There is a new iceberg in the Southern Ocean, and it's big: 50 miles long by 20 miles wide. Until February, it had been the tongue of the Mertz Glacier, sticking out from the East Antarctic ice sheet into the Southern Ocean. Large cracks had been forming over the last few years, crossing the tongue almost from one side to the other, but it took a nudge from a similarly huge iceberg to finally set the chunk of ice free. The new Mertz iceberg will travel westwards on the currents around Antarctica for decades, sometimes running aground on shallow parts of the continental shelf, eventually breaking up into smaller bergs and slowly melting. 

Here aboard the scientific drilling shipJOIDES Resolution, which just finished drilling not far from the Mertz glacier in the waters off Wilkes Land, Antarctica, we received daily images from the U.S. National Ice Center to better navigate the ice-infested waters. It was not just the larger icebergs the ship's crew had to keep an eye on, but also the smaller pieces of ice, called growlers (the size of a car) and bergy bits (the size of a house), which were often tricky to spot on radar. But our ship was able to reach most of the planned sites, where we drilled ocean-floor sediments to study, among other things, the glaciological history of icebergs breaking away from the Mertz glacier and this sector of Antarctica. 

Today, the 20-mile-wide Mertz Glacier is one of Antarctica's great ice streams, draining the ice from this part of East Antarctica. Snowfall on the highlands upstream is gradually buried and compressed until it becomes ice, and the ice flows downhill into the glacier. The glacier is moving to the ocean at 3000 feet per year and has regularly calved icebergs. Because the snow falling in the hinterland is in balance with the icebergs calving from the glacier, the ice sheet remains stable, neither gaining nor losing mass. At least, this was the case until a few years ago. 

A Shifting Balance

Something new is happening with the ice streams and glaciers. They are getting thinner, and they are getting thinner because they are speeding up. Without satellite observations, it would be very difficult to know if the glaciers are changing, as so few people come to this most remote part of the world. But since about 1993, satellites have been measuring the height of the ice and, since 2003, ice mass. 

One such satellite, NASA's ICESat, carries a laser altimeter to measure the height of the ice surface. Between 2003 and 2007, the satellite's orbit passed over Antarctica's glaciers several times, and showed that most of the ice streams draining Antarctica are getting thinner. The Mertz glacier is thinning by a modest 1 foot per year, but some in West Antarctica, like the Pine Island glacier, are thinning by as much as 20 feet per year. 

Another NASA satellite (actually a twin pair of satellites) named GRACE measures tiny changes in gravity that result from moving masses on the Earth. If ice streams speed up and drain the ice faster, ice mass will be lost from Antarctica, and gravity will go down a fraction. Before 2000, the ice on Antarctica is thought to have been roughly in balance overall, with new snowfall balancing ice lost through surface melt and icebergs. But from 2003 to 2006, the GRACE data shows ice loss was about 27 cubic miles per year, and from 2006 to 2009 the ice loss more than doubled to about 64 cubic miles of ice per year. The ice is lost into the ocean, and at the current rate it would cause sea level to rise by about 3 inches by the end of this century. 

By itself and without faster ice loss, this would not be much of a problem. But Greenland is also losing a similar amount of ice, also at an increasing rate. If there were just a few surging glaciers, ice loss might slow down, but because the vast majority of glaciers are thinning at the same time, glaciologists conclude that the cause is warmer ocean and air temperatures in recent years. This poses some interesting questions for science. Will Antarctica and Greenland continue to lose ice at a faster rate? How much ice might be lost and how much would sea levels rise? The geological record holds clues to understand how the current ice loss will play out. 

What History Tells Us

There is a lot of ice on Antarctica—enough to cover the United States to a depth of 1.5 miles. In fact, only 18,000 years ago (a blink of an eye in geologic time), twice this amount of ice covered the northwest of North America. The Great Lakes are puddles left over from when it melted, and Long Island and Cape Cod, which mark the southeastern limit of the ice's reach, are formed from debris carried by the ice. This ice sheet took 12,000 years to melt (short in geologic time, long in human time) and the meltwater caused sea level to rise almost 400 feet at an average rate of about 3 feet per century. This gives us an idea of how fast a large ice sheet can melt. Since then, the remaining ice on Greenland and Antarctica has been pretty stable. 

In the last million years, the North American ice sheet has grown and completely melted about 10 times. During the last interglacial period, 125,000 years ago, some of the Greenland and West Antarctic ice sheets melted too, raising sea level by 24 feet compared to today. The world was only 1.5 C to 2 C (2.7 F to 3.6 F) warmer then, a temperature rise at the low end of the range predicted for the end of this century. In particular, glaciologists are watching the West Antarctica ice sheet closely because it is mostly grounded below sea level, which means that once it has started to retreat, it will be very difficult to stop until the ice edge reaches higher ground. There is 10 feet of sea level rise stored up in the unstable part of the West Antarctic ice sheet. 

For the response of the ice sheets to a still warmer world, older periods in geologic time offer parallels. The JOIDES Resolution expedition to Wilkes Land is investigating some of these, such as the Pliocene, about 3 million years ago, when the world was on average 3 C (5.4 F) warmer than today. The state of the ice sheets during the Pliocene is not well known, but it is thought that sea level ranged up to 80 feet higher than today. With the sediments drilled by the Resolution over the last month, the expedition's marine geologists will be able to get a good idea of both the warmer Pliocene temperatures in the Southern Ocean and the response of the ice sheet to this warming here on East Antarctica. 

By itself, the new Mertz iceberg is not a sign of global warming—icebergs are a normal part of the system. But satellite data shows that Antarctica is losing ice, and from the geological record we know that just a few degrees of warming makes parts of the ice sheets unstable. Glaciologists are racing to understand better how the great ice streams will flow in a warmer world, and the future rate of sea level rise is still uncertain. But from what we know already, 3 feet of sea level rise by the end of the century is a fair estimate. The 145 million people living within 3.3 feet of present sea level have time to move before 2100. But ports, coastal airports, cities like Venice and New Orleans and low-lying island states will have a more difficult time adapting—and so they have already begun to plan for rising seas and a changing coastline.