Scientists have definitive new evidence that shows all but one of the world's major ice sheets are shrinking.
The study, which will be published in the magazine Science on Friday, marks the first time scientists have come up with a way to measure the changing size of the ice sheets in Greenland and Antarctica that they can all agree on.
Ice sheets are massive continental glaciers larger than 50,000 square kilometres that are found only in Greenland and Antarctica.
The study is a key step towards understanding how those massive melting glaciers are causing rising sea levels and how those levels can be measured in the future.
"Our new estimates are the most reliable to date and provide the clearest evidence yet of polar ice sheet losses," said the head researcher of the international study, Dr. Andrew Shepherd from the University of Leeds in the U.K.
"They also end 20 years of uncertainty regarding changes to the Greenland and Antarctic ice sheets. They are intended to be the benchmark data set for climate scientists from now on."
The study shows Greenland's ice sheets are melting at a rate five times faster than they were in 1990s. In contrast, Antarctica is more or less stable, although the research shows there has been a 50 per cent increase in ice loss since 1992. The melting is affecting ice sheets in the west side of Antarctica and the Antarctic peninsula.
The only ice sheet that hasn't melted significantly is on the east side of Antarctica, which appears to gradually be growing in size, partly because climate change is causing more snow to fall in that region. However, this increase isn't enough to make up for the larger losses in the rest of the continent.
Until now, there has been disagreement about what is actually happening to the mass of ice stored in polar ice sheets because of conflicting research. Since 1989, there have been 30 separate studies that have interpreted satellite data in different ways and over varying lengths of time.
This study brought together 47 scientists from 26 key labs that have conducted studies in the past. They used data gathered from 10 different satellites, including Canada's RADARSAT, over the last two decades. They ended up with 50 years of overlapping satellite observations and then calibrated the information so that they were comparing "apples to apples."
Rising sea levels
The result is confirmation that the ice sheets are indeed melting at an accelerated rate and that sea levels are rising.
"When combined, the full record of satellite data shows that the Antarctic and Greenland ice sheets have contributed just over 11 millimetres to global sea levels since 1992," said Shepherd. "This amounts to one-fifth of all sea level rise over the same period."
Scientists say the Greenland ice sheet is being influenced by climate change that is dramatically affecting Arctic regions in particular.
This year, rising temperatures have caused the ice cap on the Arctic Ocean to shrink to the smallest size ever recorded. However, melting arctic ice doesn't contribute to sea level rise because it's already floating on the surface of the ocean, just as melting ice cubes in your drink don't cause the glass to overflow.
Even so, 11 millimetres doesn't appear to be a large rise in sea level globally. But the scientists involved in the study say it can have huge effects on coastal cities.
They use the example of the recent damage to the eastern U.S. and Canada by superstorm Sandy. It's the most costly natural disaster in U.S. history, estimated at $50 billion US. Dr Erik Ivins from NASA's Jet Propulsion Laboratory, one of the co-authors of the study in Science, says 11 millimetres can translate into massive damage because of the storm surge that can develop in extreme storms.
"When you have 11 millimetres of increased sea level, if you compute the amount of mass that's capable of coming onshore during a storm surge, that's a lot of mass," Ivins told reporters. "And small changes in sea levels in certain places mean very big changes in the kind of protection of infrastructure that you need to have in place."
The data from this new study will also be included in the next major report by the Intergovernmental Panel on Climate Change that will be published in 2014.
Ultimately the scientists are hoping these measurements will help them better understand how the ice sheets are being affected by climate change. But they say they still need to do a lot more research into the complicated dynamic between the melting glaciers at opposite ends of the world and rising sea levels before they can start to predict what might happen in the future.