research Council (grant
GR3/09721). Much of the data collectionof climatological
(1961-1990)data hasbeensupportedby the U.K. Department
of Environment,Transport, and the Regions(contractEPG
1/1/48).The developmentof the GISST2.2seasurfacetemperature and seaice climatologyhasbeen supportedby the U.K.
Public Meteorological Service Contract. S.M. and I.R. both
gratefullyacknowledgethe supportof NASA grant NAGS4375, Polar Exchangeat the Sea Surface.I.R. also acknowledges the support of the United States Inter-Agency Buoy
Program(USIABP) throughgrant N00014-96-C-0185.Many
of the data sets upon which some of the figures depend,
together with the climatology,are available on the Climatic
ResearchUnit, University of East Anglia, Web site (http://
www.cru.uea.ac.uk/).
The Arctic buoy data are also available
from the IABP Web site (http://iabp.apl.washington.edu).
The
DEM is availablefrom the NGDC Web site (http://www.ngdc.
noaa.gov/seg/topo).
REFERENCES
Afifi, A. A., and S. P. Azen, StatisticalAnalysis:A Computer
OrientatedApproach, 2nd ed., 442 pp., Academic, San
Diego, Calif., 1979.
Alexandersson,
H., andA. Moberg,Homogenizationof Swedish temperature data, I, A homogeneitytest for linear
trends, Int. J. Climatol., 17, 25-34, 1997.
Bradley,R. S., QuaternaryPaleoclimatology:
Methodsof PaleoclimaticReconstruction,
472 pp., Allen and Unwin, Winchester, Mass., 1985.
Bradley, R. S., and P. D. Jones,Data basesfor isolatingthe
effects of the increasingcarbon dioxide concentration,in
Detectingthe ClimaticEffectsof IncreasingCarbonDioxide,
editedby M. C. MacCrackenand F. M. Luther, pp. 29-53,
U.S. Dep. of Energy, Washington,D.C., 1985.
Bradley, R. S., and P. D. Jones, "Little Ice Age" summer
temperaturevariations:Their nature and relevanceto recent globalwarmingtrends,Holocene,3, 367-376, 1993.
Bradley,R. S., andP. D. Jones(Eds.), ClimateSinceA.D. 1500,
706 pp., Routledge,London, 1995.
Briffa, K. R., and P. D. Jones,Global surfaceair temperature
variations over the twentieth century, 2, Implications for
large-scalepalaeoclimaticstudiesof the Holocene,Holocene, 3, 165-179, 1993.
Briffa, K. R., P. D. Jones, F. H. Schweingruber,and T. J.
Osborn,Influenceof volcaniceruptionson Northern Hemisphere summer temperaturesover 600 years,Nature, 393,
450-455, 1998.
Chapman,W. L., and J. E. Walsh, Recent variationsof seaice
and air temperaturein high latitudes,Bull. Am. Meteorol.
Soc., 74, 33-47, 1993.
Christy, J. R., R. W. Spencer,and E. Lobl, Analysisof the
mergingprocedurefor the MSU daily temperaturetime
series,J. Clim., 11, 2016-2041, 1998.
Colony, R., I. Appel, and I. Rigor, Surface air temperature
observations in the Arctic Basin, Tech. Memo. TM 1-92,
Appl. Phys.Lab., Univ. of Wash., Seattle, 1992.
Conrad,V., and L. D. Pollak,Methodsin Climatology,
459 pp.,
Harvard Univ. Press,Cambridge,Mass., 1962.
Crutcher, H. L., and J. M. Meserve, Selected-levelheights,
temperatures,
anddewpointtemperatures
for theNorthern
Hemisphere,NAVAIR 50-1C-52 rev., U.S. Navy, Chief Naval Operations,Washington,D.C., 17 pp. plus charts,11
pp. + 18 figs., 144 charts,1970.
Easterling, D. R., and T. C. Peterson, A new method for
detectingand adjustingfor undocumented
discontinuities
in
climatological
time series,Int. J. Climatol.,15, 269-377, 1995.
Easterling,D. R., B. Horton, P. D. Jones,T. C. Peterson,T. R.
Karl, D. E. Parker, M. J. Salinger,V. Razuvayev,N. Plumruer, P. Jamason,and C. K. Folland, Maximum and minimum
temperaturetrendsfor the globe,Science,
277, 364-367, 1997.
Ellsaesser, H. W., M. C. MacCracken, J. J. Walton, and S. L.
Grotch, Global climatictrendsas revealedby the recorded
data, Rev. Geophys.,24, 745-792, 1986.
Folland, C. K., and D. E. Parker, Corrections of instrumental
biasesin historicalsea surfacetemperature data, Q. J. R.
Meteorol. Soc., 121,319-367,
1995.
Folland, C. K., and M. J. Salinger,Surfacetemperaturetrends
in New Zealand and the surroundingocean,1871-1993,Int.
J. Climatol., 15, 1195-1216, 1995.
Folland, C. K., T. R. Karl, and K. Ya. Vinnikov, Observed
climate variations and change, in Climate Change: The
IPCC ScientificAssessment,
edited by J. T. Houghton, G. J.
Jenkins,andJ. J. Ephraums,pp. 195-238,CambridgeUniv.
Press, New York, 1990.
Folland, C. K., T. R. Karl, N. Nicholls, B. S. Nyenzi, D. E.
Parker, and K. Ya. Vinnikov, ObservedClimate Variability
and Change,ClimateChange1992: The Supplementary
Re-
198 ß Joneset al.: SURFACE AIR TEMPERATURECHANGES
port to the IPCC ScientificAssessment,edited by J. T.
Houghton, B. A. Callander, and S. K. Varney, pp. 135-170,
Cambridge Univ. Press,New York, 1992.
Folland, C. K., M. J. Salinger,and N. Rayner,A comparisonof
annual South Pacific island and ocean surface temperatures, Weather Clim., 17, 23-42, 1997.
Grove, J. M., TheLittle Ice Age, 498 pp., Methuen, New York,
1988.
Hansen, J. E., and S. Lebedeff, Global trendsof measuredsurface
air temperature,J. Geophys.Res.,92, 13,345-13,372,1987.
Hughes, M. K., and H. F. Diaz, Eds., The Medieval Warm
Period, Clim. Change,26, 109-342, 1994.
Hurrell, J. W., Decadal trends in the North Atlantic Oscillation
and relationshipsto regional temperature and precipitation, Science,269, 676-679, 1995.
Hurrell, J. W., and K. E. Trenberth, Satellite versus surface
37, 2 / REVIEWS OF GEOPHYSICS
Data Setfor the SouthernHemisphere,Tech.Rep. TR027, 73
pp., U.S. Dep. of Energy,Washington,D.C., 1986c.
Jones,P. D., P.M. Kelly, C. M. Goodess,and T. R. Karl, The
effect of urban warmingon the Northern Hemispheretemperature average,J. Clim., 2, 285-290, 1989.
Jones, P. D., P. Ya. Groisman, M. Coughlan, N. Plummet,
W.-C. Wang, and T. R. Karl, Assessmentof urbanization
effectsin time seriesof surfaceair temperaturesover land,
Nature, 347, 169-172, 1990.
Jones,P. D., R. S. Bradley, and J. Jouzel (Eds.), Climatic
Variationsand ForcingMechanismsof the Last 2000 Years,
649 pp., Springer-Verlag,New York, 1996.
Jones,P. D., T. J. Osborn, and K. R. Briffa, Estimating sampling errors in large-scaletemperatureaverages,J. Clim.,
10, 2548-2568, 1997a.
Jones,P. D., T. J. Osborn,T. M. L. Wigley, P.M. Kelly, and
estimatesof air temperaturesince1979,J. Clim., 9, 2222B. D. Santer,Comparisonsbetweenthe microwavesound2232, 1996.
ing unit temperature record and the surfacetemperature
Hurrell, J. W., and K. E. Trenberth,Spurioustrendsin satellite
record from 1979 to 1996: Real differencesor potential
discontinuities?,
J. Geophys.
Res.,102, 30,135-30,145,1997b.
MSU temperatures from merging different satellite
records, Nature, 386, 164-167, 1997.
Hurrell, J. W., and K. E. Trenberth, Difficultiesin obtaining
reliable temperature trends: Reconcilingthe surfaceand
satellite Microwave SoundingUnit records,J. Clim., 11,
945-967, 1998.
Hutchinson,M. F., Interpolatingmean rainfall usingthin plate
smoothingsplines,Int. J. Geogr.Inf Syst.,9, 385-403, 1995.
James,R. W., and P. T. Fox, Comparativesea-surfacetemperature measurements,Mar. Sci.Affairs Rep. 5, WMO 336, 27
pp., World Meteorol. Organ., Geneva, 1972.
Japan MeteorologicalAgency,AntarcticMeteorological
Data,
Spec.VoL VI, 364 pp., Tokyo, 1995.
Jones,P. D., Hemisphericsurfaceair temperaturevariations:A
reanalysis
andan updateto 1993,J. Clim.,7, 1794-1802,1994.
Jones,P. D., Land surfacetemperatures•Is the networkgood
enough?,Clim. Change,31,545-558, 1995a.
Jones,P. D., Recent variationsin mean temperature and the
diurnal temperature range in the Antarctic, Geophys.Res.
Jones, P. D., K. R. Briffa, T. P. Barnett, and S. F. B. Tett,
High-resolutionpalaeoclimaticrecordsfor the last millennium: Interpretation,integrationand comparisonwith general circulationmodel controlrun temperatures,Holocene,
8, 455-471, 1998.
Jones, P. D., E. B. Horton, C. K. Folland, M. Hulme, D. E.
Parker, and T. A. Basnett,The use of indicesto identify
changesin climaticextremes,Clim. Change,in press,1999.
Kagan,R. L., Averagingof Meteorological
Fields,379 pp., Kluwet Acad., Norwell, Mass., 1997.
Kahl, J. D., D. J. Charlevoix, N. A. Zaitseva, R. C. Schnell, and
M. C. Serreze,Absenceof evidencefor greenhousewarming over the Arctic Ocean in the past40 years,Nature,361,
335-337, 1993.
Karl, T. R., and P. D. Jones, Urban bias in area-averaged
surfaceair temperaturetrends,Bull.Am. Meteorol.Soc.,70,
265-267, 1989.
Karl, T. R., G. Kukla, and J. Gavin, Decreasingdiurnal temLett., 22, 1345-1348, 1995b.
perature range in the United States and Canada from
1941-80, J. Clim. Appl. Meteorol.,23, 1489-1504, 1984.
Jones,P. D., Temperature changein the Arctic and the Antarctic from instrumentaland high-frequencypaleoclimatic Karl, T. R., J. D. Tarpley, R. G. Quayle, H. F. Diaz, D. A.
Robinson,andR. S. Bradley,The recentclimaterecord:What
(since 1400) reconstructions,in Wadati Conferenceon
Global Changeand the Polar Climate, pp. 5-8, Inst. of
it can and cannottell us,Rev. Geophys.,27, 405-430, 1989.
Karl, T. R., P. Ya. Groisman, R. W. Knight, and R. R. Heim
Geosci.,Univ. of Tsukuba,Tsukuba,Japan, 1995c.
Jr., Recent variations of snow cover and snowfall in North
Jones,P. D., and K. R. Briffa, Global surfaceair temperature
variationsduringthe twentiethcentury,1, Spatial,temporal
America and their relation to precipitation and temperature variations, J. Clim., 6, 1327-1344, 1993a.
and seasonaldetails, Holocene, 2, 165-179, 1992.
Jones, P. D., and K. R. Briffa, What can the instrumental Karl, T. R., P. D. Jones,R. W. Knight, G. Kukla, N. Plummet,
record tell us about longer timescalepaleoclimaticreconV. Razuvayev,K. P. Gallo, J. Lindesay,R. J. Charlson,and
structions?,in Climatic Variationsand ForcingMechanisms
T. C. Peterson,A new perspectiveon recent global warmof the Last 2000 Years,edited by P. D. Jones,R. S. Bradley,
ing--Asymmetric trends of daily maximum and minimum
andJ. Jouzel,pp. 625-644, Springer-Verlag,
New York, 1996.
temperatures,
Bull.Am. Meteorol.Soc.,74, 1007-1023,1993b.
Jones,P. D., and M. Hulme, Calculatingregionalclimatictime Karl, T. R., R. W. Knight, and J. R. Christy, Global and
seriesfor temperature and precipitation:Methods and ilhemispherictemperaturetrends:Uncertaintiesrelated to
lustrations,Int. J. Climatol., 16, 361-377, 1996.
inadequatespatialsampling,J. Clim., 7, 1144-1163, 1994.
Jones,P. D., S.C. B. Raper, B. D. Santer, B. S. G. Cherry, Kukla, G., T. R. Karl, and M. Riches(Eds.), Minimax WorkC. M. Goodess,P.M. Kelly, T. M. L. Wigley, R. S. Bradley,
shop,Atmos. Res.,37, 1-275, 1995.
and H. F. Diaz, A Grid Point SurfaceAir TemperatureData Livezey,R. E., and W. Y. Chen, Statisticalfield significance
Setfor the NorthernHemisphere,Tech.Rep. TR022, 251 pp.,
and its determinationby Monte Carlo techniques,Mort.
WeatherRev., 111, 46-59, 1983.
U.S. Dep. of Energy, Washington,D.C., 1985.
Madden, R. A., D. J. Shea, G. W. Brantstator, J. J. Tribbia, and
Jones,P. D., S.C. B. Raper, R. S. Bradley, H. F. Diaz, P.M.
R. Webber, The effects of imperfect spatial and temporal
Kelly, and T. M. L. Wigley, Northern Hemispheresurface
sampling on estimates of the global mean temperature:
air temperaturevariations:1851-1984,J. Clim. Appl. Meteorol., 25, 161-179, 1986a.
Experimentswith model and satellite data, J. Clim., 6,
1057-1086, 1993.
Jones,P. D., S.C. B. Raper, and T. M. L. Wigley, Southern
HemiSpheresurfaceair temperaturevariations:1851-1984, Mann, M. E., R. S. Bradley, and M. K. Hughes,Global-scale
J. Clim. Appl. Meteorol.,25, 1213-1230,1986b.
temperaturepatternsand climate forcingover the past six
centuries, Nature, 392, 779-787, 1998.
Jones,P. D., S.C. B. Raper, C. M. Goodess,B. S. G. Cherry,
and T. M. L. Wigley,A Grid Point SurfaceAir Temperature Mann, M. E., R. S. Bradley, and M. K. Hughes, Northern
37, 2 / REVIEWSOF GEOPHYSICS
Hemispheretemperaturesduring the past millennium:Inferences,uncertaintiesand limitations,Geophys.Res.Lett.,
26, 759-762, 1999.
Martin, S., and E. A. Munoz, Propertiesof the Arctic 2-Meter
Air temperaturefield for 1979 to the presentderivedfrom
a new gridded dataset,J. Clim., 10, 1428-1440, 1997.
Martin, S., E. A. Munoz, and R. Drucker, Recent observations
Joneset al.: SURFACEAIR TEMPERATURECHANGES ß 199
Smith, T. M., R. W. Reynolds,and C. F. Ropelewski,Optimal
averagingof seasonalsea surfacetemperature and associated confidenceintervals(1860-1989),J. Clim., 7, 949-964,
1994.
Stearns, C. R., L. M. Keller, G. A. Weidner, and M. Sievers,
Monthly mean climatic data for Antarctic automatic
weather stations,in AntarcticMeteorologyand Climatology:
StudiesBased on Automatic Weather Stations,Antarct. Res.
of a spring-summer
surfacewarmingoverthe Arctic Ocean,
Geophys.Res.Lett., 24, 1259-1262, 1997.
Set., 61, edited by D. H. Bromwichand C. R. Stearns,pp.
1-21, AGU, Washington,D.C., 1993.
Mitchell, J. M., Jr., On the causesof instrumentallyobserved
seculartemperaturetrends,J. Meteorol.,10, 244-261, 1953. Sverdrup, H. U., The NorwegianNorth Polar ExpeditionWith
Moberg, A., and H. Alexandersson,Homogenization of a
theRS Maud 1918-1925,Scientific
Results,
vol. II, Meteorology,
Swedishtemperature data, II, Homogenized gridded air
part 1, Discussion,331 pp., GeofysikInst., Bergen, 1933.
temperaturecomparedwith a subsetof global gridded air Taljaard, J. J., H. van Loon, H. C. Crutcher, and R. L. Jenne,
Climate of the UpperAir, SouthernHemisphere,vol. I, Temtemperaturesince 1861,Int. J. Climatol.,17, 35-54, 1997.
peratures,Dew Pointsand Heightsat SelectedPressure
Levels,
New, M., M. Hulme, and P. D. Jones,Representingtwentieth
centuryspace-timeclimatevariability,1, Developmentof a
NAk'AIR 50-1C-55, 135 pp., Chief Nav. Operations,Washington, D.C., 1969.
1961-90 mean monthlyterrestrialclimatology,J. Clim., 12,
829-856, 1999.
Thorndike, A. S., and R. L. Colony, Arctic Ocean Buoy ProNicholls,N., G. V. Gruza, J. Jouzel,T. R. Karl, L. A. Ogallo,
gram data report, 19 January 1979-31 December 1979,
and D. E. Parker, Observedclimatevariability and change,
Polar ScienceCenter Data Rep., 131 pp., Univ. of Wash.,
Seattle, 1980.
in The IPCC SecondScientificAssessment,
edited by J. T.
Houghton et al., pp. 133-192, CambridgeUniv. Press,New Torok, S., The development of a high-quality temperature
York, 1996.
database for Australia, Ph.D. thesis, Univ. of Melbourne,
Parkville, Victoria, Australia, 1996.
Oke, T. R., Reviewof urbanclimatology,1969-1973,WMO Tech.
Note 169, WMO 539, World Meteorol. Organ.,Geneva,1974. Trenberth, K. E., Recent observed interdecadal climate
Parker, D. E., The statisticaleffectsof incompletesamplingof
changesin the Northern Hemisphere,Bull. Am. Meteorol.
coherent data series,J. Climatol., 4, 445-449, 1984.
Parker, D. E., Effects of changingexposureof thermometersat
land stations,Int. J. Climatol., 14, 1-31, 1994.
Parker, D. E., P. D. Jones, A. Bevan, and C. K. Folland,
Interdecadalchangesof surfacetemperaturesincethe nineteenth century,J. Geophys.Res., 99, 14,373-14,399, 1994.
Parker, D. E., C. K. Folland, and M. Jackson, Marine surface
Soc., 71, 988-993, 1990.
Trenberth, K. E., J. R. Christy, and J. W. Hurrell, Monitoring
global monthly mean surface temperatures, J. Clim., 5,
1405-1423, 1992.
van Loon, H., and J. Williams, The connection between trends
of mean temperature and circulation at the surface, I,
Winter, Mort. WeatherRev., 104, 365-380, 1976.
temperature:Observedvariationsand data requirements, Vinnikov, K. Ya., P. Ya. Groisman, and K. M. Lugina, The
Clim. Change,31,559-600, 1995a.
empiricaldata on modernglobalclimatechanges(temperParker, D. E., M. Jackson, and E. B. Horton, The GISST2.2
ature and precipitation),J. Clim., 3, 662-677, 1990.
seasurfacetemperatureand sea-iceclimatology,Clim. Res. Walsh, J. E., The elusiveArctic warming,Nature, 361, 300301, 1993.
Tech. Note 63, Hadley Cent., Meteorol. Off., Bracknell,
England, 1995b.
Wentz, F. J., and M. Schabel, On the discrepancybetween
Parker, D. E., M. Gordon, D. P. N. Cullum, D. M. H. Sexton,
observedin situ surfacewarming and the coolingtrend in
C. K. Folland, and N. Rayner, A new globalgriddedradioMSU tropospheric
temper.
atures,Nature,394,661-664,1998.
sonde temperature database and recent temperature Wigley, T. M. L., J. K. Angell, and P. D. Jones,Analysisof the
trends,Geophys.Res.Lett., 24, 1499-1502, 1997.
temperature record, in Detecting the Climatic Effects of
Peterson, T. C., H. Daan, and P. D. Jones, Initial selection of
IncreasingCarbon Dioxide, edited by M. C. MacCracken
a GCOS surface network, Bull. Am. Meteorol. Soc., 78,
and F. M. Luther, pp. 55-90, U.S. Dep. of Energy, Wash2145-2152, 1997.
ington, D.C., 1985.
Peterson, T. C., et al., Homogeneity adjustmentsof in-situ Wigley, T. M. L., P. D. Jones, and P.M. Kelly, Empirical
climate studies: Warm world scenarios and the detection of
atmosphericclimate data: A review, Int. J. Climatol., 18,
1495-1517, 1998a.
climatic changeinducedby radiatively active gases,in The
Peterson,T. C., T. R. Karl, P. F. Jamason,R. Knight, and D. R.
GreenhouseEffect, Climatic Change,and Ecosystems,
pp.
Easterling,The first differencemethod:Maximizing station
271-323, SCOPE series,John Wiley, New York, 1986.
densityfor the calculationof long-termglobal temperature Wigley,T. M. L., P. D. Jones,and S.C. B. Raper, The observed
change,J. Geophys.Res., 103, 25,967-25,974, 1998b.
global warming record: What does it tell us?, Proc. Natl.
Acad. Sci. U.S.A., 94, 8314-8320, 1997.
Reynolds,R. W., A real-time global sea surfacetemperature
analysis,J. Clim., 1, 75-86, 1988.
Rigor, I. G., R. L. Colony, and S. Martin, Statisticsof surface
air temperatureobservations
in the Arctic,J. Clim., in press,
P. D. JonesandM. New, ClimaticResearch
Unit, University
1999.
Santer, B. D., J. J. Hnilo, T. M. L. Wigley, J. S. Boyle,
C. Doutriaux, M. Fiorino, D. E. Parker, and K. E. Taylor,
Uncertaintiesin observationallybasedestimatesof temperature changein the free atmosphere,J. Geophys.Res.,104,
6305-6333, 1999.
Schwerdtfeger,W., L. M. de la Canal, and J. Scholter,MeteorologiaDescriptivadel SectorAntarcticoSudamericano,
Publ.
7, 425 pp., Inst. Antarct. Argentino, BuenosAires, 1959.
of EastAnglia,NorwichNR4 7TJ,England,United Kingdom.
([email protected])
S. Martin, Schoolof Oceanography,
Universityof Washington,Seattle,WA 98195-7940.
D. E. Parker,HadleyCentre,MeteorologicalOffice, Bracknell, BerkshireRG12 2SY, England.
I. G. Rigor, Applied PhysicsLaboratory,Universityof
Washington,Seattle,WA 98105-6698.