Book 2.indb - US Climate Change Science Program

Abrupt Climate Change1. The record of past changes in ice volumeprovides important insight to theresponse of large ice sheets to climatechange.• Paleorecords demonstrate that there isa strong inverse relation between atmosphericcarbon dioxide (CO 2 ) and globalice volume. Sea level rise associated withthe melting of the ice sheets at the end ofthe last Ice Age ~20,000 years ago averaged10–20 millimeters per year (mm a –1 )with large “meltwater fluxes” exceedingSLR of 50 mm a –1 and lasting several centuries,clearly demonstrating the potentialfor ice sheets to cause rapid and large sealevel changes.2. Sea level rise from glaciers and ice sheetshas accelerated.• Observations demonstrate that it isextremely likely that the Greenland IceSheet is losing mass and that this has verylikely been accelerating since the mid-1990s. Greenland has been thickening athigh elevations because of the increasein snowfall that is consistent with highlatitudewarming, but this gain is morethan offset by an accelerating mass loss,with a large component from rapidlythinning and accelerating outlet glaciers.The balance between gains and lossesof mass decreased from near-zero in theearly 1990s to net losses of 100 gigatonsper year (Gt a –1 ) to more than 200 Gt a –1for the most recent observations in 2006.• The mass balance for Antarctica is a netloss of about 80 Gt a –1 in the mid-1990s,increasing to almost 130 Gt a –1 in themid-2000s. Observations show that whilesome higher elevation regions are thickening,substantial ice losses from WestAntarctica and the Antarctic Peninsulaare very likely caused by changing icedynamics.• The best estimate of the current (2007)mass balance of small glaciers and icecaps is a loss that is at least three timesgreater (380 to 400 Gt a –1 ) than the netloss that has been characteristic since themid-19th century.3. Recent observations of the ice sheets haveshown that changes in ice dynamics canoccur far more rapidly than previouslysuspected.• Recent observations show a high correlationbetween periods of heavy surfacemelting and increase in glacier velocity.A possible cause is rapid meltwaterdrainage to the base of the glacier, whereit enhances basal sliding. An increase inmeltwater production in a warmer climatewill likely have major consequences onice-flow rate and mass loss.• Recent rapid changes in marginal regionsof the Greenland and West Antarcticice sheets show mainly accelerationand thinning, with some glacier velocitiesincreasing more than twofold. Many ofthese glacier accelerations closely followedreduction or loss of their floatingextensions known as ice shelves. Significantchanges in ice-shelf thickness aremost readily caused by changes in basalmelting induced by oceanic warming. Theinteraction of warm waters with the peripheryof the large ice sheets representsone of the most significant possibilities forabrupt change in the climate system. Thelikely sensitive regions for future rapidchanges in ice volume by this process arethose where ice is grounded well belowsea level, such as the West Antarctic IceSheet or large outlet glaciers in Greenlandlike the Jakobshavn Isbræ that flowthrough a deep channel that extends farinland.• Although no ice-sheet model is currentlycapable of capturing the glacier speedupsin Antarctica or Greenland that have beenobserved over the last decade, includingthese processes in models will very likelyshow that IPCC AR4 projected sea levelrises for the end of the 21st century aretoo low.3