Book 2.indb - US Climate Change Science Program

Abrupt Climate Change1. Summary1.1 PaleorecordThe most recent time with no appreciable iceon the globe was 35 million years ago duringa period when the atmospheric carbon dioxide(CO 2 ) was 1,250 ± 250 parts per million byvolume (ppmV) and a sea level 73 meters (m)higher than today. During the last interglacialperiod (~120 thousand years ago, ka) withsimilar CO 2 levels to pre-industrial valuesand arctic summer temperatures warmer thantoday, sea level was 4–6 m above present. Mostof that sea level rise (SLR) is believed to haveoriginated from the Greenland Ice Sheet, but therate of SLR is unknown. Sea level rise averaged10–20 millimeters per year (mm a –1 ) during thelast two deglaciation periods (130–116 ka and21–14 ka, respectively), with large “meltwaterfluxes” with rates of SLR exceeding 50 mm a –1lasting several centuries (Fairbanks, 1989;Rohling et al., 2008). Each of these meltwaterfluxes added 1.5–3 times the volume of thecurrent Greenland Ice Sheet (7 m) to the oceans.The cause, ice-sheet source, and mechanism ofthe meltwater fluxes are not well understood,yet the rapid loss of ice must have had an effecton ocean circulation resulting in a forcing ofthe global climate.1.2 Ice SheetsRapid changes in ice-sheet mass have surelycontributed to abrupt changes in climate andsea level in the past. The mass balance lossof the Greenland Ice Sheet increased in thelate 1990s to 100 gigatons per year (Gt a –1 ) oreven more than 200 Gt a –1 for the most recentobservations in 2006. It is extremely likely thatthe Greenland Ice Sheet islosing mass and very likelyon an accelerated path sincethe mid-1990s. The massbalance for Antarctica is anet loss of about 80 Gt a –1in the mid-1990s, increasingto almost 130 Gt a –1 inthe mid-2000s. The largestlosses are concentratedalong the Amundsen andBellinghausen sectors ofWest Antarctica and thenorthern tip of the AntarcticPeninsula. The potentiallysensitive regions for rapid changes in ice volumeare those with ice masses grounded below sealevel such as the West Antarctic Ice Sheet, with7 m sea level equivalent (SLE), or large glaciersin Greenland like the Jakobshavn, also knownas Jakobshavn Isbræ and Sermeq Kujalleq (inGreenlandic), with an over-deepened channelreaching far inland. There are large massbudgetuncertainties from errors in both snowaccumulation and calculated ice losses forAntarctica (~ ± 160 Gt a –1 ) and for Greenland(~ ± 35 Gt a –1 ). Mass-budget uncertainties fromaircraft or satellite observations (i.e., radar altimeter,laser altimeter, gravity measurements)are similar in magnitude. Most climate modelssuggest that climate warming would resultin increased melting from coastal regions inGreenland and an overall increase in snowfall.However, they do not predict the substantialacceleration of some outlet glaciers that we areobserving. This results from a fundamentalweakness in the existing models, which areincapable of realistically simulating the outletglaciers that discharge ice into the ocean.Observations show that Greenland is thickeningat high elevations, because of the increase insnowfall, which was predicted, but that thisgain is more than offset by an accelerating massloss, with a large component from rapidly thinningand accelerating outlet glaciers. Althoughthere is no evidence for increasing snowfallover Antarctica, observations show that somehigher elevation regions are also thickening,likely as a result of high interannual variabilityin snowfall. There is little surface melting inAntarctica, and the substantial ice losses fromWest Antarctica and the Antarctic Peninsula areThe potentiallysensitive regions forrapid changes in icevolume are thosewith ice massesgrounded below sealevel such as theWest Antarctic IceSheet, with 7 m sealevel equivalent.31