Ice is melting at an accelerating pace in Greenland and the Antarctic. You may know all about this. But maybe like me you’re still just catching up on the basics. If so, here’s a quick intro.
If all the ice in Antarctica melted, it would raise the sea by 61 meters! Such mammoth sea level changes do happen:
But it seems they take millennia. In the shorter term, meaning the next century or two, people tend to focus their concern on the West Antarctic Ice Sheet, or WAIS:
If the entire WAIS melted, it would make sea levels rise 4.8 meters. Another region of concern is Greenland: if all the ice there melted, it would cause a global sea level rise of 7.2 meters. This would inundate many of the world’s coastal cities.
Luckily, it seems no reputable glaciologists think the WAIS or Greenland will completely melt in the coming century. But amount of melting of these ice sheets has been a big challenge to predict.
The last International Panel on Climate Change report, back in 2007, took a pretty conservative stance, and assumed these ice sheets would melt at a slow and more or less constant rate until 2100. Their conclusion was that about 75% of sea level rise would be caused by the oceans expanding as they warmed. The melting of small glaciers, ice caps and Greenland would account for most of the rest. The Antarctic, they believed, would actually provide a small net reduction in sea levels, with increases in snowfall more than enough to outweigh the effects of melting. They predicted an overall sea level rise of between 0.18 and 0.59 meters, with most of the uncertainty arising from different assumptions about what the world economy will do.
However, almost as soon as the 4th IPCC report was released, evidence started accumulating that the melting of Greenland and the West Antarctic Ice Sheet were speeding up. For example:
This graph, taken from Skeptical Science, shows Isabella Velicogna’s estimates of the mass of the Greenland ice sheet. Unfiltered data are blue crosses. Data filtered to eliminate seasonal variations are shown as red crosses. The best fit by a quadratic function is shown in green. The data came from the Gravity Recovery and Climate Experiment—or GRACE, for short. This remarkable project uses a pair of satellites to accurately measure small variations from place to place in the Earth’s gravitational field. When ice sheets melt, GRACE can detect it.
The big news, of course, was that the melting is speeding up. Here’s the same sort of graph for Antarctica, again created by Velicogna:
• I. Velicogna, Increasing rates of ice mass loss from the Greenland and Antarctic ice sheets revealed by GRACE, Geophysical Research Letters, 36 (2009), L19503.
More recently, Eric Rignot et al compared GRACE data to another way of keeping track of these ice sheets:
Satellites and radio echo soundings measure ice leaving these sheets, while regional atmospheric climate model data can be used to estimate the amount of snow being added. The difference should be the overall loss of ice.
These graphs show Rignot’s results:
Graph a is Greenland, graph b is Antarctica and graph c is the total of both. These graphs show not the amount of ice, but the rate at which the amount of ice is changing, in gigatonnes per year. So, a line sloping down would mean that the ice loss is accelerating at a constant rate.
By fitting a line to satellite and atmospheric data, Rignot’s team found that over the last 18 years, Greenland has been losing an average of 22 gigatonnes more ice each year. Antarctica has been losing an average of 14.5 gigatonnes more each year.
But also note the black versus the red on the top two graphs! The GRACE data is in red. The other approach is in black. They match fairly well, though of course not perfectly.
The upshot? Rignot says:
That ice sheets will dominate future sea level rise is not surprising—they hold a lot more ice mass than mountain glaciers. What is surprising is this increased contribution by the ice sheets is already happening. If present trends continue, sea level is likely to be significantly higher than levels projected by the United Nations Intergovernmental Panel on Climate Change in 2007.
But how much sea level rise, exactly? Opinions still vary. A recent National Academy of Sciences report said at least 0.6 meters by 2100. But this still doesn’t include any melting of Greenland or the Antarctic!
This paper tries to take Greenland and the Antarctic into account:
• S. Jevrejeva, J. C. Moore and A. Grinsted, How will sea level respond to changes in natural and anthropogenic forcings by 2100?, Geophysical Research Letters 37 (2010), L07703.
The authors say their estimates are in line with past sea level responses to temperature change, and they suggest that estimates based on ice and ocean thermal responses alone may be misleading. With six different IPCC scenarios they estimate a sea level rise of 0.6–1.6 meters by 2100, and are confident the rise will be between 0.59 and 1.8 meters.
This paper suggests an upper bound on sea level rise 2 meters per century (if you max out everything) and a more realistic upper bound of 1 meter/century for this century (it could accelerate later):
• W. T. Pfeffer, J. T. Harper and S. O’Neel, Kinematic constraints on glacier contributions to 21st-century sea-level rise, Science 321 (2008), 1340-1343.
So, except for James Hansen, it sounds like most people would agree on an upper bound of about 2 meters of sea level rise by 2100. This is considerably more than the 4th IPCC report: remember, that gave an upper bound of about 0.6 meters.
I guess one moral is: stay tuned for further developments.
Everything you just read here, and more, was put together by the Azimuth Project team:
• Sea level rise, Azimuth Library.
I thank everyone who contributed, especially Staffan Liljgeren and Frederik de Roo.