Cynthia Rosenzweig and Daniel Hillel

Climate and Biosphere

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Some ancient societies believed that climatic phenomena - especiaUy rain~storms - represented a primal mating of the sky with the earth. This intuitive

depiction has long been supplanted by scientific inquiry. Yet the essential factremains: climate and its short~term manifestations, called weather, areconditioned by interactions of the atmosphere with the earth's surface.

Although it covers only about30 percent of the terrestrial realm,the land surface, with its varioussoil and vegetation zones, affectsthe global radiation balance, hydro~logical cycle, carbon and nutrientcycles, and momentum transfer(wind) of the entire earth and itsnear atmosphere. Land~surface phe~nomena thus playa major role indetermining climate, and are them~selves likely to be affected recipro~cally by its changes. Three interact-ing processes - the energy ex~change, the hydrological cycle, andthe carbon cycle - dominate land~

surface/atmosphere dynamics.Different biomes - rainforests,boreal and deciduous forests, savan-nas, grasslands, and deserts - varyin the rates of these processes andcreate a variegated palette over theface of our planet (see Figure 1 onthe next page).

balance. The land surface plays animportant role in reflecting,absorbing, and partitioning thereceived solar radiation. First, howmuch radiation does the surfacereflect and how much does itabsorb? The albedo, which is thepercentage of light that a surfacereflects back into the atmosphere,varies according to the color,roughness, and inclination of thesurface. It is on the order of 5 to 10percent for water, 10 to 30 percentfor vegetation, 15 to 40 percent fora bare soil, and up to 90 percent forfresh snow. The smoother and drierthe land surface (either vegetatedor bare soil), and the brighter itscolor, the higher is its albedo andthe smaller ~ the fraction of short-wave radiation (light) it absorbs.

The land surface also partici-pates in the exchange of long-waveradiation (heat). The earth's sur-face, which converts incomingshortwave radiation to sensibleheat, emits long-wave (thermal,infrared) radiation. At the sametime, the atmosphere also absorbsshortwave and emits long-wave

Solar radiation received onthe earth's surface is the

major component of the energy

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CLIMATE AND BIOSPHERE

radiation, part of which reaches thesurface. The difference betweenthese outgoing and incoming fluxesis called the net long-wave radia-tion. Calculating the net long-wave radiation at the earth's sur-face is not unlike calculating ahousehold budget: "Income minusOutgo equals Net Change inValue." In our case, however, thecurrency involved is not money butradiant energy. Incoming sunlightis called "shortwave solar radia-tion," while heat outradiated fromthe earth's surface is called "long-wave radiation." During daytime,incoming solar radiation typicallyexceeds the emission of heat by theearth's surface, so often there is anet gain of energy and the surfacetends to warm up. During thenight, when no incoming solarradiation is present, there is a netloss of energy by long-wave radia-tion. Some of the emitted radiationis blocked by the atmosphere, espe-

of the solar radiation is used in thevital process of photosynthesis, onwhich all life on earth ultimatelydepends. Part of the net radiationreceived by the land surface istransformed into heat, whichwarms the soil, plants, and theatmosphere near the ground. Thisis called "sensible heat." A majorpart is absorbed as latent heat inthe twin processes of evaporationand transpiration (the latter denot-ing evaporation from plants). Theallocation of the net radiation intothese different forms of energydepends on the nature of the sur-face, especially on the availabilityof water for evaporation. In humidecosystems, much of the energygoes ~nto evapotranspiration. Inarid r~gions, there is little moisturefor evaporation so the major por-tion of net radiation goes intoheating the surface. This is one rea-son why, for example, surface tem-peratures in Arizona are usually

cially if it is humid or cloudy. Thatis why we notice that cloudy nightstend to be warmer than clearnights.

The overall difference betweentotal incoming and total outgoingradiation (including both theshortwave and the long-wave com-ponents) is termed net radiation,expressing the rate of radiant-ener-gy absorption per unit area of landsurface. The complete equation forthe earth's net radiation can bestated very simply: IncomingSunlight minus Reflected Sunlight,plus Incoming Heat minus Out-going Heat, equals Net EnergyAbsorption.

The land surface affects theapportionment of the net radiationinto different forms of energy.Green plants are able to convert apart of the received radiation intochemical energy. Thus, a small por-tion (generally less than 5 percent)

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higher than in Alabama, despite thefact that both locations are at thesame latitude and, therefore, receivethe same amount of sunlight.

and density from season to seasonand from year to year, bothresponding to and affecting climatevariability. On longer time scales(decades), the carbon cycle isaffected by ecological successionand changes in the composition ofthe plant community, and thus mayT~~e:~r~~ea~~g~~~d~~~s~e- - "

cip,rocal passing ga~e whose main rhe three cycles - energy, water, and carbonarticle of e~change ~s water. The - are inextricabl

y /inked because they involveforces that impel this exchange are.. . . .

the sun's radiant energy and the IdentIcal or overlappIng processes wIthIn theearth's gravitational pull. Precipi- same environmental domain. The content oftation in the form of rain and s~ow water in the soil affects the way the flux offalls on the land surface. Some is ... .intercepted by trees, such as energy streamIng Into the soil-plant complex ISconifers, while most strikes the partitioned and used. Reciprocally, the energyground, eithe: t? run off on the flux affects the state and movement of water.surface or to mftltrate to the storeof soil moisture. Some of the infil-trated water joins aquifers and] ~~~_.~ _.~'--~- ... ]

slowly winds its way to rivers and ,the sea. Another fraction of soil ~ -.. ..moisture evaporates from the soil ~ ._~ - I I

surface or from the foliage of grow- 1

ing plants. In addition, water evap- 1orates from lakes, reservoirs, and iseas. The vapor carried in the air 1

eventually recondenses as liquid \

and falls as rain, or freezes to form 1

snow. The water thus returnedfeeds the rivers, which flow to the 'ocean. Evaporation from land and ]ocean sends water back to the 1atmosphere, and the exchange goes ]on continuously. 1

climate, and that the interactionsof soil and.vegetation processeswith the atmosphere must beincorporated in GCMs morerealistically.

As GCMs have improved dur-ing the last two decades, the repre-sentation of land-surface orocesses

:lave important feerlh~rir pftprt~ In

~ rhanging climate

rhe three cyclp~ energy,water, and carbon - are inextnca.,bly linked because they involveldentical or overlapping processeswithin the same environmentaldomain. The content of water inthe soil affects the way the flux ofenergy streaming into the soil-plant:omplex is partitioned and used.Reciprocally, the energy flux affectsthe state and movement of water.Likewise, the carbon cycle is linkedto the water and energy balancesbecause plant photosynthesis bothcontrols stomatal resistance, whichdetermines transpiration rate, andgoverns the time evolution of vege-tative canopies.T he terrestrial carbon cycle

interacts with the climatesystem at various time scales. Onshort time scales (minutes), plantsregulate their stomates, the smallopenings in leaf surfaces throughwhich CO2 is absorbed and water isreleased. The stomatal aperturedepends on the atmosphericdemand for water and on the sup-ply of soil water to the plant. Onseasonal time scales (months),plants create vegetative canopiesover which transpiration occurs.These canopies vary in structure

W hen atmospheric scien-

tists first created general

circulation models, now known asglobal climate models (GCMs),the land surface was representedvery simply as the "boundary con-dition" for the atmosphere.However, atmospheric modelerssoon realized that the land surfaceplays an active role in weather and

llas indeed become more realistic.The primary aim has been toimprove the simulation of relevantfluxes to the atmosphere, especiallythe latent and sensible heat fluxes.A second aim has been to enhancethe physical realism of the land-surface component of GCMs forearth-system global-change studies,such as the role of the carbon cyclein climate change, the effects ofland degradation and deforestationon the hydrologic cycle, and thecontribution of river discharge toocean salinity.

Land-surface processes inGCMs include rainfall interceptionby vegetation and its throughfall,surface and subsurface runoff, infil-tration, soil-water flow, transpira-tion, and direct evaporation fromintercepted precipitation, dew, andbare soil. Latent and sensible heatfluxe~ between the land surface andan atmospheric reference layer arecalculated.

One example of the inclusionof land-surface processes in anoverall global climate model is theland-surface model developed atthe NASA/Goddard Institute forSpace Studies (Figure 2). In thismodel the major relevant processesare simulated in terms of mathe-

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CLIMATE AND BIOSPHERE

matical equations. The variablesinteract in accordance with well-established physical and physiologi-cal relationships. An effort is madeto validate the predictions of thismathematical model by comparisonwith observations made on theground and by means of remotesensing (satellite imagery).

logical succession. Models alsoinclude short-term carbon andnitrogen dynamics in vegetativebiomass, litter, and soil organicmatter. Figure 3 shows the projec-tion of annual net primary produc-tion (photosynthesis minus respira-tion) of global vegetation simulatedwith a new version of the GISSland-surface model.

Modelers are also actively seek-ing to improve the simulation of thewater cycle by including the role oftopography. Topography plays astrong role in routing both above-and below-ground runoff, and increating areas of enhanced soilmoisture. The latter, in turn, affectsevapotranspiration and partitioningof latent and sensible heat fluxes.

C urrent research is extend-ing the land-surface mod-

els primarily in three ways. First,models are now including a moredetailed and comprehensive repre-sentation of ecosystem process,especially the carbon cycle.Second, other modelers seek to

include the role of topography insoil-moisture heterogeneity, evapo-transpiration and partitioning ofsurface fluxes, timing and partition-ing of runoff, andbaseflow onwatershed scales. Third, thedynamic role of land-use change isnow beginning to be represented inthe models. All of these researchareas are vital because GCMs areincreasingly applied in earth-systemstudies that include land-surfacechanges such as shifts in natural aswell as agricultural vegetation withglobal warming.

Some models now include theprediction of vegetation types onthe basis of climatic, physiological,and ecological processes, recogniz-ing the processes of long-term eco-

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Finally, because humans arerapidly and radically altering theland surface, modes of land use andmanagement are also being takeninto consideration. An example isthe transformation of the vastsavannas known as the Cerrados inBrazil into commercial soybean andmaize production. Simulations arenow designed to predict crop suit-ability, agricultural zones, and cropproduction on global scales, basedon climate, soil resources, andmanagement. The aim of the workis two-fold: 1) To provide morerealistic characterization of land-cover types within global climatemodels, thus improving the calcu-lation of water and carbon fluxes atgrid-box scales; and 2) To imple-ment a new generation of globalland-use/land-cover models withthe capability of simulating theeffects of climate change on agri-cultural production in a self-con-tained manner.

Figure 3. Annual net primary productivity (photosynthesis minus respiration) calculatedwith the GISS Land-Surface Model. (Source: Tubiello and Rosenzweig, 1999).

changes that are subject to ouraction, while understanding andadjusting to those processes (suchas. the El N ifio phenomenon) thatare beyond our control.

mCynthia Rosenzweig is a re~search scientist at the NASAlGoddard Institute for SpaceStudies, where she is the leaderof the Climate Impacts Group.

extensive ecological domainsresulting from the human usurp~-tion of the greater fraction of theplanet's terrestrial surface. Somespecies, perhaps even entire bioticcommunities, may not survivechanges in climate if they are pre-vented from migration.Consequently, the role played bynatural ecosystems in environmen-tal processes (Oz-COz exchange,runoff, evaporation, groundwaterrecharge, nutrient recycling, andmore) may be jeopardized.

These and other threats to theintegrity of the biosphere require usnot merely to continue but indeedto intensify the effort to understandland surface-climate interactions interms of quantitative dynamicprocesses. That is the aim andchallenge of current modelingefforts, accompanied by continuousmonitoring and data collection. Inancient times, our forebearsattempted to affect climate byappeal (in prayer and sacrifice) tothe gods whom they believed tocontrol both storms and droughts.Today we are no less affected bythe same forces of nature. Thoseforces are in part conditioned byour actions, and are in part beyondour control. It is our task to pre-vent or mitigate those deleterious

Daniel Hillel, a senior researchscientist at Columbia Univer~sity, has served as a professorof soil physics, hydrology, andthe environmental sciences atleading universities in theUnited States and abroad, andhas been a consultant to theWorld Bank and the UnitedNations.

Rosenzweig and Hillel are theco,authors of Climate Changeand the Global Harvest: Po'tential Impacts of the Green,house Effect on Agriculture(Oxford University Press,1998).

G lobal climate change,

arguably the most pro-

found environmental issue of ourtime, makes it imperative for us tounderstand climate-biosphere inter-actions in quantitative terms. Theland surface is an active participantin, not merely a passive recipientof, global climate change. Defor-estation through burning releasescarbon dioxide (CO2) and methane(CH4) from existing biomass, whilecultivation of "new" land speedsthe decomposition of the initiallypresent soil organic matter. Theseprocesses contribute to the accu-mulation of greenhouse gases in theatmosphere. As global warmingoccurs, ecosystems will respond byalteration of species compositionand shifts in location. However,these changes are likely to bethwarted by the fragmentation andisland-like isolation of the once-

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