20150723_IS

Source: BBC Radio 4

URL: N/A

Date: 23/07/2015

Event: Arctic sea ice increased by 41% in 2013 but "not to get hopes up"

Credit: BBC Radio 4, Inside Science

People:

• Dr. Adam Rutherford: Geneticist, author and broadcaster
• Rachel Tilling: PhD student at CPOM-UCL

Adam Rutherford: That ice sheet on Pluto is pretty stable, we think, but nearer to home, the Arctic ice cap has been shrinking - key evidence, over the decades, that global temperatures are on the rise. So the publication, in Nature Geoscience this week, of a new paper that shows that in 2013, which was a slightly cooler summer than average, Arctic ice had grown, not just a tad but by a whopping 41% on the previous year. On the surface, this looks like good news. The study uses data from ESA's CryoSat-2, which incorporates not just the surface area of ice but the all-important number - the volume. As lead author Rachel Tilling, from University College London, told me.

Rachel Tilling: The thickness and the volume are actually the most important measurements we can get, 'cause the area has been really, really useful in giving us, you know, an overview of how the Arctic is changing, but it only really gives us half the picture. Arctic sea ice is incredibly complicated, you know, it doesn't just grow in area, it also grows in thickness. And it's the overall volume of the sea ice, and the mass of the sea ice, that is going to have the most effect on our climate, so we really need to know how that's changing.

Adam Rutherford: And in this new paper there is a result which is pretty surprising to everyone, which is that in 2013 the Arctic sea-ice volume went up by a pretty staggering number.

Rachel Tilling: Yes, so it was quite surprising to us as well, because we've got model studies of the Arctic sea-ice volume, and you can see from the model studies that the long-term trend in Arctic sea-ice volume is downwards. And we started our measurements in 2010, and over the period of 2010 to 2012 we saw about a 14% decrease in sea-ice volume.

Adam Rutherford: Which was predicted and in line with what's been happening.

Rachel Tilling: Yeah, and it wasn't surprising, in line with what we would expect. And then we got to 2013 and we got this 41% increase, with respect to 2012, and we thought "Oh"... You know, that's not necessarily what we were expecting, but in all systems you do have some natural variability about a long-term trend, and that's what we think it is.

Adam Rutherford: But even though it's been going down, though - 14% is a very significant drop - but 41% increase - that's massive, it more than balances out several years' decrease, right?

Rachel Tilling: Yeah, it balances out the decrease of the previous three years, which is all we can really say from our measurements, but if we did have measurements going back further than that, then it would more than likely balance out, you know, the last few years before that, as well. And it was something that we found quite surprising, and that was, kind of, the main motivation of this paper, was to explore in more detail what actually drove that increase in 2013.

Adam Rutherford: Right, so what does it mean? Where is this increase coming from?

Rachel Tilling: What we started to do was look at various factors that might be influencing the changes in sea-ice volume, from year to year. And we looked at things like the snow load and the wind, because often in the Arctic you have very strong winds that can really bring lots of large sea-ice floes, which are these blocks of sea-ice that we have in the ocean. They bring them together and that allows more ice to grow in other areas. But we found the most significant factor by far was the temperature. There was this really high correlation between the sea-ice volume at the end of the summer and the number of days on which melting actually occurred, and we believe that had the most significant impact on the sea-ice volume.

Adam Rutherford: And how does that number and the sea-ice volume that you've just published - how does that relate to the overall global trends about global warming and climate change?

Rachel Tilling: It's quite hard to say specifically how that relates to the whole of the global climate, just from these five years of observational data. But what we would hope is we can use these measurements now, of sea-ice thickness and volume, in the models that do predict future climate change, and hopefully improve those.

Adam Rutherford: But those images of the Arctic sea-ice shrinking over the last few decades are pretty iconic. They're iconic in our understanding of how the planet is getting warmer, and given that how controversial this is outside of science - there's a ridiculously high consensus within science that it's a real phenomenon, but - you can see how this is going to be interpreted. You know, with 30 years of Arctic sea-ice shrinking and then suddenly it gets bigger, you're still okay with climate change being a real thing, and global warming being a real thing?

Rachel Tilling: Yeah, and that's something we were quite concerned about, when we released the paper, was that these results might be misinterpreted. That's absolutely something that we're not suggesting - we're not suggesting that this is a reversal at all of a long-term decline. We've been very careful, throughout the paper, not to use this term "reversal" or not to use the term "recovery", because it's not a recovery, in any sense, it's just an anomalous year, it's natural variability. And if you look at the long-term trend in volume in the Arctic, it is downwards, and we've got no reason to believe that that won't continue.

Adam Rutherford: So I guess one of the things that this shows is the Arctic sea ice is incredibly resilient, and it doesn't take much for it to grow back pretty quickly.

Rachel Tilling: That's true, and if you look at the number of days on which melting occurred in each year, this 41% increase in 2013 actually only equated to 7 days less, so that's just a week less on which melting occurred, which isn't actually that much of a difference, if you're thinking that you actually have about, you know, 140 days of melt over the year, on average. So, it does show perhaps that the Arctic sea ice is more resilient than we first assumed, and that there is - not to get hopes up, as such, but there is this kind of hope that if we can curb what we're doing in terms of our warming of the Arctic climate, there is still hope for the ice pack. But like I say, the long-term trends would suggest that temperatures are going to continue increasing, so the ice pack is probably going to dwindle further.

Adam Rutherford: Rachel Tilling from UCL.