Linkagesand connectivitiesmajor pathway by which carbon, phosphorus, andother elements cycle on geologic timescales.PriorOne ofthe mostubiquitous features ofthe planetarymachineryisthesuiteoflinkagesthatbindprocessesto significant human disturbance,nutrient cycling interrestrialecosystemswasusuallytightlyconstrainedin oneregionto consequences inothersthousandswithintheecosystemitselfwithmuchrecycling;of kilometers away.Atmosphericand oceanicthere was little leakage to river networks.Leakagecirculation play a major role in the transport ofoccurred only when terrestrial nutrient cycles wereheat from the tropics to the poles.The horizontalbriefly perturbed by rare events or sequences ofmovement of water in rivers is another importantrare events.transportprocess thatcouples seemingly isolatedpartsoftheplanet.Abruptchangesand criticalthresholdsThe atmospheric transport of materials, oftenBecause human societies have developed andconsidered only in thecontext of air pollution,alsoflourished over a very short period of timefrom anplays a role in natural biogeochemical cycles by lin-Earth Systemperspective,and becausetheperiodkinglandandoceanprocesses acrosslongdistances.ofinstrumentalobservationandmodernscientificIn southern Africa, for example,recirculating airenquiryiseven shorter,anarrowviewof theEarth'sflowspickupdustfromaridlandsandsmokefromenvironmenthasdeveloped.Thenotionthatasingleindustrial areas and eventuallvtransport themstable equilibrium is the natural state of Earth'sover the southernIndianOceantowardsAustralia.environment is notsupported byobservations ofPortions of these plumes subside regularly overpastglobal changes.The behaviour of the Earthcertain patches in the ocean, depositing iron-ladenSystem is typified not by stable equilibria, but bydust on the sea surface.The iron,a micro-nutrientfor phytoplankton,probablyacts as an intermittentOcean Co, source and sink areasfertiliser triggering planktonic blooms and accoun-ting for the observed‘hot spots'of carbon uptakeinprecisely theseareas of the southern IndianOcean(Fig.6)Horizontal transport of materials also occurs viariver networks,whichprovidecorridorsthatlinkmountainsand coastal areas,land andwaterecosys-tems,acrosslandscapesregional tosubcontinentalin scaleRiverinefluxes havebeenaNet Flux (10ugranh5aIron deposition on the south Indian Ocean Co,sinks20°530°S40°S........50°SNCAR..CSIROCarbon sinkTrajectorymodelareas20°40°60°80°100°120°EFigureTop panel:Mean annual exchange of co,across the sea surface.Blueand purple colours denoteregions in which the oceantakes up large amounts of co, Bottom panel: The trajectory of iron-laden aerosols from southern Africa over the southernIndianOceanandtheirdepositiononareasofobservedcarbonuptakeSources: Takahashi et al. (1999) Proc. 24 Int. Symp. on co, in Oceans, pp. 9-15. Piketh et al. (2000) Sth. Afr. J. Sci. 96, 244-246.9Planetary MachineryIGBP SCIENCE No.4
Planetary Machinery IGBP SCIENCE No. 4 9 Net Flux (1012 grams C y-1 in each 4o x 5o area) 0° 20° 40° 60° 80° 100° 120° 140° 160° 180° 160° 140° 120° 100° 80° 60° 40° 20° 0° 0 °20° 40° 60° 80° 100° 120° 140° 160° 180° 160° 140° 120° 100° 80° 60° 40° 20° 0° -10 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 10 80° 60° 70° 40° 50° 20° 30° 0° 10° 60° 70° 40° 50° 20° 30° 10° 80° 60° 70° 40° 50° 20° 30° 0° 10° 60° 70° 40° 50° 20° 30° 10° Figure 2 Ocean CO2 source and sink areas major pathway by which carbon, phosphorus, and other elements cycle on geologic timescales. Prior to signifi cant human disturbance, nutrient cycling in terrestrial ecosystems was usually tightly constrained within the ecosystem itself with much recycling; there was little leakage to river networks. Leakage occurred only when terrestrial nutrient cycles were briefl y perturbed by rare events or sequences of rare events. Abrupt changes and critical thresholds Because human societies have developed and fl ourished over a very short period of time from an Earth System perspective, and because the period of instrumental observation and modern scientifi c enquiry is even shorter, a narrow view of the Earth’s environment has developed. The notion that a single stable equilibrium is the natural state of Earth’s environment is not supported by observations of past global changes. The behaviour of the Earth System is typifi ed not by stable equilibria, but by Iron deposition on the south Indian Ocean CO2 sinks Figure 6 Top panel: Mean annual exchange of CO2 across the sea surface. Blue and purple colours denote regions in which the ocean takes up large amounts of CO2 . Bottom panel: The trajectory of iron-laden aerosols from southern Africa over the southern Indian Ocean and their deposition on areas of observed carbon uptake. Sources: Takahashi et al. (1999) Proc. 2nd Intl. Symp. on CO2 in Oceans, pp. 9-15. Piketh et al. (2000) Sth. Afr. J. Sci. 96, 244-246. 20° 40° 60° 80° 100° 120°E 50°S 40°S 30°S 20°S Trajectory NCAR model CSIRO Carbon sink areas Linkages and connectivities One of the most ubiquitous features of the planetary machinery is the suite of linkages that bind processes in one region to consequences in others thousands of kilometers away. Atmospheric and oceanic circulation play a major role in the transport of heat from the tropics to the poles. The horizontal movement of water in rivers is another important transport process that couples seemingly isolated parts of the planet. The atmospheric transport of materials, often considered only in the context of air pollution, also plays a role in natural biogeochemical cycles by linking land and ocean processes across long distances. In southern Africa, for example, recirculating air fl ows pick up dust from arid lands and smoke from industrial areas and eventually transport them over the southern Indian Ocean towards Australia. Portions of these plumes subside regularly over certain patches in the ocean, depositing iron-laden dust on the sea surface. The iron, a micro-nutrient for phytoplankton, probably acts as an intermittent fertiliser triggering planktonic blooms and accounting for the observed ‘hot spots’ of carbon uptake in precisely these areas of the southern Indian Ocean (Fig. 6). Horizontal transport of materials also occurs via river networks, which provide corridors that link mountains and coastal areas, land and water ecosystems, across landscapes regional to subcontinental in scale. Riverine fl uxes have been a
Box 2:Thegreen SaharaA7460J/M450440-1:B1.0 io0.60.40.9C0.0(e/w0/5)10658CTerr.%m-658C Terr.Flux40003000200010008000700060005000000Age (yrBP)The dramatic desertification of the Sahelian region inrainfall (part c),and the region became the present-day desert.prehistoric times demonstrates several important features ofModel predictions of theresulting increase in wind erosion andEarthSystemfunctioning.About6000yearsagotheclimatedepositionofsand offtheWestAfricancoastagreeremarkablyin the Sahel-Sahara region was much more humid than today,wellwithobservations (partd).The model simulations suggest that it was an interplay ofwithvegetationcoverresemblingthatofamodern-dayAfricansavanna. About 5500 years ago, an abrupt change in theatmosphere,ocean,vegetation and sea ice changes inwidelyregional climate occurred,triggering a rapid conversion of theseparated parts of the planet that amplified the original orbitalSahara intoits present desert conditionforcing.Theabrupt change fromsavannato desert in NorthTheultimate causewas a small,subtle change in Earth'sAfrica demonstrates that (i)abrupt changes can occur whenorbit,leading toa small changeinthedistribution ofthresholdsare crossed.(ii)thebiosphereplavs a critical rolesolar radiation on Earth's surface (part a of figure).Modelin Earth System functioning,and (ii)teleconnections areansimulations suggest that this small change nudged the Earthessential feature of the planetary machinery.Systemacrossathresholdthattriggeredaseriesof biophysicalPhotos:D.Parsons.FigurefromClaussen.etal.(1999)24,2037-2040andfeedbacks that led, in turn, to a drying climate (part b).deMenocal etal. (2000)Quat.Sci.Rev.19,347-361.Vegetation changedmoresharplyinresponsetochanqingstrong nonlinearities, where relatively small changesespecially in lower latitudes,in a forcing function can push the system acrossthe changes demonstrate widespread spatiala threshold and lead to abrupt changes in keycoherence,but are not always globally synchro-system functions (Box 2).More specifically, thenous,and complex inter-hemispheric leads and lags occurpalaeo-record showsthat:major switches inEarth Systemfunctioningthat require feedback mechanisms for amplifyingoccurred onmuch shortertimescalesthan theand propagating changes in both space andtime.glacial/interglacialcycles,therecorded changes were often rapidandIn terms ofthe present and future,these observationsofhigh amplitude;in some cases temperatureover large regions changed by up to 10°C in aare especially important. Tbey raise tbe possibilitydecadeor less,tbat antbropogenically induced global cbangealthough major, abrupttransitions,reflectingcould trigger sudden, dramatic switcbes inreorganization of the Earth System, are mostclimateand otbercomponentsoftheEartbSystemevidentinpredominantly cold,glacial periods,comparable to tbose tbat bave occurred in thethey are not absent in the last 12,000 years,past.10IGBP SCIENCE No.4Planetary Machinery
10 IGBP SCIENCE No.4 Planetary Machinery strong nonlinearities, where relatively small changes in a forcing function can push the system across a threshold and lead to abrupt changes in key system functions (Box 2). More specifically, the palaeo-record shows that: • major switches in Earth System functioning occurred on much shorter timescales than the glacial/interglacial cycles, • the recorded changes were often rapid and of high amplitude; in some cases temperature over large regions changed by up to 10°C in a decade or less, • although major, abrupt transitions, reflecting reorganization of the Earth System, are most evident in predominantly cold, glacial periods, they are not absent in the last 12,000 years, especially in lower latitudes, • the changes demonstrate widespread spatial coherence, but are not always globally synchronous, and • complex inter-hemispheric leads and lags occur that require feedback mechanisms for amplifying and propagating changes in both space and time. In terms of the present and future, these observations are especially important. They raise the possibility that anthropogenically induced global change could trigger sudden, dramatic switches in climate and other components of the Earth System comparable to those that have occurred in the past. Box 2: The green Sahara The dramatic desertifi cation of the Sahelian region in prehistoric times demonstrates several important features of Earth System functioning. About 6000 years ago the climate in the Sahel-Sahara region was much more humid than today, with vegetation cover resembling that of a modern-day African savanna. About 5500 years ago, an abrupt change in the regional climate occurred, triggering a rapid conversion of the Sahara into its present desert condition. The ultimate cause was a small, subtle change in Earth’s orbit, leading to a small change in the distribution of solar radiation on Earth’s surface (part a of fi gure). Model simulations suggest that this small change nudged the Earth System across a threshold that triggered a series of biophysical feedbacks that led, in turn, to a drying climate (part b). Vegetation changed more sharply in response to changing rainfall (part c), and the region became the present-day desert. Model predictions of the resulting increase in wind erosion and deposition of sand off the West African coast agree remarkably well with observations (part d). The model simulations suggest that it was an interplay of atmosphere, ocean, vegetation and sea ice changes in widely separated parts of the planet that amplifi ed the original orbital forcing. The abrupt change from savanna to desert in North Africa demonstrates that (i) abrupt changes can occur when thresholds are crossed, (ii) the biosphere plays a critical role in Earth System functioning, and (iii) teleconnections are an essential feature of the planetary machinery. Photos: D. Parsons. Figure from Claussen et al. (1999) 24, 2037-2040 and deMenocal et al. (2000) Quat. Sci. Rev. 19, 347-361. Terrigenous (%) fraction mm/d W/m2 A B C D (g/cm2/ka) 460 1.0 0.8 0.4 1.2 0.6 440 470 450 0.6 0.0 0.9 0.3 40 60 50 8 12 6 10 9000 Age (yr BP) 8000 7000 6000 5000 4000 3000 2000 1000 0 658C Terr. Flux 658C Terr. %