Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 2total ice discharge rates. Recent observationshave shown that changes in dynamics can occurfar more rapidly than previously suspected,and we discuss causes for these in more detailin Section 4.Because there issummer meltingover ~50% ofGreenland already,the ice sheetis particularlysusceptible tocontinued warming.Figure 2.8. Correlation of the anomaly (relative tothe 1961–1990 average) in pentadal mean annual massbalance B (Kaser et al., 2006) with the correspondinganomaly in T, surface air temperature over land(CRUTEM3; Trenberth et al., 2007). The fitted line suggestsa proportionality dB/dT of –297±133 Gt a –1 K –1 forthe era of direct balance measurements (1961–2004).loss (we begin to run out of small-glacier ice).Against that certain development must be setthe probability that peripheral ice caps wouldalso begin to detach from the ice sheets, thus“replenishing” the inventory of small glaciers.Meier et al. (2007), by extrapolating the currentacceleration, estimated a total contribution tosea level of 240 ± 128 mm by 2100, implying anegative balance of 1,500 Gt a –1 in that year.These figures assume that the current accelerationof loss continues. Alternatively, if losscontinues at the current rate of 400 Gt a –1 , thetotal contribution is 104 ± 25 mm. In contrastRaper and Braithwaite (2006), who allowedfor glacier shrinkage, estimated only 97 mmby 2100. Part of the difference is due to theirexclusion of small glaciers in Greenland andAntarctica. If included, and if they were assumedto contribute at the same rate as theother glaciers, these would raise the Raper andBraithwaite (2006) estimate to 137 mm.3.4 Causes of ChangesPotential causes of the observed behavior ofthe ice sheets include changes in snowfall and/or surface melting, long-term responses to pastchanges in climate, and changes in the dynamics,particularly of outlet glaciers, that affect3.4.1 Changes in Snowfall andSurface MeltingRecent studies find no continentwide significanttrends in Antarctic accumulation over theinterval 1980–2004 (Monaghan et al., 2006; vanden Broeke et al., 2006), and surface meltinghas little effect on Antarctic mass balance.Modeling results indicate probable increasesin both snowfall and surface melting overGreenland as temperatures increase (Hannaet al., 2005; Box et al., 2006). Model resultspredict increasing snowfall in a warmingclimate in Antarctica and Greenland, but onlythe latter could be verified by independentmeasurements (Johannessen et al., 2005.) Anupdate of estimated Greenland Ice Sheet runoffand surface mass balance (i.e., snow accumulationminus runoff) results presented in Hannaet al. (2005) shows significantly increasedrunoff losses for 1998–2003 compared withthe 1961–90 climatologically “normal” period.But this was partly compensated by increasedprecipitation over the past few decades, so thatthe decline in surface mass balance betweenthe two periods was not statistically significant.Data from more recent years, extending to 2007,however, suggest a strong increase in the netloss from the surface mass balance. However,because there is summer melting over ~50% ofGreenland already (Steffen et al., 2004b), theice sheet is particularly susceptible to continuedwarming. Small changes in temperaturesubstantially increasing the zone of summermelting and a temperature increase by morethan 3 °C would probably result in irreversibleloss of the ice sheet (Gregory et al., 2004).Moreover, this estimate is based on imbalancebetween snowfall and melting and would beaccelerated by changing glacier dynamics ofthe type we are already observing.In addition to the effects of long-term trendsin accumulation/ablation rates, mass-balanceestimates are also affected by interannual variability.This increases uncertainties associatedwith measuring surface accumulation/ablationrates used for mass-budget calculations, and it54