On Thin Ice

NASA’S new satel­lite will give sci­en­tists their best pic­ture yet of how much and how fast seas will rise

Newsweek International - - HORIZONS -

When hur­ri­cane Florence struck the u.s. last month, rain and wind weren’t the only causes of dam­age. Ris­ing seas con­trib­uted to storm surges that reached nearly 20 feet in some es­tu­ar­ies. In the past 20 years, sea lev­els have risen about 3 mil­lime­ters per year, which has had an ef­fect on coastal flood­ing. How much more will the seas rise in com­ing decades?

The North and South poles are a good place to look for an an­swer. Melt­ing ice in Green­land and Antarc­tica is a big cause of ris­ing seas; Green­land has enough ice to raise sea lev­els by 21 feet. Loss of ice at the

North Pole could shut down the Gulf Stream, plung­ing North­ern Europe and Scan­di­navia into a deep freeze—a sce­nario de­picted in the 2004 sci-fi movie The Day Af­ter To­mor­row. “It wouldn’t hap­pen as dra­mat­i­cally as in the movie,” says physi­cist Thorsten Markus, who stud­ies po­lar ice at NASA’S God­dard Space Flight Cen­ter in Green­belt, Mary­land. “But that is ba­si­cally the idea. If the cir­cu­la­tion shuts down, ev­ery­thing changes dra­mat­i­cally.” Newsweek spoke to Markus, a project sci­en­tist for the NASA satel­lite ICESAT-2. Launched in Septem­ber, it mea­sures po­lar ice by bounc­ing laser beams off the Earth’s sur­face. The el­e­va­tion read­ings it ob­tains are ac­cu­rate within the width of a pen­cil.

How does ICESAT-2 help in un­der­stand­ing the rise of sea lev­els?

The main ob­jec­tive is to mea­sure the height of float­ing ice on the ocean, as well as the height of the ice sheets over Green­land and Antarc­tica. Bet­ter un­der­stand­ing of those pro­cesses will ul­ti­mately im­prove our pre­dic­tions of what will hap­pen in the fu­ture. The only way you can make ac­cu­rate pre­dic­tions is by con­tin­ued mon­i­tor­ing and data ac­qui­si­tion.

How does the satel­lite com­pare with its pre­de­ces­sor?

The orig­i­nal ICESAT fired a sin­gle beam of light 40 times a sec­ond, al­low­ing it to mea­sure el­e­va­tion about ev­ery 560 feet along Earth’s sur­face. ICESAT-2’S laser is split into six beams and fires 10,000 pulses a sec­ond, scan­ning the ground ev­ery 2.5 feet. That’s very, very high-den­sity data. We get much higher res­o­lu­tion—it’s un­be­liev­able.

What’s the dif­fer­ence be­tween sea ice and ice sheets?

Sea ice forms when ocean wa­ter freezes. It drifts around, and it can last for a sea­son or maybe longer. We have a very good han­dle on the cov­er­age from the satel­lite im­ages, but what’s eluded us is the thick­ness.

Sheets, like those found in Green­land and Antarc­tica, are com­pletely dif­fer­ent beasts. They are miles thick, and the only source for this is snow­fall. It falls, com­pacts over time and be­comes ice. Ice sheets move slowly down­stream over the course of decades, and even­tu­ally pieces break off to form ice­bergs. If this is a sta­ble sys­tem, then the amount of snow­fall will equal the amount of ice that breaks off.

Are the ice sheets sta­ble now?

No. They are los­ing a lot of mass each year; Green­land alone loses 250 gi­ga­tons a year. Roughly one-third of ob­served sea level rise comes from melt­ing ice sheets, so if they go away, sea level will in­crease. That spells trou­ble for coastal ar­eas like Florida.

What hap­pens when sea ice, which floats on the ocean, melts?

There is no ef­fect on sea level be­cause it’s al­ready in the ocean— think of a melt­ing ice cube in your drink—but it can change cli­mate and weather pat­terns far beyond the poles. Sea ice is white, which means it re­flects the sun. When it melts, the dark ocean ab­sorbs more en­ergy from the sun, ul­ti­mately rais­ing global tem­per­a­tures, which in turn cause glaciers and ice sheets on land to melt fur­ther. As ocean wa­ter gets warmer, it ex­pands. What’s hap­pen­ing in the po­lar re­gions is very im­por­tant to our cli­mate, our cities and our [gross do­mes­tic prod­uct] in gen­eral.

WHAT GOES AROUND... Clock­wise from top: The new ICESAT-2, launched in Septem­ber, mea­sures sur­face el­e­va­tion; Markus stud­ies po­lar ice; ICESAT-2’S or­bit will take it over the North and South poles.

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