Understanding the contribution outlet glaciers have on the global sea level rise is tricky business, and as a new study shows, we are seriously lacking in long-term information.
A lack of long-term data often results in faulty assumptions and hypotheses
Previous studies looking at the contribution outlet glaciers have on the global sea level rise have looked at the situation without proper long-term information. One such study saw scientists create two scenarios; one in which the glaciers in Greenland would double their speed towards the ocean between 2000 and 2010 and then stablize, and a second scenario in which their speed would increase tenfold and then stabilize.
The study found that at the lower rate, the Greenland ice would contribute approximately four inches to the rising sea level by 2100, and at the higher rate would end up contributing around 19 inches by the same time.
A worthy study, if the rates by which the velocity increased were anywhere near accurate, but the scientists had very little data available for how major ice regions were starting to behave in the wake of a rising global temperature.
Find some long-term data and we’ll begin to get an idea
Twila Moon, a University of Washington doctoral student in Earth and space sciences and lead author of a paper documenting the changes in speed of 200 outlet glaciers in Greenland, wanted to make use of the massive store of data acquired by the Canadian Space Agency’s Radarsat-1 satellite, Germany’s TerraSar-X satellite and Japan’s Advanced Land Observation Satellite.
So Moon, along with colleagues, set about creating a decadelong record of changes in the Greenland outlet glaciers by producing velocity maps using the data acquired by the satellites. Starting with the winter of 2000-01, and then repeating the process for each winter from 2005-06 through 2010-11, they found that the outlet glaciers had not increased in velocity as much as had originally been theorised.
With answers, come more and more questions
“In some sense, this raises as many questions as it answers. It shows there’s a lot of variability,” said Ian Joughin, a glaciologist in the UW’s Applied Physics Laboratory who is a coauthor of the Science paper and is Moon’s doctoral adviser.
The scientists found absolutely no indication that the glaciers were going to stop gaining speed during the rest of this century, and so could not say one way or the other whether the glaciers could reach or exceed the above mentioned scenario where they contribute four inches to sea level rise.
“There’s the caveat that this 10-year time series is too short to really understand long-term behavior,” said Ian Howat, an assistant professor of earth sciences at Ohio State University. “So there still may be future events – tipping points – that could cause large increases in glacier speed to continue. Or perhaps some of the big glaciers in the north of Greenland that haven’t yet exhibited any changes may begin to speed up, which would greatly increase the rate of sea level rise.”
What the record showed
The velocity record created by the scientists showed a complex pattern of behaviour. Almost all of the glaciers that end on land move at top speeds of 30 to 325 feet per year, with small changes in speed because they are already moving slowly.
Glaciers that terminate in fjord ice shelves however move at 1,000 feet to a mile a year, but don’t gain much speed during the decade.
Thus, in the east, southeast and northwest areas of Greenland, glaciers that end in the ocean can travel seven miles or more in a year.