New Scientist - One of the two new studies shows that last report of the Intergovernmental Panel on Climate Change (IPCC), published in 2007, underestimated actual sea level rise. That's because the IPCC's fourth assessment report (AR4) did not include contributions from the melting of the Greenland and Antarctic ice sheets.
So, for the years 1993-2011, the IPCC estimated that sea level would rise by about 2 millimetres a year. But the satellite data from that period now tell a different story.
Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany and colleagues compared IPCC AR4 projections with actual measurements and found the projections lagging behind what was happening in the real world. Global sea level has been rising at about 3.2 millimetres a year over the past two decades
Environmental Research Letters - Comparing climate projections to observations up to 2011
We analyse global temperature and sea-level data for the past few decades and compare them to projections published in the third and fourth assessment reports of the Intergovernmental Panel on Climate Change (IPCC). The results show that global temperature continues to increase in good agreement with the best estimates of the IPCC, especially if we account for the effects of short-term variability due to the El Niño/Southern Oscillation, volcanic activity and solar variability. The rate of sea-level rise of the past few decades, on the other hand, is greater than projected by the IPCC models. This suggests that IPCC sea-level projections for the future may also be biased low.
There are four major mechanisms: the thermal expansion of oceans in a warming world; the loss of ice from the Greenland and Antarctic ice sheets; the melting of mountain glaciers and ice caps (such as those in the Himalayas); and the extraction and discharge of groundwater.
The likely culprits are continental ice sheets. "[In IPCC models], the two big ice sheets in Greenland and Antarctica contribute nothing to future sea level rise, because they assume that the mass loss from Greenland is balanced by ice gain in Antarctica due to higher snowfall rates," says Rahmstorf.
But satellite data show that the ice sheets are losing ice to the oceans.
If the models have not accurately reproduced what happened in recent years, it is likely that their projections for the future are not correct either. Since 2007, the IPCC has recognised this. Its initial projection of a maximum sea level rise of 60 centimetres by 2100 has been upped to include an additional 20-centimetre rise due to ice sheets melting.
And the data are clear: from 1990 to 2000, the melting of Greenland and Antarctic ice sheets added about 0.25 millimetres a year to global sea level rise. For 2005-2010, that number has increased to about 1 millimetre a year (Science, http://dx.doi.org/10.1126/science.1228102"
Greenland is losing the most ice, causing sea level to rise by about 0.75 millimetres per year. What's happening in Antarctica is more nuanced. East Antarctica is gaining mass because of increased snowfall, but this is more than offset by the loss of ice from West Antarctica, particularly along the Amundsen Coast, where warm water is melting ice shelves from beneath. This is leading to thinning and speed-up of glaciers, such the Pine Island and Thwaites glaciers.
Until now, sea level rise from the extraction of groundwater (which eventually ends up in the sea) has been countered by dams built on rivers over the last century, which hold water back on land. But the best sites for dams have now been utilised, so we can't expect to store more water on land.
As we extract more groundwater for irrigation – a trend that could increase as climate change causes droughts – it could add up to 10 centimetres to the sea level by 2100, according to Rahmstorf. "This will become a net contribution to sea level rise in the future," he says. "Not big, but not negligible."
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