Eric Wolff: climate change in ice cores

Eric Wolff describes how cores of ice drilled from Antarctica and Greenland are used to determine climate changes (and other changes) over the past 800,000 years.

I’m Eric Wolff and I work at the British Antarctic Survey here in Cambridge, England, and I work mainly on studying ice cores to understand past climates. They come from all over Antarctica and even in a few cases from Greenland in the Northern Hemisphere. A lot of ice cores come from the Antarctic Peninsula region here, which is where most of the British activity is focused, and actually the ones in the cold room at the moment do come from here, I think. A lot of the ice cores that I’ve worked on, though, have been from the deep ice cores drilled by European projects, and they’ve actually come from the other side Antarctica, in particular from Dome C, which is just here, which is now a French/Italian station, but it was a big European project involving ten different countries, where I was the science leader in the field. Essentially for the deep cores there a drill barrel that’s, well, in total about ten metres long, and it’s got different sections. It’s got a section that grips the bit of hole you’ve already drilled, and then another section that unwinds to drill out the next bit of core, which would be typically a ten centimetre/four inch diameter core. And then above that there are other sections with batteries and power supplies and so on, and then a cable on the end. So you drill two or three metres at a time. So the process is lowering the drill down the hole, which might take to the bottom of a – a 3,000 metre hole might take an hour. Then a couple of minutes of drilling the next two metres of ice, and then another hour bringing it up to the surface. So there is actually a lot waiting around. The bandsaw is one of the main tools of ice core scientists, funnily enough, both in the field and in the laboratory here, because you’ve got a section of core that’s, as I said, about ten centimetres diameter, and you’ve got cut it into sections that different laboratories are going to operate on and the different samples, and that’s done to the first degree with a bandsaw. There may be other ways of cutting it to clean samples and so on. So actually we spent a lot of the time the field and a lot of time here just standing at minus twenty degrees cutting things into samples that can be put into bottles for analysis. Here you can see there’s a piece of ice from the Antarctic Peninsula, and you can see as it starts to get clear you can the bubbles, which is where the gases, not only the CO2 and the methane, but all the normal gases like nitrogen and oxygen, are trapped. And we can break those bubbles open and analyse them in some of the instruments. The most important analyses that you do on ice cores is the water isotopes, the oxygen isotopes and deuterium, because they’re the thing that tells you what the temperature was at the ice core site when the snow fell. And that, again until very recently, used to be done with very expensive mass spectrometers that were quite time consuming to use. But now there are new instruments, using essentially laser absorption methods, and that’s what this instrument is. So what it’s doing is it’s taking discrete samples in bottles, that we bottled up from the ice, and the needle is going into the bottle to suck up the sample into the instrument. It only requires a few tenths of a millilitre to do it. And that measures the ratio of oxygen isotopes and of hydrogen isotopes in water, which then just gives us a readout which essentially tells us what the temperature was. And it can do this with several samples an hour. So what you see there is the needle going into the bottle, taking a sample, it’s doing the analysis, and when it’s finished it goes and takes the next one. It’s just breaking the seal on the top of the bottle to take a fresh sample each time. The trick with these instruments is that they’re what’s called 'cavity enhanced', which means they’ve got very, very reflective mirrors at each end. So the light actually travels backwards and forwards many thousands of times, giving you a light path of hundreds of metres or kilometres essentially, for the small number of water molecules in there to absorb the light. And the different isotopes of water absorb slightly different wavelengths of light, so by looking at the ratio between the amount of light that’s absorbed at different wavelengths, you can calculate what the isotope ratio is in the water. And the numbers that are coming out are those ratios, they’re actually turned mathematically into something that looks like either a positive or a negative number, but essentially they’re telling you what the ratio of the different isotopes is. Once the data have come off the instruments downstairs, then someone will hopefully send me a file with them, if I haven’t generated it myself. And then the very first thing I would do is just to plot the data, whatever it is the oxygen isotopes calcium, sodium, against depth, and just get a feeling for what the data looks like. But then of course what we’ve got to do with it, is put it into spreadsheets and manipulate it in order to get it against age, to understand what it means, and see what, for instance, periodicities might emerge from it. And eventually write a paper from it, which after all is the main currency of science, is papers containing ideas. So the data are very useful and they help other people to have ideas, but it’s our own ideas that are the most exciting thing to produce. So staring at the data, doing things with it, and writing about it, are the three main activities that I ought to be doing here. Well actually I use a very simple programme called EasyPlot, which was first produced, I don’t know, ten or fifteen years ago, which I’m sure is completely out of date, but it is a very, very easy way of just taking data and manipulating it and dragging and dropping it from place to place. So I tend to spend a lot of time playing with plots in EasyPlot, comparing different datasets and so on. A lot of my research is really to try and understand this question of why does Earth’s climate work the way it does; why do we go in and out of Ice Ages roughly every 100,000 years? Because actually until we know that, we can’t really say that we understand today’s climate. So here we have 800,000 years of climate record, from Dome C, the EPICA ice core in Antarctica. All we’ve got on vertical axis is temperature, compared to the present. So here’s the present, which is relatively warm, and then between 10,000 and 110,000 years ago, we’ve got the last Ice Age when the temperature at the maximum was about ten degrees colder in Antarctica than it is today. And then what we call the Last Interglacial, when it was slightly warmer than today in Antarctica, and so on. Lots of cycles of Ice Ages coming and going, natural cycles of Ice Ages coming and going over 800,000 years, which is as deep as the ice core goes. And then in parallel with that, which was perhaps the most exciting result from the Dome C ice core, is the carbon dioxide record over the same period, which looks remarkably similar, showing the way that carbon dioxide and temperature interact with each. And then right at the end you’ve got this multicoloured bit, which is modern measurements of CO2, showing how the carbon dioxide level has risen way out of the range that it had over the last 800,000 years. Firstly I would say that as any scientist I just find it fascinating. So even if there was no benefit to it at all, I would still love to do it as a hobby. It’s just – I just want to know, just as people want to know what stars are out there or what’s at the bottom of the ocean, I really want to know why Ice Ages come and go. Of course there is also a much more practical motivation, that if we want to understand how climate’s going to evolve in the future, in response to the slightly unusual things that human activity is doing to the atmosphere, we really need to understand how that whole climate system works. And by climate system I mean everything: the atmosphere, the ocean, the ice sheet, and so on. And we can actually only get that from looking at palaeoclimate records such as ice cores, because some of the timescales over which things happen, for example ice sheets coming and going, we just don’t have observations of. We weren’t even looking at ice sheets a few hundred years ago, let alone understanding how they worked. So from that point of view the motivations is to understand the past as the only evidence really of whether the things that we’re putting into models are correct attributions of how it works, and therefore can take us into the future. Mainly to me, ice cores are a way of understanding how the Earth works, so that’s probably the most important aspect of them, which isn’t quite about the ice itself but, although I must say I do like ice, in general I like ice. I like snow and ice.

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