Trac­ing warm­ing through past sea lev­els

China Daily (Hong Kong) - - SCIENCE & TECHNOLOGY - By JUSTIN GIL­LIS

Thirty-five years ago, a sci­en­tist named John H. Mercer is­sued a warn­ing. By then it was al­ready be­com­ing clear that hu­man emis­sions would warm the earth, and Dr. Mercer had be­gun think­ing about the con­se­quences.

His pa­per, in the jour­nal Na­ture, was ti­tled “West Antarc­tic Ice Sheet and CO Green­house Ef­fect: A Threat of Disas­ter.” In it, Dr. Mercer pointed out the un­usual to­pog­ra­phy of the ice sheet sit­ting over the western part of Antarc­tica. Much of it is be­low sea level, in a sort of bowl, and he said that a cli­matic warm­ing could cause the whole thing to de­grade rapidly on a ge­o­logic time scale, lead­ing to a pos­si­ble rise in sea level of about five me­ters.

While most sci­en­tists agree that the planet is in the early stages of what is likely to be a sub­stan­tial rise in sea level, they still do not know if Dr. Mercer was right about a danger­ous in­sta­bil­ity that could cause that rise to hap­pen rapidly, in ge­o­logic time. Re­searchers may be get­ting closer to fig­ur­ing that out.

A new pa­per comes from Michael J. O’Leary of Curtin Univer­sity in Aus­tralia and five col­leagues from around the world. Dr. O’Leary has spent more than a decade ex­plor­ing the re­mote western coast of Aus­tralia, con­sid­ered one of the best places in the world to study sea lev­els of the past.

The pa­per, pub­lished in Na­ture Geo­science, fo­cuses on a warm pe­riod in the earth’s his­tory that pre­ceded the most re­cent ice age. In that epoch, some­times called the Eemian, the plan­e­tary tem­per­a­ture was sim­i­lar to lev­els the world may see in com­ing decades as a re­sult of hu­man emis­sions, so it is con­sid­ered a pos­si­ble in­di­ca­tor of things to come.

Ex­am­in­ing el­e­vated fos­sil beaches and coral reefs along more than 1,600 kilo­me­ters of coast, Dr. O’Leary’s group con­firmed some­thing sci­en­tists pretty much al­ready knew. In the warmer world of the Eemian, sea level sta­bi­lized for sev­eral thou­sand years at three to four me­ters above mod­ern sea level.

Dr. O’Leary’s group found what they con­sider to be com­pelling ev­i­dence that near the end of the Eemian, sea level jumped by an­other five me­ters or so, to set­tle at close to nine me­ters above the mod­ern level, be­fore fall­ing as the ice age set in.

Dr. O’Leary said in an in­ter­view that he was con­fi­dent the five-me­ter jump hap­pened in less than a thou­sand years — how much less, he can­not be sure.

This find­ing is some­thing of a vin­di­ca­tion for one mem­ber of the team, a North Carolina field ge­ol­o­gist, Paul J. Hearty. He had ar­gued for decades that the rock record sug­gested a jump of this sort, but it was only re­cently that mea­sure­ment and mod­el­ing tech­niques reached the level of pre­ci­sion needed to be cer­tain.

If the work does hold up, the im­pli­ca­tions are pro­found. The only ex­pla­na­tion for such a large, rapid jump in sea level is the col­lapse of a po­lar ice sheet, on ei­ther Green­land or Antarc­tica.

Dr. O’Leary is not pre­pared to say which; fig­ur­ing that out is the group’s next pro­ject. But a five-me­ter rise in less than a thou­sand years, a ge­o­logic in­stant, has to mean that one or both ice sheets con­tain some in­sta­bil­ity that can be set off by a warmer cli­mate.

An­other pa­per, by An­ders Lev­er­mann of the Pots­dam In­sti­tute for Cli­mate Im­pact Re­search in Ger­many, im­plies that even if emis­sions were to stop to­mor­row, the world has prob­a­bly locked in sev­eral me­ters of sea level rise over the long term.

Ben­jamin Strauss and his col­leagues at Cli­mate Cen­tral, an in­de­pen­dent group of sci­en­tists and jour­nal­ists in Prince­ton, New Jersey, cal­cu­lated that by 2100, con­tin­u­ing on our cur­rent path would mean a long-term sea level rise of seven me­ters, but ag­gres­sive emis­sion cuts could limit that to two me­ters.

On that cru­cial point science has not as­cer­tained the an­swer. Sci­en­tists can look at the rocks and see in­dis­putable ev­i­dence of jumps in sea level, and they can as­so­ciate those with rel­a­tively mod­est in­creases in global tem­per­a­ture.

But the na­ture of the ev­i­dence is such that it is hard to tell the dif­fer­ence be­tween some­thing that hap­pened in a thou­sand years and some­thing that hap­pened in a hun­dred.

On the hu­man time scale, of course, that is all the dif­fer­ence in the world.

If sea level is go­ing to rise by 17 me­ters over a thou­sand years, that is quite a lot of time to ad­just — to pull back from the beaches, to re­in­force ma­jor cities, and to de­velop tech­nolo­gies to help us cope.

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