In 2007, the IPCC projected a maximum sea level rise of 59 centimetres by 2100. The IPCC acknowledged that this was likely to be an under-estimate because understanding of the processes happening on ice sheets was inadequate to enable reliable estimates to be made.

A team of researchers led by Eric Rignot from Nasa’s Jet Propulsion Laboratory has now reported that ice loss from Antarctica and Greenland has accelerated over the last 20 years and the increase in sea levels will, indeed, be significantly higher than the 2007 estimate.

By 2006, the Greenland and Antarctic sheets were losing a combined mass of 475 billion tonnes of ice a year.

Loss from the Greenland sheet alone is now increasing by an average of almost 22 billion tonnes year, while the Antarctic sheet is shedding an additional 14.5 billion tonnes each year.

If these increases persist, water from the two polar ice sheets will raise the average global sea level by 15 centimetres by 2050. About another 15 centimetres will come from a combination of melt water from mountain glaciers and thermal expansion of the seawater.

By 2100, the sea level rise from the two ice sheets alone is estimated to be 56 centimetres.

The new research combined calculations of ice gain and loss through combining various types of satellite reading and data taken on the ground, such as the thickness of the ice sheet and the speed at which glaciers are moving, with data from Nasa’s Grace mission, which uses twin satellites to measure variations in the Earth’s gravitational pull.