Science Interviews


Sun, 15th Nov 2009

Monitoring the Greenland Ice Sheets

Jonathan Bamber, Bristol University

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Chris - Well also in the news this week.  Weíve got some worrying news emerging from Greenland because scientists have shown that the ice there is melting, and the time that itís doing that at, the rate which it is melting at is increasing. So the melt rate is accelerating.  But how do we actually quantify how fast ice is melting from a land mass with any accuracy?  Well, thereís a paper in the journal Science this week, itís by Bristol University scientist, Professor Jonathan Bamber and his colleagues, and it might be able to help us.  And Jonathanís with us now to tell us a bit more.  Hi, Jonathan.

Jonathan -   Hi.

Chris -   Welcome to the Naked Scientists.  So tell us, first of all, what the issue is with Greenland.

Jonathan -   So Greenland is the biggest ice mass in the northern hemisphere.  Itís got enough ice in it to Ė if it melted, if you took away a whole ice sheet.  It would raise global sea level by about seven meters.

Chris -   To put that into perspective then we would be looking at, the Pennines would be about the only bit of Britain left above water, wouldnít they?

Jonathan -   No, no.  Itís not quite that bad, but you can say bye-bye to the Houses of Parlaiment which might be a good thing but, you know, I couldnít comment on that.  But seven meters, thatís what Ė about 25 feet, so I'm not suggesting that thatís going to happen tomorrow or anything like that, but there is a huge potential for sea level rise in the Greenland ice sheet.  I think the other thing about big ice masses like Greenland and Antarctica is that once you set them on a certain course, they're like the super tankers of the climate world.  Once youíve pointed them in a certain direction, itís very, very difficult to turn them another way.

Chris -   How do you quantify how much weight ice, water, is going from Greenland?

Jonathan -   Well, thereís a variety of techniques.  But what a lot of scientists have been very excited about in about the last five, six years is a satellite mission called GRACE, which stands for Gravity Recovery and Climate Experiment.  It doesnít matter what the acronym is, but itís an absolutely amazing mission.  Itís actually two satellites and itís able to measure very, very accurately, small changes in the gravity field of the Earth.  And so, if an ice sheet like Greenland loses mass or gains mass for that matter, it can actually measure those variations, and it does it on a, roughly, monthly timescale.

Chris -   And what has this told you?

Jonathan -   So a number of scientists have looked at this problem with GRACE and with other satellite data as well.  And the problem is being with all the previously published results is that thereís been a lot of variability in the numbers.  In fact, the numbers have differed from each other by about a factor of  two, you know, some have been double others.  But what weíve done is actually compared two different approaches using GRACE and an entirely independent approach for measuring the mass loss of the ice sheet, and they tie up pretty well.  So it gives us a lot of confidence on our results, and we think that weíve sorted out a lot of the issues that existed with earlier observations.  And yeah, I guess itís a pretty disturbing picture.  In the early Ď90s, the ice sheet looked like it was relatively close to balance, maybe losing 50 gigatons of ice.  A gigaton is one billion tons.  And in the last few years, that rate has increased to something like 273 gigatons a year and thatís a lot of ice.

Chris -   Thatís Ė well, 273 gigatons, thatís a cubic kilometre per gigaton.  So thatís 273 cubic kilometres.

Jonathan -   So, I mean, just kind of try and, itís pretty hard with numbers that big to really know what youíre talking about here, but one gigaton is about the volume of Lake Windermere.  So weíre talking about 273 Lake Windermeres.

Chris -   Per year?

Jonathan -   Per year.  But I think the other interesting statistic I like is that 4 gigatons is enough water to supply the entire domestic water supply of the UK.  So 273 is pretty much the water supply of the whole planet.

Chris -   And thatís just melting every year.  Has that changed though because one of the points you make in your paper is that there appears to be an acceleration going on?  This, one would presume, would be secondary to global climate change.  So whatís the pattern of that acceleration?

Jonathan -   Weíve seen Ė GRACE only went up in 2002 and the reliable measurements are only about six years of observations.  So we donít have a very long record from GRACE, but just in that time, we have seen the rate of increase, increase by about 2.5 times.  So I think the mean for the period for 2003 to 2008 is about 180.  But the last two years, itís gone up to something like 270 gigatons a year.  So thatís a big acceleration in mass loss.

Chris -   So itís almost like Greenland is sort of a barometer of whatís happening potentially in other bits of the world, isnít it?

Jonathan -   Well, thatís one of the interesting things because I think until we started making some of these observations, most glaciologists and climate scientists felt that the ice sheets responded very, very slowly, and not very dramatically to climate change.  What weíre seeing in Greenland is completely the converse, and I think itís surprised a lot of scientists.


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