Titans of Science: Susan Solomon
PodcastsThe Naked ScientistsTitans of Science: Susan SolomonHow to change the world for the better with science16 July 2024Today's Titans interview is with the key figure in one of science's modern triumphs. . . .
PodcastsThe Naked ScientistsTitans of Science: Susan SolomonHow to change the world for the better with science16 July 2024Today's Titans interview is with the key figure in one of science's modern triumphs. Susan Solomon and her team were the first to theorise and prove what was causing the hole in the ozone layer above the Antarctic, and why it was growing: chlorofluorocarbon pollutants humans were emitting. What followed is a testament to what can be achieved in the face of significant challenges with international collaboration. What lessons does it hold for how we tackle climate change?Susan Solomon: Finding the hole in the ozone layerSusan SolomonOn today's edition of Titans of Science, we're going to hear from Susan Solomon at the National Oceanic and Atmospheric Administration. Susan is an atmospheric chemist and she was one of the first people in the world to respond to reports in the early 80's of deterioration of the planet's ozone layer. She joined the faculty at MIT in 2011 where she serves as the Ellen Swallow Richards professor of atmospheric chemistry and climate science. She's frequently cited as one of the most important women in science, and in 2008 was selected by Time Magazine as one of the top 100 most influential people in the world. Time went on to say, 'All scientists like to believe they will leave the world better than they found it. Susan Solomon surely will. Having helped save the Earth's atmosphere already, she's now playing a role in trying to do it again. 'Chris - Susan, welcome to the programme. Pleasure and a privilege to have you with us. The first thing that you did for the atmosphere concerned the ozone layer, and a hole that turned up therein. How is the health of the ozone layer now?Susan - Well, the ozone layer is really the world's signature environmental success story. We have already seen the chemicals that were causing the ozone depletion to start dropping and drop quite a bit, and we've actually seen the Antarctic ozone holes start to get a little bit smaller and form a little bit later, particularly in the month of September. So that's a huge win. It would've been so much worse by now if we had not stopped using those dangerous chemicals. Chris - It's so nice to start a piece like this with some good news that we're going to tell people about, and maybe we'll change that later on. But let's back up very slightly. What is the ozone layer? The clue is in the name, obviously, but what does it do for us and why does it matter?Susan - The evolution of life on this planet wouldn't have happened until the ozone layer actually evolved. So ozone is a form of oxygen that has a somewhat different structure. Instead of having two oxygen atoms together, it's got three, and that allows it to absorb certain wavelengths of ultraviolet light that would otherwise make it all the way down to the ground. And as anyone who's ever been sunburned knows, we really need to be careful about ultraviolet. Chris - Where is the ozone layer?Susan - Most of the ozone layer is located between about 12 and 30 kilometres above our heads. So there's not much ozone in the lower atmosphere, and that's a good thing because ozone is actually quite toxic when you breathe it. You want it up in the upper atmosphere protecting you from ultraviolet, but you don't want it down at the ground in the air that you're breathing. People who had to breathe it in smog episodes, for example, in Los Angeles in the 60's and 70's, or even now in places like India, really do suffer. We want it up there, we don't want it down here. Chris - And how did it come to light that ozone had a problem?Susan - Well, the earliest work highlighting the idea that the ozone layer might be in danger actually goes back to the mid 1970's when two chemists pointed out that we were using these dangerous chemicals in larger and larger amounts, and the concentrations were building up in the atmosphere and that they could destroy ozone. The idea that something could last in the atmosphere for a hundred years was critical to that concern and people began to be concerned that if we kept using these compounds, we might see a few percent depletion or loss of the ozone layer in about a hundred years. It was a small change, far in the future - It was kind of a future problem if you want to call it that - but then in 1985 the British Antarctic Survey discovered the Antarctic ozone hole, which was a huge surprise. We didn't expect to see big ozone losses that early over Antarctica. You lose not a few percent, but more like 50% of the total amount of ozone over your head. That protective layer is reduced by a massive fraction and we didn't expect it in Antarctica. So it was a huge mystery, initially. Chris - It was Jonathan Shanklin and Brian Gardiner and others who spotted that, wasn't it, that hole?Susan - Yes. And Joe Farman who unfortunately is no longer with us, but they did some amazing work there. They managed to show something that the satellites had not seen, which was quite a shocker to the science community. The truth of what they said has obviously stood the test of time and then some. Chris - When we say hole, is it really a hole and how big is it?Susan - Well, it's a 50% loss of ozone. It's not 100%. So that's good news in a way. But the reason that it is 50% is that in a layer between about 14 and 24 kilometres, there is indeed a hole. The ozone is completely gone in that layer. So the half that you have left is only what's above and below that region where it's completely gone. The reason that it began to be called the hole is how dramatically it looks isolated when you look at it on satellite imagery. The reason for that is that the antarctic atmosphere, you can think of it as being kind of like a whirlpool, literally what we call a vortex, but think of a whirlpool like when you drain your bathtub or whatever. That keeps the air inside the middle of the whirlpool very isolated and the ozone just gets eaten away and eaten away and eaten away. So it does look like a hole. It's a very big hole. It's twice the size of the continental United States. Chris - Does it actually extend beyond the margins of Antarctica?Susan - Slightly, yeah, it's about 25 million square kilometres at its height in a bad year. Chris - Are we just fortunate in a way that because of the geography of Antarctica with that polar vortex, that whirlpool that has concentrated the chemicals that are depleting the ozone down there, and we can perhaps come onto that in a minute. Is it convenient and fortunate that it's happened there where there aren't so many people, just research scientists chiefly and holidaymakers, and not over more populated parts of the planet?Susan - Yes but I'll hasten to add that we also see significant ozone losses over the Arctic where there are a lot more people. We also see significant ozone losses over mid-latitudes, both southern and northern mid-latitudes. So although the ozone hole can be thought of as like the canary in the coal mine, it showed us first that there was going to be a big problem. Not too long after that we started measuring bigger changes at other latitudes than what we expected to. Susan Solomon: Chemistry of the ozone holeSusan SolomonChris - Can you talk us through what the chemistry of the ozone hole is? Because you alluded to the fact there were some toxic chemicals that those scientists pointed out in the 1970's as a possible future problem, that they could be storing up trouble. But what were those chemicals and what is the mechanism by which they degrade ozone and why specifically concentrate down in Antarctica?Susan - Most of the problem comes from chlorofluorocarbons and, interestingly enough, these molecules were being used in large amounts in things like refrigeration, air conditioning, foam blowing, and especially as the propellants in spray cans. So back in the 1970s, actually about 75% of the global use of these molecules was in hairspray and underarm deodorant of people all around the world. The idea that it was building up and building up every year, that it had a hundred year lifetime and it might deplete the ozone layer someday, was enough to actually motivate the American public to give up spray cans. What happened was that hairstyles changed and people literally went on the stick to save the ozone layer. They started using stick deodorant. It's also really interesting that this country was really first on that, the American consumer gave up these chemicals before there was even any regulation. That was mainly motivated by the environmental groups urging people to do that and people voluntarily did it. It really shows you the power of public opinion and the power of the consumer on certain environmental problems when you can in fact take action and we can take action on all kinds of things in my view. But certainly in this case the action was huge because 75% of the market is a real punch in the stomach to any industry. The American chemical industry certainly took note of that. Chris - Ironically, in a programme talking about atmosphere and air quality, I think even asthma inhalers were using CFCs, these chlorofluorocarbons, to propel the drugs down the lungs of people who had airway problems, weren't they? But I think you are referring to the Montreal protocol and treaty 1986 which led to a massive reduction in use worldwide of these chemicals. But why are they so bad for ozone? Do we understand what they do to the ozone to make it fall apart so we get a hole in the first place?Susan - Yeah, we certainly do. It took us a few years, but actually remarkably only a few years. I think the whole scientific community deserves a lot of credit for being so fast and figuring this out after the British pointed it out. The way it works is that Antarctica really is the coldest place on earth. It's so cold that clouds form in the very dry Antarctic stratosphere. The stratosphere is dry everywhere and normally we don't have clouds at stratospheric altitudes. We have them down here in the lower atmosphere, but you don't get them where the ozone layer is normally. You get them very frequently in the Antarctic and sometimes in the Arctic. It's just much less common to get the frequency of polar stratospheric clouds being as much in the Arctic as it is in the Antarctic. So the coldest place on Earth with all those clouds provided a surface for chemistry to happen that simply doesn't happen when all you have are gases. That fundamentally changed the extent to which these compounds could deplete ozone. So it's not that there's more chlorofluorocarbon down there than anywhere else, it's just that it's more potent because of this surface chemistry on those cloud particles. Chris - It's also seasonal, the hole, isn't it? We see headlines that this year it's grown, then it's shrunk a bit. Why is it seasonal?Susan - That is such a cool thing. So you need not only cold temperatures, but you also need sunlight to drive the chemistry that's involved. That's a little complicated. The main point is that the air, when it's cold and dark, the chemistry just can't get cranked up. What happens is that the chlorine on the surfaces of these clouds makes a molecule called Cl2, which is actually chlorine gas. It's a very strong absorber of any kind of light. It Immediately breaks it down and those chlorine atoms then go on and destroy ozone and some further reactions. But that's why you need sunlight as well as cold temperatures. Then, when it gets too warm, the chemistry can't happen anymore. So basically it's sort of a goldilocks effect where you need a combination of cold and light and you get that sweet spot right in the Antarctic in their spring which, because it's the southern hemisphere, is August and September, not what we would think of like February and March in the Northern Hemisphere. We do see ozone depletion in the Arctic to some extent in February and March too but nothing like the Antarctic in almost all years. Chris - You mentioned, and it was a surprise to me because I hadn't heard that before, that we're also seeing ozone depletion at other latitudes, not just down on the South Pole. You just mentioned the Arctic. If it's down to temperature, that's not going to be the case at these other latitudes so much, is it? So how do we account for that loss?Susan - Well, these surface reactions happen to a limited extent everywhere. Fundamentally, that's a big part of this and there's also some mixing of the ozone depleted air from the Antarctic down to, say, New Zealand and Australia. Same thing happens in the Arctic: there's some communication of that depleted air, and the gases are also doing some loss of ozone, but it is mostly that surface chemistry operates to a more limited degree at warmer temperatures.