Cynthia Rosenzweig
and Daniel Hillel
Climate and Biosphere
.~.y.~.~.~.i.~...~.~.~.t.~~~.~
Someancient societiesbelievedthat climatic phenomenastorms -
especiaUyrain~
representeda primal mating of the sky with the earth. This intuitive
depiction has long been supplanted by scientific inquiry. Yet the essentialfact
remains: climate and its short~term manifestations, called weather, are
conditioned by interactions of the atmosphere with the earth's surface.
Although it covers only about
30 percent of the terrestrial realm,
the land surface, with its various
soil and vegetation zones, affects
the global radiation balance, hydro~
logical cycle, carbon and nutrient
cycles, and momentum transfer
(wind) of the entire earth and its
near atmosphere. Land~surfacephe~
nomena thus playa major role in
determining climate, and are them~
selveslikely to be affected recipro~
cally by its changes.Three interacting processes
- the energy ex~
change, the hydrological cycle, and
the carbon cycle - dominate land~
surface/atmospheredynamics.
Differentbiomes- rainforests,
boreal and deciduousforests,savannas,grasslands,
and deserts- vary
in the rates of these processesand
create a variegated palette over the
face of our planet (seeFigure 1 on
the next page).
S
olar radiation received on
the earth's surface is the
major component of the energy
NATIONALFORUMNol. 79, No.2
balance.The land surfaceplaysan
important role in reflecting,
absorbing,and partitioning the
receivedsolarradiation. First,how
much radiation doesthe surface
reflect and how much doesit
absorb?The albedo,which is the
percentageof light that a surface
reflectsback into the atmosphere,
variesaccordingto the color,
roughness,and inclination of the
surface.It is on the orderof 5 to 10
percentfor water, 10 to 30 percent
for vegetation,15 to 40 percentfor
a baresoil, and up to 90 percentfor
freshsnow.The smootherand drier
the land surface(either vegetated
or baresoil), and the brighter its
color, the higher is its albedoand
the smaller~the fractionof shortwaveradiation (light) it absorbs.
The land surfacealsoparticipatesin the exchangeof long-wave
radiation (heat). The earth'ssurface,which convertsincoming
shortwaveradiation to sensible
heat, emits long-wave(thermal,
infrared) radiation. At the same
time, the atmospherealsoabsorbs
shortwaveand emits long-wave
23
CLIMATE AND BIOSPHERE
radiation, part of which reachesthe
surface.The differencebetween
theseoutgoingand incoming fluxes
is called the net long-waveradiation. Calculatingthe net longwaveradiation at the earth'ssurfaceis not unlike calculatinga
householdbudget:"Income minus
Outgo equalsNet Changein
Value."In our case,however,the
currencyinvolved is not moneybut
radiant energy.Incoming sunlight
is called"shortwavesolarradiation," while heat outradiatedfrom
the earth'ssurfaceis called "longwaveradiation." During daytime,
incomingsolarradiation typically
exceedsthe emissionof heat by the
earth'ssurface,so often there is a
net gain of energyand the surface
tendsto warm up. During the
night, when no incoming solar
radiation is present,there is a net
lossof energyby long-waveradiation. Someof the emitted radiation
is blockedby the atmosphere,espe-
cially if it is humid or cloudy.That
is why we notice that cloudy nights
tend to be warmerthan clear
nights.
The overall differencebetween
total incoming and total outgoing
radiation (including both the
shortwaveand the long-wavecomponents)is termednet radiation,
expressingthe rate of radiant-energy absorptionper unit areaof land
surface.The completeequationfor
the earth'snet radiation can be
statedvery simply: Incoming
Sunlight minusReflectedSunlight,
plus Incoming Heat minusOutgoing Heat, equalsNet Energy
Absorption.
The land surfaceaffectsthe
apportionmentof the net radiation
into different formsof energy.
Green plants are ableto convert a
part of the receivedradiation into
chemicalenergy.Thus, a smallportion (generallylessthan 5 percent)
of the solarradiation is usedin the
vital processof photosynthesis,on
which all life on earth ultimately
depends.Part of the net radiation
receivedby the land surfaceis
transformedinto heat, which
warmsthe soil, plants,and the
atmospherenear the ground.This
is called "sensibleheat." A major
part is absorbedaslatent heat in
the twin processes
of evaporation
and transpiration(the latter denoting evaporationfrom plants). The
allocation of the net radiation into
thesedifferent formsof energy
dependson the nature of the surface,especiallyon the availability
of waterfor evaporation.In humid
ecosystems,
much of the energy
goes~ntoevapotranspiration.In
arid r~gions,there is little moisture
for evaporationso the major portion of net radiation goesinto
heating the surface.This is one reasonwhy,for example,surfacetemperaturesin Arizonaareusually
SPRING 1999/PHI KAPPA PHI JOURNAL
higher than in Alabama, despite the
fact that both locations are at the
samelatitude and, therefore, receive
the sameamount of sunlight.
T~~e:~r~~ea~~g~~~d~~~s~ecip,rocal passing ga~e whose main
article of e~change ~swater. The
and density from seasonto season
and from year to year, both
responding to and affecting climate
variability. On longer time scales
(decades), the carbon cycle is
affected by ecological succession
and changes in the composition of
the plant community, and thus may
three cycles -rheare
inextricabl
y
forces
that
impel
this
exchange
are..
climate, and that the interactions
of soil and.vegetation processes
with the atmosphere must be
incorporated in GCMs more
realistically.
As GCMs have improved during the last two decades,the representation
of land-surface orocesses
"
energy, water, and carbon
/inked
.
thesun's
radiant
energy
andthe IdentIcal or overlappIng
because
they
.
involve
.
processes wIthIn the
same environmental domain. The content of
tation in the form of rain and s~ow water in the soil affects the wa
y the flux of
earth'sgravitationalpull. Precipi-
falls on the land surface.Someis
intercepted
by trees,suchas
conifers,whilemoststrikesthe
ground,
eithe:t?runoffonthe
surfaceor to mftltrate to the store
of soil moisture.Someof the infiltrated waterjoins aquifersand]
slowly
winds
its way
to rivers
and
the sea.
Another
fraction
of soil
moisture evaporatesfrom the soil
surfaceor from the foliageof growing plants.In addition, water evaporatesfrom lakes,reservoirs,and
seas.The vaporcarriedin the air
eventually recondensesas liquid
and falls asrain, or freezesto form
snow. The water thus returned
feeds the rivers, which flow to the
ocean. Evaporation from land and
oceansendswaterback to the
atmosphere, and the exchange goes
on continuously.
T
he terrestrial carbon cycle
interacts with the climate
systemat various time scales.On
short time scales(minutes), plants
regulate their stomates, the small
openings in leaf surfacesthrough
which CO2 is absorbed and water is
released.The stomatal aperture
dependson the atmospheric
demand for water and on the supply of soil water to the plant. On
seasonaltime scales(months),
plants create vegetative canopies
over which transpiration occurs.
These canopies vary in structure
NATIONAL FORUMNol. 79, No.2
...
.
energy streamIng Into the soil-plant complex IS
partitioned
and used. Reciprocally, the energy
flux affects the state and movement of water.
:lave important
,
~ rhanging
-..
~
_.~'--~-In
...
~~~_.~
feerlh~rir
climate ..
._~ -
pftprt~
I
rhe three cyclp~
energy,
are inextnca.,
1water, and carbon 1bly linked becausethey involve
ildentical or overlapping processes
within the same environmental
\domain. The content of water in
1the soil affects the way the flux of
energy streaming into the soil-plant
':omplex is partitioned and used.
]Reciprocally, the energy flux affects
1the state and movement of water.
]Likewise, the carbon cycle is linked
1to the water and energy balances
becauseplant photosynthesis both
controls stomatal resistance, which
determines transpiration rate, and
governs the time evolution of vegetative canopies.
1
W
hen
atmospheric
scien-
tists
first
general
created
circulation models,now known as
global climate models(GCMs),
the land surfacewasrepresented
very simply asthe "boundarycondition" for the atmosphere.
However,atmosphericmodelers
soon realizedthat the land surface
playsan active role in weatherand
]llas indeedbecomemore realistic.
The primary aim hasbeento
improvethe simulation of relevant
I
fluxesto the atmosphere,especially
the latent and sensibleheat fluxes.
A secondaim hasbeento enhance
the physicalrealismof the landsurfacecomponentof GCMs for
earth-systemglobal-changestudies,
suchasthe role of the carboncycle
in climate change,the effectsof
land degradationand deforestation
on the hydrologiccycle, and the
contribution of river dischargeto
oceansalinity.
Land-surfaceprocesses
in
GCMs include rainfall interception
by vegetationand its throughfall,
surfaceand subsurface
runoff, infiltration, soil-waterflow, transpiration, and direct evaporationfrom
interceptedprecipitation, dew,and
baresoil. Latent and sensibleheat
fluxe~betweenthe land surfaceand
an atmosphericreferencelayerare
calculated.
One exampleof the inclusion
of land-surfaceprocesses
in an
overall global climate model is the
land-surfacemodel developedat
the NASA/Goddard Institute for
SpaceStudies(Figure2). In this
model the major relevantprocesses
are simulatedin termsof mathe25
CLIMATE AND BIOSPHERE
matical equations.The variables
interact in accordancewith wellestablishedphysicaland physiological relationships.An effort is made
to validatethe predictionsof this
mathematicalmodelby comparison
with observationsmadeon the
groundand by meansof remote
sensing(satellite imagery).
C
urrent research is extending the land-surface mod-
els primarily in three ways. First,
models are now including a more
detailed and comprehensive representation of ecosystemprocess,
especially the carbon cycle.
Second, other modelers seek to
26
include the role of topographyin
soil-moistureheterogeneity,evapotranspirationand partitioning of
surfacefluxes,timing and partitioning of runoff, andbaseflowon
watershedscales.Third, the
dynamicrole of land-usechangeis
now beginningto be representedin
the models.All of theseresearch
areasare vital becauseGCMs are
increasinglyappliedin earth-system
studiesthat include land-surface
changessuchasshifts in natural as
well asagriculturalvegetationwith
global warming.
Somemodelsnow include the
prediction of vegetationtypeson
the basisof climatic, physiological,
and ecologicalprocesses,
recognizing the processes
of long-termeco-
logical succession.Models also
include short-term carbon and
nitrogen dynamics in vegetative
biomass, litter, and soil organic
matter. Figure 3 shows the projection of annual net primary production (photosynthesis minus respiration) of global vegetation simulated
with a new version of the GISS
land-surface model.
Modelers are also actively seeking to improve the simulation of the
water cycle by including the role of
topography. Topography plays a
strong role in routing both aboveand below-ground runoff, and in
creating areasof enhanced soil
moisture. The latter, in turn, affects
evapotranspiration and partitioning
of latent and sensibleheat fluxes.
SPRING 1999/PHI KAPPA PHI JOURNAL
Finally, becausehumansare
rapidly and radically altering the
land surface,modesof land useand
managementare alsobeing taken
into consideration.An exampleis
the transformationof the vast
savannasknown asthe Cerradosin
Brazil into commercialsoybeanand
maizeproduction.Simulationsare
now designedto predict crop suitability, agriculturalzones,and crop
productionon global scales,based
on climate, soil resources,
and
management.The aim of the work
is two-fold: 1) To provide more
realisticcharacterizationof landcovertypeswithin global climate
models,thus improving the calculation of water and carbonfluxesat
grid-boxscales;and 2) To implement a new generationof global
land-use/land-cover
modelswith
the capabilityof simulatingthe
effectsof climate changeon agricultural production in a self-contained manner.
G
lobal
climate
arguably
the
change,
most
pro-
found environmentalissueof our
time, makesit imperativefor us to
understandclimate-biosphereinteractionsin quantitative terms.The
land surfaceis an active participant
in, not merelya passiverecipient
of, globalclimate change.Deforestationthrough burning releases
carbondioxide (CO2) and methane
(CH4) from existingbiomass,while
cultivation of "new" land speeds
the decompositionof the initially
presentsoil organicmatter.These
processes
contribute to the accumulation of greenhousegasesin the
atmosphere.As global warming
occurs,ecosystems
will respondby
alterationof speciescomposition
andshifts in location. However,
thesechangesare likely to be
thwartedby the fragmentationand
island-likeisolation of the onceNATIONALFORUMNol. 79, No.2
2.0-1
1.8-,
1.S-1
1.41.21.0.8-,
.S-!
.4-1
.I
.2-1
.0-
Figure 3. Annual net primary productivity (photosynthesis minus respiration) calculated
with the GISS Land-Surface Model. (Source: Tubiello and Rosenzweig, 1999).
extensiveecologicaldomains
resultingfrom the human usurp~tion of the greaterfraction of the
planet'sterrestrialsurface.Some
species,perhapseven entire biotic
communities,may not survive
changesin climate if they arepreventedfrom migration.
Consequently,the role playedby
natural ecosystems
in environmental processes
(Oz-COzexchange,
runoff, evaporation,groundwater
recharge,nutrient recycling,and
more) maybe jeopardized.
changes that are subject to our
action, while understanding and
adjusting to those processes(such
as.the El N ifio phenomenon) that
are beyond our control.
Theseand other threatsto the
integrity of the biosphererequireus
not merelyto continue but indeed
to intensify the effort to understand
land surface-climateinteractionsin
termsof quantitative dynamic
processes.
That is the aim and
challengeof current modeling
efforts,accompaniedby continuous
monitoring and data collection. In
ancient times,our forebears
attemptedto affectclimate by
appeal(in prayerand sacrifice)to
the godswhom they believedto
control both stormsand droughts.
Todaywe areno lessaffectedby
the sameforcesof nature.Those
forcesare in part conditionedby
our actions,and are in part beyond
our control. It is our task to prevent or mitigate thosedeleterious
Daniel Hillel, a senior research
scientist at Columbia Univer~
sity, has served as a professor
of soil physics, hydrology, and
the environmental sciences at
leading universities in the
United States and abroad, and
has been a consultant to the
World Bank and the United
Nations.
m
Cynthia Rosenzweig is a re~
search scientist at the NASAl
Goddard Institute for Space
Studies, where she is the leader
of the Climate Impacts Group.
Rosenzweigand Hillel are the
co,authors of Climate Change
and the Global Harvest: Po'
tential Impacts of the Green,
houseEffect on Agriculture
(Oxford University Press,
1998).
27