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Thursday, October 15, 2009

Andy Reisinger: Polar ice keeps melting – at a faster and faster rate

Polar ice keeps melting – at a faster and faster rate

by Andy Reisinger, degrees of change blog, October 12, 2009

A key rea­son for con­cern about cli­mate change is that it could lead to grad­ual melt­ing of the polar ice sheets in Green­land and Antarc­tica, result­ing in an inex­orable rise in global sea lev­els. A new set of high-precision mea­sure­ments (paper in press with the jour­nal ‘Geo­phys­i­cal Research Let­ters’) now con­firms that the ice sheets are not only melt­ing but that their melt rate has more than dou­bled over the past seven years. This sug­gests that sea lev­els could rise a lot faster than indi­cated in ear­lier stud­ies. Increases in sea level by more than 1m dur­ing the 21st cen­tury now have to be seri­ously considered.

Numer­ous stud­ies since the early 2000s indi­cated that sev­eral glac­i­ers that drain the polar ice sheets into the ocean have accel­er­ated as a result of local warm­ing. These obser­va­tions trig­gered con­cerns that the polar ice sheets could loose ice more quickly and sea level could rise more rapidly than cli­mate mod­els had sug­gested. But it was unclear how rep­re­sen­ta­tive these rather strik­ing changes were – was it just a tem­po­rary rip­ple run­ning through some glac­i­ers or were the entire ice sheets loos­ing ice at an increas­ing rate?

High-precision mea­sure­ments of the change in the total mass of the polar ice sheets, using a pair of orbit­ing satel­lites (the Grav­ity Recov­ery and Cli­mate Exper­i­ment), have now revealed that both ice sheets are indeed loos­ing mass, and at an accel­er­at­ing pace. The satel­lite data cap­ture the changes in total ice mass and hence give a truly com­pre­hen­sive pic­ture of changes in the polar ice sheets rather than only spot mea­sure­ments for par­tic­u­lar glaciers.

The assess­ment by the Inter­gov­ern­men­tal Panel on Cli­mate Change (IPCC) pub­lished in 2007 found that over the period 1993 to 2003, Green­land and Antarc­tica had both lost about 75 bil­lion tonnes of ice every year. If this loss of ice con­tin­ued at that rate for a cen­tury, this would result in sea level rise of 4.2cm. The new satel­lite data now reveal that by 2002/2003, the Green­land ice sheet was already loos­ing as much as 137 bil­lion tonnes of ice per year, and that this rate more than dou­bled again to 286 bil­lion tonnes of ice per year dur­ing 2007–2009. Mean­while, Antarctica’s ice loss had increased from 75 to 104 bil­lion tonnes per year dur­ing 2002–2006, and more than dou­bled to 246 bil­lion tonnes per year by 2006–2009. These most recent rates of ice loss would raise global sea lev­els by 15cm if they con­tin­ued at cur­rent rates over a century.

The trou­ble is of course that there is lit­tle rea­son to assume that the loss of ice will con­tinue only at cur­rent rates while the atmos­phere and oceans con­tinue to warm. It would be extremely sur­pris­ing if the observed melt­ing processes did not increase fur­ther, imply­ing a sig­nif­i­cant risk that sea lev­els would increase by more – and poten­tially a lot more – than the 0.6m that the IPCC had pro­jected for the 21st century.

Unfor­tu­nately, we still do not suf­fi­ciently under­stand all the processes that take place within and under­neath the polar glac­i­ers to under­stand which aspects of warm­ing are respon­si­ble for this accel­er­a­tion. Can­di­dates for the accel­er­ated ice loss include warm­ing of the atmos­phere, increases in local ocean tem­per­a­tures, and the cre­ation of melt­wa­ter on top of the ice sheet that is drain­ing into its base and acts as lubri­cant for the glac­ier flow. Until those processes are bet­ter under­stood, extrap­o­la­tions of the recent trends have to be treated with caution.

How­ever, these recent data are con­sis­tent with a raft of other obser­va­tions and model stud­ies that sug­gest that we now have to seri­ously con­sider that sea lev­els could rise by more than 1m dur­ing the 21st cen­tury as a result of cli­mate change. A report ear­lier this year by Pro­fes­sor Will Stef­fen at ANU for the Aus­tralian Depart­ment of Cli­mate Change advised that sea level rise could lie some­where between 0.5 m and per­haps up to 1.5 m by 2100. It’ll be a busy time for car­tog­ra­phers as we are in the process of re-designing the world’s coastlines.

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