Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 1After carbon dioxide(CO 2 ), methane(CH 4 ) is the nextmost importantgreenhouse gas thathumans directlyinfluence.to seasonally ice-free Arctic conditions. Inone climate model simulation, a transitionfrom conditions similar to pre-2007 levels toa near-ice-free September extent occurred in adecade (Holland et al., 2006). Increasing oceanheat transport was implicated in this simulatedrapid ice loss, which ultimately resulted fromthe interaction of large, intrinsic variability andanthropogenically forced change. It is notablethat climate models are generally conservativein the modeled rate of Arctic ice loss ascompared to observations (Stroeve et al., 2007;Figure 1-3), suggesting that future ice retreatcould occur even more abruptly than simulated.This nonlinear response occurs because seaice has a strong inherent threshold in that itsexistence depends on the freezing temperatureof seawater. Additionally, strong positive feedbacksassociated with sea ice act to accelerate itschange. The most notable of these is the positivesurface albedo feedback in which changes in icecover and surface properties modify the surfacereflection of solar radiation. For example, in awarming climate, reductions in ice cover exposethe dark underlying ocean, allowing more solarradiation to be absorbed. This enhances thewarming and leads to further ice melt. Becausethe AMOC interacts with the circulation of theArctic Ocean at its northern boundary, futurechanges in the AMOC and its attendant heattransport thus have the potential to furtherinfluence the future of sea ice.SummaryOur analysis indicates that it is very likely thatthe strength of the AMOC will decrease overthe course of the 21st century. In models wherethe AMOC weakens, warming still occursdownstream over Europe due to the radiativeforcing associated with increasing greenhousegases. No model under plausible estimates offuture forcing exhibits an abrupt collapse of theMOC during the 21st century, even accountingfor estimates of accelerated Greenland ice sheetmelting. We conclude that it is very unlikely thatthe AMOC will abruptly weaken or collapseduring the course of the 21st century. Basedon available model simulations and sensitivityanalyses, estimates of maximum Greenland icesheet melting rates, and our understanding ofmechanisms of abrupt climate change from thepaleoclimatic record, we further conclude that itis unlikely that the AMOC will collapse beyondthe end of the 21st century as a consequence ofglobal warming, although the possibility cannotbe entirely excluded.The above conclusions depend upon our understandingof the climate system and on theability of current models to simulate the climatesystem. An abrupt collapse of the AMOC in the21st century would require either a sensitivityof the AMOC to forcing that is far greaterthan current models suggest or a forcing thatgreatly exceeds even the most aggressive ofcurrent projections (such as extremely rapidmelting of the Greenland ice sheet). While weview these as very unlikely, we cannot excludeeither possibility. Further, even if a collapse ofthe AMOC is very unlikely, the large climaticimpacts of such an event, coupled with thesignificant climate impacts that even decadalscale AMOC fluctuations induce, argue for astrong research effort to develop the observations,understanding, and models required topredict more confidently the future evolutionof the AMOC.7. Abrupt Change inAtmospheric MethaneConcentrationAfter carbon dioxide (CO 2 ), methane (CH 4 ) isthe next most important greenhouse gas thathumans directly influence. Methane is a potentgreenhouse gas because it strongly absorbsterrestrial infrared (IR) radiation. Methane’satmospheric abundance has more than doubledsince the start of the Industrial Revolution(Etheridge et al., 1998; MacFarling Meure etal., 2006), amounting to a total contribution toradiative forcing over this time of ~0.7 watts24