MELTING ANTARCTICA

THE TOWERING GLACIERS of West Antarctica hold the fate of the world’s coasts in their flanks. Their collapse could send sea levels up by at least a foot by 2100—and potentially much more.

Human-driven global warming has changed the character of the winds that blow over the ocean near some of the most fragile glaciers in West Antarctica. Sometimes, those winds have weakened or reversed, which in turn causes changes in the ocean water that laps up against the ice in a way that caused the glaciers to melt.

The massive West Antarctic ice sheet holds something like 6 percent of the world’s fresh water frozen in its guts. If it all melted away, global sea levels would rise by about 10 feet or more. That’s not likely to happen anytime particularly soon, scientists think, but some parts of the ice sheet are particularly vulnerable, in danger of crossing a crucial “tipping point” if they retreat too far.

The glaciers have been receding because their snouts spill over the edge of the continent into the surrounding ocean, which is warmer than the ice. The warm water melts away the ice.

Over decades, the temperature of the water has waxed and waned, driven in part by natural climate cycles that send different water masses close to the edge of the ice sheet at different times, cycling through from cold to a little less cold every five years or so.

The main thing that controls whether warm water makes it to the edge of the ice sheet, it turns out, is the strength of the winds a little bit farther offshore, in the heart of the icy, bitter Amundsen Sea. Sometimes, those winds—cousins of the famous raging band of Southern Ocean winds known as the Roaring 40s—slacken or even reverse. When they do, more warm water ends up near the edge of the ice sheet, which means more ice melts away.

ICE OUT OF BALANCE

In the past, and even up to the early part of the record the scientists looked at in the 1920s, ice melted during warm phases and grew back during cold phases. But over the last century, that balance has come undone. The shifting winds and warm ocean phases have eaten away at the ice more quickly than it’s being replaced.

The ultimate cause of the wind patterns, they found, is human-caused climate change. The extra greenhouse gases humans have pumped into the atmosphere over the past few hundred years have changed the way heat moves around the planet so thoroughly that they’ve changed the shape of the basic wind patterns at the poles.

The Antarctic ice sheet sat more or less stable in shape and size for many thousands of years. But about a century ago, pieces of it started to retreat in measurable ways. That’s well within the time frame when carbon dioxide and other greenhouse gases had started to accumulate thickly in the atmosphere, so it seemed logical to think that human influence was affecting the ice. But Antarctica is a complicated place that changes a lot because of natural variability, so it has been challenging to pinpoint the extent of human influence on the changes.

But a warming planet has very clearly changed the way winds move around Antarctica—and that change is likely to continue, unless something drastic happens to slow or reverse the warming process. If we carry this pattern forward, we may move to a situation where we’re flipping between warm and very warm. And that could be devastating for the ice.

Keeping future greenhouse gas emissions in check would go a long way toward keeping those crucial winds from weakening further, the water under the edge of the ice chilly, and the ice frozen.