By Jonathan Amos
Science reporter, BBC News
Nasa's Icesat spacecraft tracks changes in ice sheet height
Greenland and parts of Antarctica are losing large volumes of ice to the oceans as their glaciers get thinner, a Nasa satellite has revealed.
Many glaciers have increased their flow rates in recent years, and the Icesat mission now allows scientists to measure their thickness in detail.
A UK team studying the data told the journal Nature that the findings had implications for future sea-level rise.
A full melt of the Greenland ice would push sea level up by about 7m (20ft).
The extent of "dynamic thinning", observed by the satellite, has been a major source of uncertainty in projections of sea-level rise.
"All of the glaciers that are changing rapidly are ones that flow into the sea," said Hamish Pritchard from the British Antarctic Survey (BAS).
"The fact that they end in the sea means a buoyancy effect is working on them," he told BBC News.
"Normally, they're heavy things and they rest on the sea-bed and friction slows them down. But as you start to thin glaciers, they start to float off the sea-bed more and more; there's less friction and the glaciers can speed up."
The swiftness with which some of the glaciers now move towards the sea far outstrips the rate at which ice can be restored to the land through precipitation.
As a consequence, these glaciers are shown in the Icesat data to be falling in height - some dramatically so.
Fore example, the giant Pine Island and Thwaites Glaciers in the West Antarctic are thinning by up to nine metres per year.
The Pine Island Glacier is thinning by up to several metres per year
The US space agency satellite uses a laser altimeter to measure the elevation of the ice sheets.
It can acquire high-resolution data, especially in the steeply sloping coastal areas, where the radar altimeters flown on many other Earth observation satellites struggle to see important details.
Dr Pritchard and colleagues looked through six years of data to assess the behaviour of the ice along the entire margins of the two continents.
They monitored fast-moving and nearby "stagnant" ice streams at the same altitude to try to distinguish between the various possible causes of thinning.
The findings re-affirm what many suspect - that the reduced elevation of these glaciers is not the result of changes in precipitation or melt, but the increased speed at which they now move.
"If these were changes solely related to atmospheric conditions, either changes in temperature or snowfall, we would expect the glaciers and the nearby ice to be showing the same signal; and they don't," explained Professor David Vaughan, also at BAS.
"The glaciers are thinning because they are speeding up. Because they're flowing more rapidly to the ocean, they are discharging more ice into the ocean than is being replaced by new snowfall. The balance is loss."
The researchers tell Nature that this dynamic thinning, as they call it, now covers all latitudes in Greenland.
Of 111 fast-moving Greenland glaciers studied, 81 were shown to be thinning at twice the rate as the slow-moving ice beside them.
In Antarctica, the picture is more complicated, with Icesat reporting significant ice growth in places. Nonetheless, the researchers say, thinning has intensified at key locations where ice streams enter the ocean.
Some of the most graphic examples come from the Antarctic Peninsula, a region that is known to be warming faster than the rest of the continent.
Little thinning was seen on the massive, cold East Antarctic ice sheet, while the West Antarctic ice showed a mixed picture.
In many places in both Antarctica and Greenland, glaciers are being confronted by warmer waters which are eroding their fronts.
The break-up of floating ice shelves that would normally constrict glacier flow has also contributed to the observed acceleration. And in some regions, increased air temperatures are having an effect.
The team says better tools are needed to understand how changes in glacier behaviour will impact sea level.
"One of the big issues in glaciology is trying to get the right models to predict the ice sheets and to explain these observations," said Dr Pritchard.
"This is a set of measurements the modellers can use to improve their models and to predict future sea-level rise."