An Integrated Earth SystemOver the last two decades a newimperative has come to dominateenvironmental concerns.Witha rapidlyincreasingunderstandingofthe nature of Earth's life support system,a growing awareness hasemerged that human activities are exerting an ever acceleratinginfluence on aspects of Earth System functioning upon which thewelfare and the future of human societies depend.Thehuman-nature relationshipcomponent.Intermsofasportinganalogy,lifeisaplayer,not a spectator.Second,human activities areTHEINTERACTIONS BETWEEN ENVIRONMENTALChangEandnowsopervasiveandprofoundintheirconsequen-human societieshavealongandcomplexhistory,ces that they affect the Earth at a global scalespanning many millennia.They vary greatly throughin complex,interactive and acceleratingways;time and fromplacetoplace.Despitethese spatialhumans nowhavethe capacity to alter the Earthand temporal differences, in recent years a globalSystem in ways thatthreaten thevery processes andperspective has begun to emerge that forms thecomponents,both biotic and abiotic,upon whichframeworkforagrowingbodyofresearchwithinthehumansdependenvironmentalsciences.CrucialtotheemergenceofthisperspectivehasbeenthedawningawarenessofSystems thinkingand its application tothetwofundamental aspectsofthenatureoftheplanet.environment are not new.However,until veryThe first is that the Earth itself is a single system,recently,much of the understanding about howwithin which the biosphere is an active, essentialthe Earth operates was applied to only piecesIGBP SCIENCE No. 4An Integrated Earth System
4 IGBP SCIENCE No. 4 An Integrated Earth System Over the last two decades a new imperative has come to dominate environmental concerns. With a rapidly increasing understanding of the nature of Earth’s life support system, a growing awareness has emerged that human activities are exerting an ever accelerating infl uence on aspects of Earth System functioning upon which the welfare and the future of human societies depend. An Integrated Earth System The human-nature relationship THE INTERACTIONS BETWEEN ENVIRONMENTAL change and human societies have a long and complex history, spanning many millennia. They vary greatly through time and from place to place. Despite these spatial and temporal differences, in recent years a global perspective has begun to emerge that forms the framework for a growing body of research within the environmental sciences. Crucial to the emergence of this perspective has been the dawning awareness of two fundamental aspects of the nature of the planet. The fi rst is that the Earth itself is a single system, within which the biosphere is an active, essential component. In terms of a sporting analogy, life is a player, not a spectator. Second, human activities are now so pervasive and profound in their consequences that they affect the Earth at a global scale in complex, interactive and accelerating ways; humans now have the capacity to alter the Earth System in ways that threaten the very processes and components, both biotic and abiotic, upon which humans depend. Systems thinking and its application to the environment are not new. However, until very recently, much of the understanding about how the Earth operates was applied to only pieces
(subcomponents) of the Earth. What is really newThe Vostok ice core recordabout the understanding of the Earth System over4glacial cyclesrecorded intheVostokicecorethe last 10-15 years is a perspective that embracestheSystemasawbole.Severaldevelopmentshave蛋品服店led tothis significant change inperception:The view of Earth from a spaceship, a blue-8green sphere floating in blackness, triggersemotional feelings of a hometeeming with lifeset in a lifeless void, as well as more analytical#品perceptions of a materially limited and self-contained entity.GlobalobservationsystemsallowtheapplicationAge (kyr BP)ofconceptsthatwereonlypreviouslyapplicableJ.R.PetitetaL.Not399,429-36,1999atsubsystemlevel,orregional orlocal scales,toFigure The 420,000 year Vostok ice core record, showingthe Earth as a whole.the regular pattern of atmosphere CO, and CH,Globaldatabases allowglobal scalephenomenaconcentrationsandinferredtemperaturethroughfourtobe addressed with consistently acquired dataglacial-interglacial cycles. The upper and lower bounds ofthat have the potential for harmonization andall three variables are tightly constrained. These featurescomparison at aglobal scale.are typical of a self-regulating system.Dramaticadvances inthepowertoinfercharacAdapted from Petit et al. (1999) Nature 399,429-436 by the PAGESteristics of Earth System processes in thepast(PastGlobal Changes)International ProjectOffice.allow contemporary observationstobeviewedin a coherent time continuum.Enhancedcomputingpowermakespossiblenotonly essential data assimilation,but increasinglycarbon dioxide(CO,)and methane(CH),aresophisticatedmodels improveunderstanding oftightly coupled and show very similar patternsfunctionalinteractionsandsystemsensitivities.throughout the record.The main maxima and minima of temperatureScience has crossed the threshold of a profoundand atmospherictracegasconcentrationfollowshift in the perception of the human-environmenta regular pattern through time, each cyclerelationship,operating across humanity as a wholespanning approximately100,000 years.and at the scale of the Earth as a single systemThe range over which temperature and tracegasconcentrations varied is bounded at upper andTheEarthasasystemlowerlimits:thevaluesfallrecurrentlywithinthesame envelope through four cycles of the EarthThe fact that the Earth behaves as a single,interlinked,self-regulating system was put into dramatic focusSystem over the last half million years.in 1999withthe publicationof the420,000-yearThis systemic behaviour of Earth's environment isrecord from the Vostok ice core (Fig.1).These data,duetoa combination ofexternal forcing-primarilyarguablyamongthemostimportantproduced byvariations in solar radiation levels near the Earth'sthescientificcommunityinthe2ohcentury,providesurface-andalargeand complex arrayoffeedbacksa powerful temporal context and dramatic visualand forcings witbin Earth's environment itself.Theevidencefor an integrated planetaryenvironmentalinternal dynamics ofthe System,ratherthan externalsystem.forcings,undoubtedlykeep theplanethabitableTheVostok ice coredatagiveawealthofinsightsfor life.For example, without the thin layer ofintotheEarthSystem.Threestrikingcharacteristicsozoneintheupperatmosphere,muchmoreharmfuldemonstrate beyond any doubt that the Earth isultraviolet radiation would penetrate to the Earth'sa system,with properties and behaviour that aresurface;and without the thin layer of heat-absorbingcharacteristic of the System as a whole.greenhouse gases in the lower atmosphere,the.The temporal dynamicsof global temperatureand oftheglobalcarboncycle,as representedbyplanet's mean surface temperature would be about33°Clower than itisnow.the atmosphericconcentration ofthetracegases5IGBP SCIENCE No. 4An Integrated Earth System
An Integrated Earth System IGBP SCIENCE No. 4 5 Figure 1 The 420,000 year Vostok ice core record, showing the regular pattern of atmosphere CO2 and CH4 concentrations and inferred temperature through four glacial-interglacial cycles. The upper and lower bounds of all three variables are tightly constrained. These features are typical of a self-regulating system. Adapted from Petit et al. (1999) Nature 399, 429-436 by the PAGES (Past Global Changes) International Project Offi ce. The Vostok ice core record 4 glacial cycles recorded in the Vostok ice core J.R. Petit et al., Nature, 399, 429–36, 1999. Age (kyr BP) inferred temperature °C ppmv CO2 ppbv CH2 280 260 240 220 200 700 600 500 400 -8 -6 -4 -2 0 2 4 400 300 200 100 350 250 150 50 0 (subcomponents) of the Earth. What is really new about the understanding of the Earth System over the last 10 - 15 years is a perspective that embraces the System as a whole. Several developments have led to this signifi cant change in perception: • The view of Earth from a spaceship, a bluegreen sphere floating in blackness, triggers emotional feelings of a home teeming with life set in a lifeless void, as well as more analytical perceptions of a materially limited and selfcontained entity. • Global observation systems allow the application of concepts that were only previously applicable at subsystem level, or regional or local scales, to the Earth as a whole. • Global databases allow global scale phenomena to be addressed with consistently acquired data that have the potential for harmonization and comparison at a global scale. • Dramatic advances in the power to infer characteristics of Earth System processes in the past allow contemporary observations to be viewed in a coherent time continuum. • Enhanced computing power makes possible not only essential data assimilation, but increasingly sophisticated models improve understanding of functional interactions and system sensitivities. Science has crossed the threshold of a profound shift in the perception of the human-environment relationship, operating across humanity as a whole and at the scale of the Earth as a single system. The Earth as a system The fact that the Earth behaves as a single, interlinked, self-regulating system was put into dramatic focus in 1999 with the publication of the 420,000-year record from the Vostok ice core (Fig.1). These data, arguably among the most important produced by the scientifi c community in the 20th century, provide a powerful temporal context and dramatic visual evidence for an integrated planetary environmental system. The Vostok ice core data give a wealth of insights into the Earth System. Three striking characteristics demonstrate beyond any doubt that the Earth is a system, with properties and behaviour that are characteristic of the System as a whole. • The temporal dynamics of global temperature and of the global carbon cycle, as represented by the atmospheric concentration of the trace gases carbon dioxide (CO2 ) and methane (CH4 ), are tightly coupled and show very similar patterns throughout the record. • The main maxima and minima of temperature and atmospheric trace gas concentration follow a regular pattern through time, each cycle spanning approximately100,000 years. • The range over which temperature and trace gas concentrations varied is bounded at upper and lower limits; the values fall recurrently within the same envelope through four cycles of the Earth System over the last half million years. This systemic behaviour of Earth’s environment is due to a combination of external forcing – primarily variations in solar radiation levels near the Earth’s surface – and a large and complex array of feedbacks and forcings within Earth’s environment itself. The internal dynamics of the System, rather than external forcings, undoubtedly keep the planet habitable for life. For example, without the thin layer of ozone in the upper atmosphere, much more harmful ultraviolet radiation would penetrate to the Earth’s surface; and without the thin layer of heat-absorbing greenhouse gases in the lower atmosphere, the planet’s mean surface temperature would be about 33°C lower than it is now
The recent human influence on the carbon cycle360b)350400133403308Human perturbation350(d) o asone320W31019601970198019902000300250200150-450-400-350-300-250-2000-150-100-50050Thousands of yearsFigureAtmospheric Co,concentration from the Vostok ice core record with therecent human perturbation superimposed.Theinsetshows the observed contemporary increase in atmospheric co, concentration from the Mauna Loa (Hawai) Observatory.Sources: Petit et al. (1999) Nature 399, 429-436 and National Oceanic and Atmospheric Administration (NOAA), USAGlobal Changestrong.It is increasingly clear that the Earth Systemis being subjected to an ever-increasing diversity ofOverthepastfewdecades,evidencehasmountednew planetary-scale forces that originate in humanthat planetary-scale changes are occurring rapidly.activities,ranging from the artificial fixation ofThese are, in turn, changing the patterns of forcingsnitrogen and the emission of greenhouse gasesandfeedbacksthatcharacterisetheinternaldynamicsto the conversion and fragmentation of naturalof theEarthSystem (Figs.2.3).Keyindicators,suchvegetation and the loss of biological species.It isas the concentrations of cO,inthe atmosphere,these activities and others like them that give rise toare changing dramatically,and in many cases thethe phenomenon of global cbange.linkages of these changes to human activities areBox 1:Global change is more than climate change1000 years of climate changeTheterm Earth System refers to the suite of interactingphysical,chemical,biological and human processesthatNorthern Hemispheretransport and transform materials and energy and thusAverage Surface Temperatureprovide the conditions necessary for life on the planet.Climate refers to the aggregation of components ofweather-precipitation,temperature,cloudiness,forexample-buttheclimatesystemincludesprocessesinvolving ocean,land and seaiceinaddition to theatmosphere.TheEarthSystemencompassestheclimatesystem,and many changes in Earth Systemfunctioning200n1600800directlyinvolvechangesin climate.However,thetion (AD 1000-1980)calibration period (AD 1902-1980) mew data (AD 1902-1998)Earth System includes othercomponents and processes,reconstruction(40yearsmooothed)..lineartrend (AD1000-1850)biophysical and human,important for its functioning.Mann et al.,1999:Geo. Res. Let.,26, 6, 759Some EarthSystemchanges,naturalorhuman-driven,can have significant consequences without involving anychanges in climate.Global change should not be confusedFigure Mean annual temperature variations over the northernwith climate chanqe,itis significantlymore.hemisphere for the last 1000 years.IGBP SCIENCE No.4An Integrated EarthSystem
6 IGBP SCIENCE No. 4 An Integrated Earth System Global Change Over the past few decades, evidence has mounted that planetary-scale changes are occurring rapidly. These are, in turn, changing the patterns of forcings and feedbacks that characterise the internal dynamics of the Earth System (Figs. 2,3). Key indicators, such as the concentrations of CO2 in the atmosphere, are changing dramatically, and in many cases the linkages of these changes to human activities are Figure 2 Atmospheric CO2 concentration from the Vostok ice core record with the recent human perturbation superimposed. The inset shows the observed contemporary increase in atmospheric CO2 concentration from the Mauna Loa (Hawaii) Observatory. Sources: Petit et al. (1999) Nature 399, 429-436 and National Oceanic and Atmospheric Administration (NOAA), USA The recent human infl uence on the carbon cycle -450 -400 -350 -300 -250 -2000 -150 -100 -50 0 50 400 350 300 250 200 150 Thousands of years atmospheric C02 (ppmV) Human perturbation 1960 1970 1980 1990 2000 360 310 320 330 340 350 b) Atmospheric CO2 Concentration (ppm) Mean annual temperature variations over the northern hemisphere for the last 1000 years. 1000 years of climate change Northern Hemisphere Average Surface Temperature 1000 1200 1400 1600 1800 2000 Year Temperature Anomaly (°C) 1.0 0.5 0.0 -0.5 -1.0 1998 Mann et al. 1999: Geo. Res. Let.,26, 6, 759 reconstruction (AD 1000-1980) raw data (AD 1902-1998) calibration period (AD 1902-1980) mean reconstruction (40 year smooothed) linear trend (AD 1000-1850) Figure 3 The term Earth System refers to the suite of interacting physical, chemical, biological and human processes that transport and transform materials and energy and thus provide the conditions necessary for life on the planet. Climate refers to the aggregation of components of weather - precipitation, temperature, cloudiness, for example - but the climate system includes processes involving ocean, land and sea ice in addition to the atmosphere. The Earth System encompasses the climate system, and many changes in Earth System functioning directly involve changes in climate. However, the Earth System includes other components and processes, biophysical and human, important for its functioning. Some Earth System changes, natural or human-driven, can have signifi cant consequences without involving any changes in climate. Global change should not be confused with climate change; it is signifi cantly more. Box 1: Global change is more than climate change strong. It is increasingly clear that the Earth System is being subjected to an ever-increasing diversity of new planetary-scale forces that originate in human activities, ranging from the artificial fixation of nitrogen and the emission of greenhouse gases to the conversion and fragmentation of natural vegetation and the loss of biological species. It is these activities and others like them that give rise to the phenomenon of global change
Planetary MachineryAn understanding of the nature of the Earth System before human activitiesbecameanimportantfactorprovidesthebackdrop of naturalcyclesandof processes,patterns and variabilityagainst whichcurrentandfutureanthropogenic changes must be evaluated.Over the past decade research hasidentified manykey characteristics of the ways in which the Earth Systemoperates to keep the global environment within limits suitable for life.of the carbon fixed by phytoplankton in the upperRoleofthebiospherelayers sinks to the interior, where it is stored awayBIOLOGICALPROCESSESINTERACTSTRONGLYWithphVSiCalfromcontactwiththeatmosphereforhundredsonandchemicalprocessatetheenvironmentthousandsofyears.Thisbiologicalpump,alongwiththatkeepsEarthhabitablefor life.Themore thatnicalconstraintsonthesolubilityofthefunctioningoftheEarthSysteninedinphysico-chemcO,controlthepatternofCO,exchangebetweendetail,theeroleplavedaancnondtheatmosphere.Intriguingly,thebylifeitsHofthephytoplanktonspeciesinvolvedintheexamDle10l09antimayhold a keyto therate of andDiologito thecarbonstorage (Fig.4)otentialfotlerrestrialbiotaarealsoanimportantcomponenttiminEarthSystemfunctioninginanumberofways.Forbyphytoplanin.theexample,thetypeofvegetationpresentonthelandsurfacettherebsurface influences the amount of watertranspiredCO,todissolefromtheatm24 Planetary MachineryIGBP SCIENCE No. 4
Planetary Machinery IGBP SCIENCE No. 4 7 Role of the biosphere BIOLOGICAL PROCESSES INTERACT STRONGLY with physical and chemical processes to create the environment that keeps Earth habitable for life.The more that the functioning of the Earth System is examined in detail, the greater is the realisation of the role played by life itself in helping to control the System. For example, biological processes contribute signifi cantly to the absorption of atmospheric CO2 by the oceans, which in turn controls atmospheric CO2 concentration on long time scales. Photosynthesis by phytoplankton reduces the amount of CO2 in the surface layer of the ocean, thereby allowing more CO2 to dissolve from the atmosphere. About 25% of the carbon fi xed by phytoplankton in the upper layers sinks to the interior, where it is stored away from contact with the atmosphere for hundreds or thousands of years. This biological pump, along with physico-chemical constraints on the solubility of CO2 , control the pattern of CO2 exchange between the oceans and the atmosphere. Intriguingly, the nature of the phytoplankton species involved in the biological pump may hold a key to the rate of and potential for carbon storage (Fig. 4). Terrestrial biota are also an important component in Earth System functioning in a number of ways. For example, the type of vegetation present on the land surface infl uences the amount of water transpired An understanding of the nature of the Earth System before human activities became an important factor provides the backdrop of natural cycles and of processes, patterns and variability against which current and future anthropogenic changes must be evaluated. Over the past decade research has identifi ed many key characteristics of the ways in which the Earth System operates to keep the global environment within limits suitable for life. Planetary Machinery
continuously on all time-scales (Fig.5).A carefulWho's there mattersexamination of evidencefrom thepast shows that:variability is reflected not onlyintemperature-variabilityinthehydrologicalcvcle,whichisoften ofmuchgreaterimportancetohumanpopulations, has been quite extreme on alltime-scalesinthepast,no single variable orregion truly reflectsglobalvariability-globalmean conditions maskimmensevariationsinregionalresponses,during thelate Holocene.when the naturalforcings and boundary conditions were similartothoseoperatingtoday,thereisstrongevidencethattherangeofvariabilitysignificantlyexceededthat captured by instrumental records.RelianceFigureLife in the oceans plays an important role in maintainingon theveryshortperiod ofinstrumental recordsgeochemical balance in the Earth System and the fate ofthe carbon that is fixed by the ocean's phytoplanktongives a false sense of the true variability of theisverymuchafunctionofthesizeandtaxonomyofEarth System.the species present. For example, in addition to fixingVariability in theclimate systemcarbonviaphotosynthesis,onegroupofphytoplankton,thecoccolithophorids,suchas Emiltiania huxleyi (shownV11.0above) produces calcium carbonate platelets (liths).Each lith is only about 2.5 μm in lengthbut many0.8are produced every year. It is estimated that bloomsOON0.6of E.huxleyi cover about 1.4millionkm2of theaz.1Poceaneveryyear.Thus,overgeologicaltime,tremendousAntarctica1Oaccumulations of carbon fixed by coccolithophorids50-100150develop.The white cliffs of Dover are, for example,hofYearsRa5largely comprised of platelets from coccolithophorids.0.3Source: K.Richardson.P0.20.15back to the atmosphere and the absorption oreino.reflection of the sun's radiation.The vegetation's20rooting patterns and activity are also importantireenloncontrollersofbothcarbonandwaterstorageandof101520253035404550fluxes between the land and the atmosphere.The02biological diversity of terrestrial ecosystems affects0.16Ethe magnitude ofkey ecosystem processes such0.120.08as productivity,and plays a role in the long-termGreenland0.04stability of ecosystem functioning in the face of a12-1416changingenvironment.TemporalvariabilityVariability and change are realities of the EarthB-year changeSystem,and static,so-called equilibrium,conditions0.00are unlikely to be a part of the System on almostFigure Variability in the climate system.Ice accumulationany time scale.The notion that unusally stablerate history at four different time scales. Data fromconditions prevailed over the past several millenia,Antarctica(toppanel)andGreenland(bottomthreewhenhumancivilisationsdeveloped,andrepresentpanels).normal conditions isfalse.The record showsthatAdapted from Jacobson et al. (eds.)(2000) Earth System Science,the functioning of the Earth System has variedAcademic Press, p. 479.8IGBP SCIENCE No. 4Planetary Machinery
8 IGBP SCIENCE No. 4 Planetary Machinery Figure 4 Life in the oceans plays an important role in maintaining geochemical balance in the Earth System and the fate of the carbon that is fi xed by the ocean’s phytoplankton is very much a function of the size and taxonomy of the species present. For example, in addition to fi xing carbon via photosynthesis, one group of phytoplankton, the coccolithophorids, such as Emiliania huxleyi (shown above) produces calcium carbonate platelets (liths). Each lith is only about 2.5 µm in length but many are produced every year. It is estimated that blooms of E. huxleyi cover about 1.4 million km2 of the ocean every year. Thus, over geological time, tremendous accumulations of carbon fi xed by coccolithophorids develop. The white cliffs of Dover are, for example, largely comprised of platelets from coccolithophorids. Source: K. Richardson. Who’s there matters back to the atmosphere and the absorption or refl ection of the sun’s radiation. The vegetation’s rooting patterns and activity are also important controllers of both carbon and water storage and of fl uxes between the land and the atmosphere. The biological diversity of terrestrial ecosystems affects the magnitude of key ecosystem processes such as productivity, and plays a role in the long-term stability of ecosystem functioning in the face of a changing environment. Temporal variability Variability and change are realities of the Earth System, and static, so-called equilibrium, conditions are unlikely to be a part of the System on almost any time scale. The notion that unusally stable conditions prevailed over the past several millenia, when human civilisations developed, and represent normal conditions is false. The record shows that the functioning of the Earth System has varied continuously on all time-scales (Fig. 5). A careful examination of evidence from the past shows that: • variability is refl ected not only in temperature – variability in the hydrological cycle, which is often of much greater importance to human populations, has been quite extreme on all time-scales in the past, • no single variable or region truly refl ects global variability – global mean conditions mask immense variations in regional responses, • during the late Holocene, when the natural forcings and boundary conditions were similar to those operating today, there is strong evidence that the range of variability signifi cantly exceeded that captured by instrumental records. Reliance on the very short period of instrumental records gives a false sense of the true variability of the Earth System. Figure 5 Variability in the climate system. Ice accumulation rate history at four different time scales. Data from Antarctica (top panel) and Greenland (bottom three panels). Adapted from Jacobson et al. (eds.) (2000) Earth System Science, Academic Press, p. 479. Variability in the climate system Thousands of Years Before Present 0 150 50 100 Normalized Accumulation Rate 1.2 0.2 0.4 0.6 0.8 1.0 Antarctica Younger Dryas 0 50 5 40 10 45 15 20 25 35 30 Accumulation Rate (m/yr) 0.3 0.05 0 0.1 0.15 0.2 0.08 0.04 0.12 0.16 0.2 0.25 Greenland Greenland Greenland 0.1 0.0 0.2 0.3 10 16 12 14 3-year change