Climate Change and Ice Cores

Figure 1: Air bubbles fixed in ice. These can tell us how gas concentrations have changed over thousands of years.

Chris - A lot of people say that all the severe weather must be evidence for climate change. On the other hand you've got people saying that the earth has always had natural cycles and this is just one of them. Which camp are your feet planted in and what's the evidence?

Eric - Clearly an individual bad day or good day doesn't tell you anything, you do get extremes. So you need to look at what happens over a long period. But I think that the arguments about whether global warming is real hinge on four aspects. The first one is the physics that tells us to expect that when we get more carbon dioxide in the atmosphere it should get warmer. The second one is whether carbon dioxide has actually increased in the atmosphere, and that's what I'm best at because that's what we can see from ice cores. The third one is whether in the past that's caused climate change. And we can see in the ice cores that at least every time carbon dioxide's changed in the past, then it has warmed. So there's no counter evidence. And the fourth one is, is it warming today? Which is actually the hardest one to do because we don't have much detailed evidence about what the climate was in the past. And in a way it's the least relevant one because if we think the physics tells us that it's going to warm, and if we find out that in the past it's always warmed, then the fact that we can't quite see it yet doesn't matter. People often want to know if it's warmer now and it is but that's the hardest one to be sure about.

Chris - How are you using ice to work out what's been going on in the past?

Eric - The Antarctic ice sheet is up to about 3 miles or 4.5 kilometres thick at the thickest part. And if you drill into it you're effectively drilling into snowfalls that go back over hundreds of thousands of years. The oldest core that we've worked on is 800,000 years old. So we drill out a cylinder of ice 10cm in diameter but 3km long,

Chris - Do you do it in an environmentally friendly way?

Eric - We do it in as environmentally friendly a way as possible. But I'm afraid there's no getting away from the fact that going to the Antarctic is not actually an environmentally friendly thing to do by itself but hopefully you'll think the benefit was worth it. So we drill this cylinder of ice and in the ice is lots of evidence about what the climate was like in the past; how much snowfall there was in the past, what the temperature was and perhaps most crucially, we have little bubbles in the ice. The snow compacts to form solid ice with air bubbles trapped in it. You can then crack open these bubbles, put them into a chemical analyser, and find out literally what the proportion of carbon dioxide to nitrogen and oxygen and all the other parts of the air.

Chris - So it's like a time capsule going back over hundreds of thousands of years. How far back can you wind that clock?

Eric - Well so far we've gone back 800,000 years. I think there is older ice around in the Antarctic, and we'd like to look at it but that's as old as we've found so far.

Figure 2: Drilling ice cores in Antarctica.

Chris - What do those 800,000 years tell you?

Eric - Well in that time we know that the earth has gone through roughly 8 ice ages. When I say that I mean that there was ice covering northern Europe and northern America. In Britain that's as far south as Norfolk at times, or even a little bit further. It was also colder over the entire globe including Antarctica. And what we've found is that every time it was colder, there was less carbon dioxide in the atmosphere. Every time it was a little bit warmer like it has been for the last 10,000 years, there's a bit more carbon dioxide in the atmosphere, but never anything like the amount that there has been over the last few decades.

Chris - How much has it changed recently?

Eric - Well let me give you some numbers. In the last 800,000 years, carbon dioxide's always ranged between 180 ppm (parts per million) in the cold periods up to 300 ppm in the very warmest times. And at the moment, it's 380 ppm. So that's already 30% higher than it's ever been. And the only possible explanation for that is human activity. There's other evidence too. We can look at the isotopic structure of the carbon and it looks like stuff that's come from fossil fuels rather than from natural systems. Also we know how much stuff we're putting into the atmosphere and all the calculations work. We're contributing to the increase.

Chris - Whilst you can demonstrate that there's lots of carbon dioxide in the atmosphere that doesn't actually mean that that's causing the warming does it? Because there's lots of other things that can be a greater greenhouse-provoking agent than carbon dioxide. Water for example, and methane are much stronger green house gases.

Eric - Yes the biggest greenhouse gas is water vapour. But we don't have any control over that, the water vapour just acts as a feedback in this system. It's the carbon dioxide that we can actually do something about. Methane, although more effective is at lower concentration. But that's also double, now, from the concentration it's ever been in the last 650,000 years. There's no comfort to be had from talking about other gases. The carbon dioxide is actually increasing very fast, about 50 times faster than any time we can see in the historical record. But you're right to ask how do we know that that's what's causing the change. We can see in the past that every time we came out of an ice age the CO2 increased. If the CO2 increased in the past and the climate didn't change then we would have a cause to question if we know what we're talking about. But the fact that it did, although not proof for, is not negative proof, which is what we want. What people have done for the last 100 years is run climate models that understand the physics of the system. And if they put in things like volcanic eruptions which cause a cooling, or changes in solar activity, then they can reproduce more or less what happened in the first half of the 20th century. But only by adding in carbon dioxide can they reproduce what's happened in the second half of the 20th century.

Chris - So that's the nail in the coffin for carbon dioxide. You mentioned earlier you found a number of ice ages and warm cycles. Given that humans have only been knocking around for 6 million years, how do you account for that?

Eric - There are certainly natural cycles in the climate and nobody's claiming that all climate change is man made. And in a certain way it almost doesn't matter whether the change is man made or natural. If we were coming out of an ice age you might be living in a village in what's now the English Channel. The fact is that all climate change is pretty bad for people. What people have done in the past is to move to places that suit their lifestyle better. Unfortunately now we have a very crowded planet and there's really nowhere for people to move to. So we're going to have to deal with this in a different way.