13
Weather and Climate (1985) 5: 13-18
ARE NEW ZEALAND'S SHEEP CAUSING AN INCREASE
IN ATMOSPHERIC METHANE?
D. C. Lowe
Institute o f Nuclear Sciences, DSIR,
Lower Hutt.
ABSTRACT
Recent measurements of various atmospheric trace gases in background
tropospheric air have shown that the atmospheric concentrations o f several
are increasing rapidly. T h e concentration o f atmospheric methane, f o r
example, is increasing globally at the rate o f 1.7% per year and extrapolation leads to an estimated "Greenhouse Effect" o f about 40% o f that
predicted for increasing atmospheric carbon dioxide. I n addition, increasing
atmospheric methane w i l l disturb t h e atmospheric chemistry o f t h e
troposphere w i t h unknown consequences f o r other trace species i n the
atmosphere.
The causes o f the increasing atmospheric methane concentration i n the
atmosphere are not understood. However the suggested sources of the gas,
for example biological, do have significantly different stable and radioactive
isotopic signatures. A feasible analysis technique based on these isotopic
differences w i l l be outlined and the possible atmospheric significance o f
increasing atmospheric methane will be discussed.
TRACE G A S C H E M I S T RY A N D
ATMOSPHERIC M E T H A N E
The desirable properties o f the atmosphere
may be impaired by a variety of natural and
anthropogenic processes; for example, through
biomass burning, the large scale injection o f
industrial waste gases, aerosols, and particulate
matter. Man's activities are now so extensive
that he can modify the composition of the atmosphere, w i t h t h e possibility o f changing
climate on a global scale. F o r example, the
release of carbon dioxide from the combustion
of fossil fuels and from forest clearing may
produce a warming effect in the troposphere
(Manabe, 1983), while the release o f chlorofluoro methane compounds and the use o f
nitrogen fertilizers m a y possibly reduce the
amount o f stratospheric ozone (Crutzen and
Ehhalt, 1977).
Many gases are found in the atmosphere in
trace amounts and a study of their abundance
leads to information on sources and sinks, residence times and chemical pathways, a l l o f
which provide an understanding o f how the
atmosphere responds t o gases released b y
anthropogenic processes o r b y n a t u r a l
phenomena. Many o f the trace gases i n the
atmosphere begin as reduced species emitted
from t h e surface o f the Earth. I n the atmosphere t h e y a r e oxidised, undergo f r e e
radical photochemistry and various products
from the reactions may return to the Earth's
surface b y rainout, washout o r b y d r y deposition. One important class o f atmospheric
trace gases is the aliphatic hydrocarbons which
have been found i n both clean and polluted
air (Rudolph and Ehhalt, 1981). They are
destroyed in the sunlit atmosphere by photochemical reactions that are generally initiated
by O H radicals o r ozone attack, and t h e
eventual products are stable trace gases like
carbon monoxide, hydrogen, carbon dioxide,
and water.
Atmospheric Methane increase
14
Methane, with concentrations of about 1.65
ppmv* i n the Northern Hemisphere and 1.5
ppmv in the Southern Hemisphere, is the most
abundant hydrocarbon found i n the atmosphere. I t is a strong absorber in the infrared
and contributes significantly t o t h e Earth's
atmospheric greenhouse effect. I t also plays a
major role in atmospheric chemistry as a sink
for O H radicals and produces small but significant amounts o f C O , and H 2 0 i n t h e
troposphere. O n the basis o f measurements
made by gas chromatography, there is growing
evidence that, a t least i n recent years, t h e
methane concentration o f t h e Earth's a t mosphere has increased a t the rate o f 1.7%
per year (Fraser et al., 1981; Rasmussen and
Khalil, 1981; see Fig. 1). Indirect evidence of
atmospheric methane increases has also come
from ice core measurements made b y Craig
ppmv = parts per million by volume.
and Chou (1982). They have shown that the
methane mixing ratio in air trapped in Greenland i c e cores decreases f r o m 1.25 p p m v
around 1910 A.D. to a baseline value of 0.70
ppmv 500 years ago corresponding to a depth
of 250 m. Below this level to a depth of 1950
m (approx. 27,000 years RP.) the mixing ratio
appears t o be constant at the baseline level
and i n agreement with the data o f Robbins
et al. (1973), measured in 700 to 2470 year
old Antarctic ice (see Fig. 2).
The methane concentration increase in the
Northern Hemisphere is about 0.25 ppmv over
the 1970 to 1980 period. Applying a radiative/
convective model developed b y Lacis e t al.
(1981), this corresponds t o a n equilibrium
greenhouse heating over the decade o f 0.054
degrees Celsius or about 40% of the warming
predicted for CO„ over the same period (Craig
and Chou, 1982). Increasing methane may
ultimately deplete O H radicals i n the Earth's
atmosphere leading to a significant change in
1
2.0
I —
"Clean air" atmospheric methane observations.
0
0
1.5
-IS
o
•••••••••'
0
o
0
°11S?
o _09 2t)
0
0
,•••• •"""-.
0
••••••..
•••••••
1.0
1965
1
1970
1
1975
(YEAR)
1980
Fig. 1 : Measurements o f t h e concentration o f atmospheric methane m a d e a t v a r i o u s m a r i n e a n d
"clean a i r " sampling sites f r o m 1965 t o 1981. T h e sources o f the various measurements are detailed in
Rasmussen and Khalil, 1983.
15
Atmospheric Methane Increase
1
I c e c o r e " methane measurements.
1981
period o f a t m o s p h e r i c — I t
observations
/ 1 9 6 5
goo
0000•
0.70 p p m v baseline • * - 1 - . 4 - ° .
ammo emo
m o n =
•
•
•
•
•
•
1
1400
1
1600
1800
2000
(YEAR)
Fig. 2 : Measurements o f the methane concentration i n a i r bubbles trapped i n A n t a r c t i c g l a c i a l i c e .
Source: Craig and Chou, 1982.
the chemistry o f t h e troposphere. Carbon
monoxide and ozone i n the troposphere are
also likely t o increase, accompanied b y a n
increase i n stratospheric ozone. (Rasmussen
and Khalil, 1983).
SOURCES A N D SINKS O F M E T H A N E
The reasons f o r the rapid increase i n atmospheric methane are not understood. For the
concentration of a trace gas to remain constant,
sources, sinks and transport processes for the
gas must be balanced. Obviously for methane,
in 1500 A D , the Elizabethan era, a change
must have occurred which led to an imbalance.
Major sources for methane are believed to
be biogenic. I n particular, ruminants l i k e
sheep and cattle produce large amounts o f
methane by a process known politely as enteric
fermentation. Other known biological sources
of methane are swamps, paddy fields a n d
sewage. M o r e recently, Zimmerman (1983)
has proposed that termites are a large source
of methane. I n addition, oil, gas and geothermal fields and volcanoes are known to produce CH,. Gold (1982) has also suggested that
the Earth's core contains a vast amount o f
primordial methane which may be being released gradually into the atmosphere.
The major sink of atmospheric methane is
an oxidation reaction with the OH radical in
the troposphere.
CH, + O H • --> CH,• + H2
,
(1)
This reaction is the first i n a series o f important reactions in the atmosphere known as
the methane oxidation series.
Since i t is unlikely that a sudden change
occurred to increase methane production from
volcanic or primordial sources in 1500 AD, it
seems likely that the atmospheric methane increase is due to a change in a biological source
and/or a decrease i n t h e atmospheric O H
radical concentration.
16
A
t
m
o
s
p
h
e
r
i
c
Methane Increase
2.0
-o
cti
60 a)
No. of sheep in N.Z.
z
c 2
—
55 am
1.5
_cc°
46
1.0
1965
50 z
1
9
7
0
1
9 7 5
(YEAR)
19 80
Fig. 3 : To t a l number o f sheep i n N e w Zealand f r o m 1965 t o 1981 ( f r o m N . Z . Yearbook) a n d the
observed global atmospheric methane increase.
ARE N.Z.'s SHEEP C A U S I N G A N INCREASE
I N AT M O S P H E R I C M E T H A N E ?
In Fig. 3 the number of sheep in New Zealand are plotted together with clean air global
atmospheric methane measurements made over
the last 15 years. Clearly there i s n o correlation in total flock numbers with the global
measurements but N.Z.'s sheep probably contribute significantly to local methane concentrations. I t has been estimated that one sheep
can produce 20 litres (STP) of methane a day
and in a confined space a large flock of sheep
could produce high concentrations of methane.
Methane i s o f course a versatile energy
source. When coupled by as yet undeveloped
technology to the carburettor o f a vehicle, i t
has been estimated that the methane output of
one sheep would be capable of powering the
vehicle for several kilometres a day. Sheep are
much more efficient at producing methane than
people. To power the same vehicle by people a
whole football team, a couple of kegs of beer
and even more sophisticated plumbing would
be required.
ISOTOPIC COMPOSITION O F M E T H A N E
Methane contains 6 different isotopes of car-
bon and hydrogen and the proportions of each
can depend o n the source o f the methane.
Several isotopic measurements have been made
of various methane sources and the results are
listed i n Table 1. Various unit systems are
used for the different isotopes and a full description of them is beyond the scope of this paper.
For a review o f the significance o f isotopic
measurements t h e r e a d e r s h o u l d consult
Gonfiantini (1978).
One o f t h e most interesting isotopes i n
methane is radiocarbon, written as t4C because
it has 14 protons and neutrons in its nucleus
instead of the 12 found in the most abundant
isotope of carbon, '2C. 14C is radioactive and
has been widely used as a method o f dating
organic materials. A l l living things contain
14C obtained from the atmosphere and are
radioactive. Once an organism dies i t ceases
to exchange atmospheric CO, and the amount
of radiocarbon contained i n i t begins t o decrease with a half-life o f 5700 years. Hence
very old biological materials like oil and gas
contain no 14C, whereas methane from sheep
for example is radioactive; i.e., "modern" (see
Table 1 ) . I f t h e cause o f increasing a t mosphere methane i s principally living bio-
Atmospheric Methane increase
1
7
TA B L E 1 : I S O T O P I C COMPOSITION A N D SOURCES O F AT M O S P H E R I C M E T H A N E . C A R B O N
ISOTOPES I N M E T H A N E A R E " N O R M A L " " C , ST ABLE ' ' C , R A D I O A C T I V E " C . H Y D R O G E N ISOTOPE A R E " N O R M A L " ' H , S TA B L E ' H ( D E U T E R I U M ) , R A D I O A C T I V E all ( T R I T I U M ) . T H E R AT I O
OF " C / " C I S G E N E R A L LY A B O U T 1 % A N D l ' C / " C F O R " M O D E R N " S A M P L E S E X C H A N G I N G
W I T H AT M O S P H E R I C C O , I S A B O U T 1 0 ' . T H E M E A S U R E M E N T S L I S T E D I N T H E T A B L E A R E
REFERRED T O A N I N T E R N AT I O N A L S TA N D A R D SYSTEM ( G O N F I A N T I N I , 1978). F O R " C A N D
214 T H E M O R E N E G AT I V E T H E N U M B E R T H E S M A L L E R T H E ISOTOPE C O N T E N T O F T H E
M E T H A N E SAMPLE.
Methane Source
"C
(PDB o/oo)
Isotopes i n Methane
"C
'
H
(% modern wood) ( S M O W o/oo)
Bacterial (marsh, peat, animals—sheep!)
-55 t o –90 ( M o d e r n )
Oil (related)
Coal (related)
Geothermal ( N . Z . )
Primordial (carbon)
Present Atmospheric
–30 t o
–25 t o
–25 t o
– 5 to
–47
–55
–35
–30
– 7
Marsh gas
– 275
– 200 -4-- 2 0
150 1 0
– 100 2 0
3/1 (TU.)
0
0
0
0
Variable
PDB — Pee Dee Belamite, a standard calcium carbonate used f o r " C measurements. 0 / 0 0 means parts p e r
thousand.
SMOW — Standard Mean Ocean Water, used as a standard f o r ' H measurements. See Gonfiantini, 1978.
— Tr i t i u m Unit. 1 T. U . = 1 3H atom i n 1 0 " ' H atoms.
logical organisms then atmospheric methane
should be almost as radioactive as atmospheric
CO,. On the other hand, i f the cause o f the
increase is very old methane, (e.g., from o i l
or gas wells) then atmospheric methane will
contain a much smaller amount of 14C.
ANALYSIS T E C H N I Q U E
A series of routine measurements of the 14C
content o f atmospheric methane will help t o
determine the possible cause of the increasing
concentration of the gas. These measurements
should be made at clean air sites so that the
air is representative of a large region and not
contaminated b y local sources o f methane.
Many coastal sites in New Zealand with frequent onshore winds fulfill this requirement.
For example, Baring Head lighthouse, southeast of Wellington, is an excellent site for making atmospheric C O , measurements representative of the southwestern Pacific (Lowe et
al., 1979).
Despite the importance of 14C measurements
of atmospheric methane no measurements i n
clean air have been reported. This is because
the concentration o f methane i n clean air is
so low that, i f conventional radiocarbon dating techniques were used, over 200 M3 o f air
would have to be processed; a formidable task.
However within the last few years a new radio-
carbon dating technique known as Tandem
accelerator mass spectrometry ( TA M S ) has
been developed i n w h i c h milligram sized
samples o f carbon can be dated. A TA M S
facility i s currently being developed a t the
DSTR's Institute of Nuclear Sciences in Lower
Hutt. This facility should enable 14C measurements to be made of various atmospheric trace
gases including methane. A n atmospheric
methane sampler is under construction and i t
is hoped that the first 14C measurements o f
methane in clean air will be available by late
1985. These measurements should play a vital
role in establishing the possible causes o f the
global increase of atmospheric methane.
CONCLUSIONS
The increasing concentration of atmospheric
methane is a problem o f global significance
comparable t o present concerns over the increasing concentration o f atmospheric carbon
dioxide. However, whereas the major cause of
increasing dioxide can be directly attributed to
the wide-spread combustion of fossil fuels, the
causes of the atmospheric methane increase are
not understood.
Because many methane sources have distinct isotopic "signatures", isotopic measurements may help to identify possible causes of
the atmospheric methane increase. I n par-
20
Wellington Wind Lore
When is a northerly not a northerly?
When is a southerly not a southerly?
When i t ' s a bent westerly
When it's a bent easterly
Fig. 1 : Effect o f topography o n w i n d f l o w through Cook Strait.
1. NOR'EASTER
This wind is not usually very strong. The
clouds thicken slowly and gradually lower, becoming leaden grey, with rain. The barometer
falls, sometimes quite dramatically.
This type o f weather is usually associated
with the advance of a depression from the mid
or north Tasman. The air is mild. A northeast
wind can bring Wellington's warmest temperatures.
2. NOR'WESTER
This is the most common type, known for its
tendency to increase in strength (sometlmes to
gale force) and become very gusty.
The "nor'wester" with its falling glass precedes troughs o f l o w pressure crossing the
country from west t o east within westerly
al rstreams.
The indications that fair weather is giving
way t o a "nor'wester" are cloud build-ups,
sometimes seen at sunset, or patches of cloud
racing fast across the sky o n a n otherwise
fine day.
Increasing cloudiness, sometimes w i t h
thunderheads, i s followed b y a period o f
showers, at the onset of which the wind reaches
its height. The wind then subsides.
3. WEST-NOR'WESTER
Following a "nor'wester", windvanes i n
Wellington sometimes continue to point northwest, but the weather clears rapidly and the
glass starts to rise. The wind behind the trough
is actually westerly now, though still appears
northwesterly through Cook Strait. The "westnor'wester" is one "northerly" which does not
indicate deteriorating weather, although t h e
clear weather associated with i t is sometimes
short-lived, depending upon the amount of shift
in the true wind direction from northwest to
west.
Tell-tale "west-nor'wester" signs are clear
skies after rain f r o m the northwest, lighter
w:nds, cooler temperatures, l o w e r relative
humid.ty, and a rising barometer in spite o f
little apparent wind change.
Any fronts or bands of showery weather in
Wellington Wind Lore
2
1
TABLE 1:
THE NORTHERLIES
Nor'easter
Nor'wester
,,x
\
o
e
$ .
Q0
,0
N N - N E lisht to m i l d to
moderote w o r m
sole zolosixty
West nor'wester NWw - lisht
WNW WSW
-
f e w
hmirS
sets in c l o u d y bright ofter
9roduolly becomes , s t o r t s
o'cost -
to
N-NW NW moderate
through to
moderote- S t r i r t S
short period port cloudy- few hoursroan after cloudy s e v e r a l clays
windfrnoxim-
IgC11
themshowers
no min c l e a r f e w hours
somethsies
cool
1-2 days
Spoiler
NWW-NW moclerote — -7cool
fresh
o
r
a west to southwest airstream over New Zealand will result in unsettled weather in most
western areas. While Wellington i s sheltered
by the South Island under these conditions,
evidence of bad weather elsewhere is often high
clouds moving from the southwest and a rather
cold wind from the northwest.
4. SPOILER
This cool and breezy wind accompanied by
extensive patches of low cloud is similar to a
"nor'wester" and represents a true wind from
west t o northwest. T h e barometer, however,
does not fall and may even rise slightly. That
the glass is not falling and wind not increasing
in strength, indicates that the main influences
on the weather conditions are anticyclonic not
cyclonic.
Usually caused by an intensifying anticyclone
in the north or northeast, the weather in most
places, especially north of Wellington, is clearer
n o
a
rain extensive o f t e n
l
s
low several cloys
cloud
than around Cook Strait's wind funnel.
THE SOUTHERLIES (Table 2)
If the northerlies are usually signs o f bad
weather, then southerlies often signal good
weather. Southerlies, however, are less understood than northerlies — and are certainly
less appreciated!
Frequently, a rising nor'wester indicates bad
weather which ends with a change to the south
and an improvement in the weather.
Also a southerly change can often provide
a welcome break during times of predominant
nor'westers such as i n the spring equinox.
But Wellingtonians have a paranoia about
southerlies! Southerlies are commonly thought
to start with a bang and to last for three days!
It is true that some southerlies bring stormy
weather, and certainly they a l l bring cooler
temperatures, b u t most o f them indicate a
change for the better, sooner or later.