The melting glaciers that might drown your city

New York City has plenty to worry about from sea level rise. But according to a new study by Nasa researchers, it should worry specifically about two major glacier systems in Greenland’s northeast and northwest - but not so much about other parts of the vast northern ice sheet.

The research draws on a curious and counterintuitive insight that sea level researchers have emphasised in recent years: As ocean levels rise around the globe, they will not do so evenly. Rather, because of the enormous scale of the ice masses that are melting and feeding the oceans, there will be gravitational effects and even subtle effects on the crust and rotation of the Earth. This, in turn, will leave behind a particular ‘‘fingerprint’’ of sea level rise, depending on when and precisely which parts of Greenland or Antarctica collapse.

Now, Eric Larour, Erik Ivins and Surendra Adhikari of Nasa’s Jet Propulsion Laboratory have teased out one fascinating implication of this finding: Different cities should fear the collapse of different large glaciers.

‘‘It tells you what is the rate of increase of sea level in that city with respect to the rate of change of ice masses everywhere in the world,’’ Larour said of the new tool his team created.

The research was published in

accompanied by an online feature that allows you to choose from among 293 coastal cities and see how certain ice masses could affect them if the ice enters the ocean. The scientists also released a video that captures some of how it works.

The upshot is that New York needs to worry about certain parts of Greenland collapsing, but not so much others. Sydney, however, needs to worry about the loss of particular sectors of Antarctica the ones farther away from it - and not so much about the ones nearer. And so on.

This is the case because sea level actually decreases near a large ice body that loses mass, because that mass no longer exerts the same gravitational pull on the ocean, which accordingly shifts farther away. This means that from a sea level rise perspective, one of the safest things is to live close to a large ice mass that is melting.

‘‘If you are close enough, then the effect of ice loss will be a sea level drop, not sea level rise,’’ said Adhikari. The effect is immediate across the globe.

Indeed, the research shows that for cities like Oslo and Reykjavik, which are close to Greenland, a collapse of many of the ice sheet’s key sectors would lower, not raise, the local sea level. (These places have more to fear from ice loss in Antarctica, even though it is much farther away.)

The risk is mainly from the northern parts of Greenland and especially from the ice sheet’s northeast, according to research.

This is revealing because while Greenland has hundreds of glaciers, three in particular are known to pose the greatest sea level risk because of their size and, if they collapse, how they could allow the ocean to reach deep into the remaining ice sheet, continually driving more ice loss. The three most threatening by far are Jakobshavn glacier on Greenland’s central western coast, Petermann glacier in its far northwest and Zachariae glacier in the far northeast. Zachariae is part of a massive feature known as the Northeast Greenland Ice Stream, which reaches all the way to the centre of the ice sheet and through which fully 12 percent of Greenland’s total ice flows.

The new research shows that Petermann, and especially the northeast ice stream, are a far bigger threat to New York than Jakobshavn is.

In a high-end global warming scenario run out for 200 years, the study reported, Petermann glacier would cause 8.2cm of globally averaged sea level rise, the northeast ice stream would cause 10.5cm, and Jakobshavn would cause

4.39cm. Of this total, New York would see 5cm of rise from Petermann, 7.1cm from the Northeast ice stream and just

1.5cm from Jakobshavn.

This all really matters because in the real world, glaciers are melting at very different rates. Jakobshavn is the biggest ice loser from Greenland and is beating a very rapid retreat at the moment. Zachariae is starting to lose ice and looking increasingly worrisome, but still nothing like Jakobshavn. Petermann is holding up the best, for now, though it has lost large parts of the floating ice shelf that stabilises it and holds it in place.

The same goes for Antarctica its melting, too, will have differential effects around the world. And that matters even more because the ice masses that could be lost are considerably larger than in Greenland.