What is the maximum amount that sea levels could rise by 2100? Much attention has been given to the numbers issued by the Intergovernmental Panel on Climate Change in 2007, and the fact that they are absolute minimums.
Now, a team of researchers has said that there may be a way of nailing the fixed upper limit – a rate of sea-level rise which physically could not be surpassed by the end of the century. That level, they say, is 2 metres: sea levels are unlikely to rise more than 2 metres by 2100.
Setting an upper limit on how fast seas will rise is important because planning for an indefinite amount of sea-level rise requires an indefinite amount of money.
"If we plan for 3 metres and only get 60 centimetres, for example, or vice versa, we could spend billions of dollars of resources solving the wrong problems," explains Tad Pfeffer of the University of Colorado at Boulder.
In 2007, the IPCC stated that sea-levels would rise by between 18 and 59 cm by 2100. The Panel underlined that this estimate only included rises caused by the expansion of sea water in a warmer climate, not the water contributed by the disintegration of Greenland and Antarctica ice sheets.
This has created predictable amounts of confusion. Some climatologists – most notably James Hansen, head of the Goddard Institute for Space Studies in New York – have warned that governments should prepare for their coastlines to disappear much faster as, although we have a good idea of the minimum sea-level rise, the maximum limit remains elusive.
In Greenland, where ice streams flow down steep valleys that end beneath the current sea-level, they searched for rocky bottlenecks in ice stream beds. These physically prevent Greenland's ice from sliding into the Atlantic, and the researchers say they must therefore limit the rate of sea-level rise from Greenland.
The team also collected information on the fastest speeds at which ice sheets have ever been seen to move over rock and into the sea, both in Antarctica and in Greenland.
They then calculated how much ice would need to be released into the oceans in order for sea-levels to rise by 2m and 5m by 2100. They conclude that, given the Greenland bottlenecks, a rise of 5m by 2100 is physically impossible.
The team calculate that rises of 0.8 to 2m are physically possible. "They underline the points that Hansen and others have been making. Sea level rises in metres rather than centimetres can't be ruled out," says Gavin Schmidt of the Goddard Institute. "And that is very scary."