Monitoring the Greenland Ice Sheets

Chris - Well also in the news this week.  We've got some worrying news emerging from Greenland because scientists have shown that the ice there is melting, and the time that it's 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, there's a paper in the journal Science this week, it's by Bristol University scientist, Professor Jonathan Bamber and his colleagues, and it might be able to help us.  And Jonathan's 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.  It's got enough ice in it to - 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, wouldn't they?

Jonathan -   No, no.  It's 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 couldn't comment on that.  But seven meters, that's what - about 25 feet, so I'm not suggesting that that's 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 you've pointed them in a certain direction, it's very, very difficult to turn them another way.

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

Jonathan -   Well, there's 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 doesn't matter what the acronym is, but it's an absolutely amazing mission.  It's actually two satellites and it's 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 there's 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 we've 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 we've sorted out a lot of the issues that existed with earlier observations.  And yeah, I guess it's a pretty disturbing picture.  In the early '90s, 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 that's a lot of ice.

Chris -   That's - well, 273 gigatons, that's a cubic kilometre per gigaton.  So that's 273 cubic kilometres.

Jonathan -   So, I mean, just kind of try and, it's pretty hard with numbers that big to really know what you're talking about here, but one gigaton is about the volume of Lake Windermere.  So we're 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 that's 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 what's the pattern of that acceleration?

Jonathan -   We've seen - GRACE only went up in 2002 and the reliable measurements are only about six years of observations.  So we don't 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, it's gone up to something like 270 gigatons a year.  So that's a big acceleration in mass loss.

Chris -   So it's almost like Greenland is sort of a barometer of what's happening potentially in other bits of the world, isn't it?

Jonathan -   Well, that's 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 we're seeing in Greenland is completely the converse, and I think it's surprised a lot of scientists.