Green­land now a ma­jor driver of ris­ing seas: study

Tehran Times - - SCIENCE - (Source: phys.org)

Ocean lev­els rose 50 per­cent faster in 2014 than in 1993, with melt­wa­ter from the Green­land ice sheet now sup­ply­ing 25 per­cent of to­tal sea level in­crease com­pared with just five per­cent 20 years ear­lier, re­searchers re­ported Mon­day.

The find­ings add to grow­ing con­cern among sci­en­tists that the global wa­ter­mark is climb­ing more rapidly than forecast only a few years ago, with po­ten­tially dev­as­tat­ing con­se­quences.

Hundreds of mil­lions of peo­ple around the world live in low-ly­ing deltas that are vul­ner­a­ble, es­pe­cially when ris­ing seas are com­bined with land sink­ing due to de­pleted wa­ter ta­bles, or a lack of ground-form­ing silt held back by dams.

Con­ser­va­tive pro­jec­tion

Ma­jor coastal cities are also threat­ened, while some small is­land states are al­ready lay­ing plans for the day their drown­ing na­tions will no longer be liv­able.

“This re­sult is im­por­tant be­cause the In­ter­gov­ern­men­tal Panel on Cli­mate Change (IPCC)” — the UN sci­ence ad­vi­sory body — “makes a very con­ser­va­tive pro­jec­tion of to­tal sea level rise by the end of the century,” at 60 to 90 cen­time­ters (24 to 35 inches), said Peter Wad­hams, a professor of ocean physics at the Univer­sity of Ox­ford who did not take part in the re­search.

That es­ti­mate, he added, as­sumes that the rate at which ocean lev­els rise will re­main con­stant.

“Yet there is con­vinc­ing ev­i­dence — in­clud­ing ac­cel­er­at­ing losses of mass from Green­land and Antarc­tica — that the rate is ac­tu­ally in­creas­ing, and in­creas­ing ex­po­nen­tially.”

Green­land alone con­tains enough frozen wa­ter to lift oceans by about seven me­ters (23 feet), though ex­perts dis­agree on the global warm­ing thresh­old for ir­re­versible melt­ing, and how long that would take once set in mo­tion.

“Most sci­en­tists now ex­pect to­tal rise to be well over a me­ter by the end of the century,” Wad­hams said.

The new study, pub­lished in Na­ture Cli­mate Change, rec­on­ciles for the first time two dis­tinct mea­sure­ments of sea level rise.

The first looked one-by-one at three con­tri­bu­tions: ocean ex­pan­sion due to warm­ing, changes in the amount of wa­ter stored on land, and loss of land-based ice from glaciers and ice sheets in Green­land and Antarc­tica.

Satel­lite al­time­try

The sec­ond was from satel­lite al­time­try, which gauges heights on the Earth’s sur­face from space.

The tech­nique mea­sures the time taken by a radar pulse to travel from a satel­lite an­tenna to the sur­face, and then back to a satel­lite re­ceiver.

Up to now, al­time­try data showed lit­tle change in sea lev­els over the last two decades, even if other mea­sure­ments left lit­tle doubt that oceans were mea­sur­ably deep­en­ing.

“We cor­rected for a small but sig­nif­i­cant bias in the first decade of the satel­lite record,” co-au­thor Xue­bin Zhang, a professor at Qing­dao Na­tional Lab­o­ra­tory of Marine Sci­ence and Tech­nol­ogy in China’s Shan­dong Prov­ince, told AFP.

Over­all, the pace of global aver­age sea level rise went up from about 2.2 mil­lime­ters a year in 1993, to 3.3 mil­lime­ters a year two decades later.

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