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Carbon Brief Staff

02.06.2013 | 8:15pm
ScienceExtra water or more heat – what’s driving sea level rise?
SCIENCE | June 2. 2013. 20:15
Extra water or more heat – what’s driving sea level rise?

Sea levels are rising as seawater warms and expands, and as the Earth’s ice melts into the sea.

Research over the last few years suggests most of the rise is a result of ‘thermal expansion’ of seawater. But a  new study in the journal Nature Geoscience suggests that in recent years melting ice may have been the main cause of sea level rise.

The study finds that between 2005 and 2011 melting ice sheets and glaciers were responsible for about 75 per cent of sea level rise, while the effect of water warming and expanding played a much smaller role.  So does the research suggest melting ice is a bigger problem than scientists previously thought?

Measuring sea levels

As sea water heats up it becomes less dense and takes up more space, a process called thermal expansion. As the water takes up more space, sea levels rise. The study’s authors used seven years of temperature data from a set of buoys called Argo to calculate how much thermal expansion has contributed to sea level rise over that time.

They also looked at data collected over the past decade by a set of special satellites that measure changes in earth’s gravity to work out how much glaciers and ice sheets are shrinking. This let the researchers calculate the amount of extra water being added to the oceans by melting ice.

Comparing the two over the seven year period this study looked at, the authors found that meltwater from glaciers and ice sheets contributed more to sea level rise than thermal expansion. The study estimates that extra meltwater caused sea levels to rise by about 1.8mm per year, three times more than thermal expansion, which caused about 0.6mm of sea level rise per year.

This is a different result to many other studies in this area, some of which were included in the 2007 climate science report from the Intergovernmental Panel on Climate Change (IPCC). The report  suggests that, over the time period scientists have had access to reliable data, thermal expansion of water has been the biggest cause of sea level rise – contributing more than all the sources of meltwater combined. The report also suggests thermal expansion will be responsible for between 70 and 75 per cent of sea level rise in the future.

Breaking it down

So why does this result differ from earlier studies?

It may be that ice melt is contributing more to sea level rise.  A number of other recent studies suggest changes in ice are driving more sea level rise than previously thought, and many of world’s glaciers and ice sheets are certainly seeing long term declines.

But there are some important caveats. This new study looks at what’s driving sea level rise over a pretty short time period. Climate studies usually draw conclusions from a few decades of data, to try and make sure that short-term natural climate cycles don’t hide long term trends – this covers just eight years.

There are also a variety of ways of interpreting the satellite gravity data, so other studies draw different conclusions about ice sheets’ contributions. Add to that the fact scientists haven’t been able to accurately monitor what’s driving sea level rise for very long – and it’s easy to see why it’s a complicated picture.

Thermal expansion probably remains the single biggest cause of sea level rise in the long run. But these results could at least suggest that ice melt is having more of an effect on sea levels as time goes on.

In the long run

The study shows that between 2005-2011, sea levels rose a little slower than the longer term trend – rising about 2.4mm per year instead of 3.1mm per year. But the authors say that’s most likely down to two big La Niña events which caused sea levels to drop temporarily, and so this research doesn’t change the overall picture for sea level rise.

Sea level rise is expected to accelerate in the future, and higher greenhouse gas emissions will speed it up further.

There’s still quite a lot of uncertainty when it comes to saying exactly how fast sea levels will rise, but the upcoming report from the IPCC looks set raise estimates of how big a role ice sheets and glaciers will play.

Chen et al. (2013) Contribution of ice sheet and mountain glacier melt to recent sea level rise. Nature Geoscience. DOI:10.1038/NGEO1829

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