The estimates do not account for the impact of dynamic effects like those seen in Antarctica, because the processes are poorly understood.
Dynamic effects are happening, Pritchard said.
"They are quite large in magnitude and are likely to get larger so that they could dominate the sea level rise signal.
"The importance of dynamic effects is that they can transfer ice very quickly into the sea, much quicker than melting the ice sheet or glacier surface and letting it run off as water."
Just how high could seas rise? Pritchard explained that no one can be sure.
"We're not yet at the stage where we can come up with new sea-level rise predictions except to say that it is very likely to be larger than the numbers in the IPCC report."
Pritchard and colleagues from British Antarctic Survey published their findings this week in the Journal of Geophysical Research.
Glacial Splashdowns Like "Stack of Dominoes"
The study suggests that faster glacier flows are caused by a thinning of the glaciers' lowest layers, the sections that extend down through fjords and into the sea.
The weight of these lower layers is supported by seawater rather than by land. As the ice thins, the glaciers become more buoyant, allowing them to flow faster toward the sea.
"What we are finding is that these glaciers are very sensitive to the conditions at their ocean boundaries," said Ian Howat, a researcher with the University of Washington's Polar Science Center who is unaffiliated with the study.
"Apparently a relatively small amount of melting at this boundary, either from the increased air or ocean temperatures, is enough to destabilize the entire glacier.
"These glaciers act like a stack of dominoes, with a slight nudge at the front causing the entire stack to fall over," he added.
Howat cautioned that many aspects of glacial dynamics, and how they could react to climate change, remain mysterious.
"The question still remains as to whether the changes we're observing are permanent or are a more regular purging of the system," he said.
In the case of Greenland the amount of ice that the glaciers has lost is very small relative to the size of the ice sheet, he said, so the ice sheet could restabilize and even grow again with a small amount of cooling or increased precipitation.
"However the glaciers in Alaska or the Antarctic Peninsula aren't supplied by a vast ice sheet," Howat added, "so for them to regrow or even stabilize for the long term would take a much more drastic reversal in current climate trends."
Free Email News Updates
Sign up for our Inside National Geographic newsletter. Every two weeks we'll send you our top stories and pictures (see sample).
SOURCES AND RELATED WEB SITES