What’s new on ice sheet melt?
- 05 Jun 2013, 18:30
- Freya Roberts
A major review of the latest research on ice sheets is published today in Nature, updating what scientists know about the world’s two biggest bodies of ice – Greenland and Antarctica.
Since the last major climate science report, published by the Intergovernmental Panel on Climate Change (IPCC) in 2007, there have been some big advances in modelling and monitoring ice sheets. That monitoring process is important, because when ice on land melts it drains into the ocean, causing sea levels to rise.
Over the last six years, satellite techniques used to observe changes in ice sheets have improved, allowing scientists to refine estimates of how much ice sheets are contributing to sea level rise. But what do the new and improved modelling techniques tell us about the ice sheets?
Greenland in decline
In Greenland, loss of ice is adding about 0.7 mm to sea levels each year. Surface melting reached record levels in the past few years, but Greenland is also losing chunks of solid ice too. Glaciers and ice flows are transporting more ice from the heart of the ice sheet out to sea where it floats, displacing water and forcing sea levels up.
It’s likely the Greenland ice sheet will drive more sea level rise in the future too, the article says. Although climate models predict more snow will fall over Greenland, those gains are likely to be outpaced by losses from melting and shedding.
The article highlights processes which might amplify ice loss from the ice sheet. For example, both the loss of nearby Arctic sea ice and melting on the surface of the ice sheet reduce the amount of sunlight reflected. Instead, that heat is absorbed, raising surface temperatures.
Antarctica’s mixed picture
At the other end of the planet, the picture is a little less clear. But overall, data indicate the Antarctic ice sheet is losing mass. The review suggests Antarctica is adding a more modest 0.2 mm per year to sea levels.
But different parts of the ice sheet are changing in different ways. The Antarctic Peninsula and West Antarctica is losing solid ice, as huge bodies of ice acting as buttresses to ice flows break up under rising temperatures. But the picture in East Antarctica is somewhat different – here the ice sheet is growing thanks to an increase in snowfall.
As co-author of the paper, Dr Ben Smith from the Polar Science Centre in Washington tells us, there’s still uncertainty about changes in Antarctica, despite the technological advances. One reason is that satellites can only collect data over a small area of the ice, so errors come from scaling up their estimates to cover the entire ice sheet.
Measuring the height of the ice is further complicated by the fact the land beneath the ice is itself changing height, the article notes.
For now at least, the losses outweigh the gains. But scientists aren’t sure what will happen in the future.
A bigger contribution to sea levels
Despite remaining uncertainties, scientists still have a better idea now of how much ice sheets are raising sea levels compared to a few years ago.
At the time of the last IPCC report (AR4) there was only about 10 years worth of reliable sea level data, from 1993 and 2003. It suggested Greenland and Antarctica were together raising sea levels by about 0.42 mm per year – that’s about 15 per cent of total sea level rise. According to the review, over the next ten years that contribution doubled to about 0.82mm per year:
Smith tells us that increase is down to two things:
“First, we now know better what’s going on in the two ice sheets […] Second, and more importantly, the ice sheets are changing faster than they were in the years leading up to AR4.”
Lead author of the article, Edward Hanna, a Professor at the University of Sheffield adds:
“The main increase [in sea level contribution] has been from the Greenland Ice Sheet due to significant warming, leading to increased melting and discharge of ice over the last 5-10 years whereas Antarctic ice sheet losses seem to be much more modest”
It’s worth bearing in mind these are still quite short time periods, which means natural climate cycles could be affecting what’s driving sea levels. Together with the uncertainties over the data, scientists are still treating these estimates with caution.
But overall, the figures demonstrate what scientists have learned from the last few years of scientific advances – that Antarctica and Greenland are losing mass and driving up sea levels, increasingly fast as time goes on.