 My name is Stephen Sherwood. I'm a professor at the University of New South Wales in Sydney, Australia. And I do research on clouds and water vapor and climate and how all those things are connected. So there's a global water cycle and globally the amount of water that evaporates from the oceans has to come out, fall out of the atmosphere as rain. I can't stay in the atmosphere for very long, maybe about a week. So that whole cycle is really driven by the radiation balance of the atmosphere. The atmosphere is losing energy because it's radiating infrared radiation to space and to the surface. And that has to be also balanced by the heat that's released by the precipitation. So if you put all these things together we can say something about the rate of this global water cycle and how it does increase as the climate warms up because the energy flows increase. And so all of that happens a bit faster. That doesn't necessarily mean though that we're going to all have more rain in a warmer climate because it also means that the land surfaces are evaporating faster as well. I think that by and large in a warmer climate we can expect it to physically be drier in the sense that if you put a pan of water out somewhere it would evaporate faster in a warmer climate. And we would also get more rain but if you take the ratio of the rain that you get to how long that water lasts in that pan you're losing in a warmer climate. And this is something that you can work through with some thermodynamic calculations but this is what comes out. And it's supported by our climate, fancy climate models when we run them they do show the aridity of most continents on earth becoming more severe in a warmer climate. Now what might help us is that with higher CO2 plants don't need as much water. So the plants, most plants, some plants might actually be okay. But this is a huge area of uncertainty and I think it's pretty important. Over oceans and land precipitation and evaporation should happen faster in a warmer climate. But what happens over land is that what we call the potential evaporation increases much faster than any of those other things. So the potential evaporation is how fast a pan of water would evaporate. And what that means is that trying to keep water around for use becomes a lot harder. You can't evaporate water that doesn't rain. So the actual evaporation might not go up that much but the atmosphere is trying to evaporate water a lot harder as the climate warms up. In the climate system the way we understand it is that there are things that we call forcings that drive changes to the global energy budget and therefore the global temperature as a response to that. And then there are things that we call feedbacks. One of the feedback works is that when the earth starts to warm up, let's say, because of an input of heat, that warming triggers other changes which have their own effects on the energy balance of the earth. And if they in turn cause the earth to absorb more sunlight, for example, then that's what we call positive feedback because it means that the whole system will end up having to warm even more before it finally comes to a new balance. And the biggest feedback that we know about comes from water vapor. This water vapor feedback has been part of thinking on climate since the very beginning. Even Arrhenius in 1896 took account of the water vapor feedback when he computed global warming. But it's always been or it has been controversial for a long time because people have come up with various ways that it might not happen and there has been some important details in what determines the water vapor concentration in the atmosphere that we haven't known. We do know that an atmosphere at equilibrium will have more water vapor when it's warmer, but our atmosphere isn't in equilibrium. So there was this problem of saying, well, you know, how do we know that the actual water vapor amount is going to do what we think? And I would say in the last ten years, we have developed a good enough understanding of what controls the actual water vapor amounts in the atmosphere as opposed to this theoretical equilibrium so that we can say confidently that, yes, we will get this doubling of climate change from water vapor feedback and even the reasonable skeptics or semi-reasonable skeptics out there who have questioned this feedback in the past that I know of have basically moved on and said, yes, okay, there is this water vapor feedback and the real focus of discussion now is in clouds and feedbacks from them because they're definitely more uncertain. So in the atmosphere, there are several greenhouse gases. Carbon dioxide is the one that you keep hearing about. There's also water vapor, which is natural. It gives us all of our rain and gives us our clouds. And if you just look at the greenhouse effect of that water vapor, it's more than the greenhouse effect of the carbon dioxide. You might think, well, why do we care about carbon dioxide? Well, the problem is that carbon dioxide, once it's in the atmosphere, stays there pretty much for centuries. Some of it stays there for millennia. Water vapor only stays there for about a week. And moreover, the amount of water vapor that's in the atmosphere is so tightly controlled by the circulation of the air and the temperature of the air that you and I could boil as much water as we wanted and we wouldn't have one iota of effect on the amount of water vapor or the greenhouse effect of what's in the atmosphere. So what actually happens is when we burn fossil fuels, it puts carbon dioxide into the atmosphere. It increases the greenhouse effect of that. And then once that warms the earth, the earth will increase the amount of water vapor in the atmosphere. We can't do anything about it. So it acts like a feedback. And because it's a strong greenhouse gas, it means that the water vapor feedback can be strong and it is strong. And that's why it can double the impact of the carbon dioxide change or any other change to the energy budget. Could you talk briefly about just some of the other climate feedbacks there are in the system and how they all add up to an overall climate sensitivity? So the water vapor feedback itself roughly doubles the sensitivity that you would have. That's really the major one. There's ice. So in a warmer climate, ice will melt. And ice is bright. It reflects sunlight to space. So if you lose that, you're absorbing more sunlight and that's another positive feedback, but it's a lot smaller than the water vapor one. And then the big unknown is clouds. Clouds reflect a lot of sunlight to space. They also exert their own contribution to earth's greenhouse effect. And either of those capacities of the clouds or characteristics of the clouds could change in a warmer climate. And one of the big unknowns and topics of research right now that occupies people like me is to try to figure out what the clouds will do. The behavior of clouds that we see in our models is that they increase warming somewhat. They add a bit more positive feedback to what's already coming from water vapor and ice melting. For a long time, people regarded that as a black box sort of result that we didn't understand and really didn't trust at all. Lately, looking more carefully at why it happens in the models, we start to see a couple of pretty straightforward things that we do understand. And the most important one of those is simply the fact that in a warmer climate, the part of the atmosphere that we call the troposphere gets thicker. And the part that we call the stratosphere gets thinner. And the clouds, which form near the boundary between those two, move up. And their greenhouse effect gets stronger. And so they then give you another positive feedback. We think we're on pretty solid ground in that one. Other things happen in our models that are more diverse or different between different models and so they're not well understood. And they're also, it's very hard to tell from observations what's going on. We don't have long records of clouds. And the records that we do have that go back a few decades suffer from the fact that a new satellite is launched every five years and its sensor is a little different and you can't exactly compare. One of the things we'd like to be able to do is look at the observed variations in today's climate and use those to tell us what clouds would do in a warmer climate. And it turns out that it's fairly tricky to do that. And the reason is that the natural variability of the system from year to year or month to month includes a lot of things going on that don't really happen over 100 year timescale when you warm the planet. And so a lot of people have tried to infer cloud feedbacks from say satellite observations over the last 10 or 20 years. But I don't think any of those studies have really nailed the problem. I don't think any studies have nailed the problem yet. We're going to have to work on this for some time to come. But what you have to do is you really have to understand all those other factors that are there in the year to year variability and take them into account. And that's something that we haven't figured out how to do yet. So we don't really have a way of proving what the feedbacks are. But I think we're making some progress in understanding the cloud ones and it's looking less and less likely that they are going to do anything to reduce climate change. So in the last IPCC report we concluded that it was likely that they exert a positive feedback. It's the first time the IPCC has made a claim about even the sign of cloud feedback. Just the fact that they made a claim about it shows that the sign to be understanding must have got fairly strong. There were definitely differences of opinion about what to say about that. But there were at least as many people who wanted a stronger statement as there were who wanted a weaker one. There's really three ways to try to put bounds on what we call climate sensitivity which is the overall amount of warming that you get in the system from something like a doubling or quadrupling of carbon dioxide or something else like that. And one of those is to try to figure it out from first principles, be really smart, put together a model of the system and use the model to tell you what it's going to do. This is tough. We're doing it and it gives us a range of results but there are a lot of feedbacks that happen in these models and they depend on knowledge of the physics and the processes that goes beyond what we currently know how to confidently capture and put in. However, it gives us an idea of what to expect. You can also go back and look at the paleoclimate record which we have evidence on paleoclimate going back depending on how strong of evidence you want through the Phanerozoic about the last 600 million years. But particularly over the last 60 or 70 million years a lot of people look 50 million years ago people look over the Quaternary, the last million years and they're getting more interested in periods of time in between. And for many of these time periods you can get an estimate of the global temperature and you can get an estimate of the influences that things like CO2 almost always was higher when the climate was warmer and almost always was lower when the climate was colder. And in some cases we know why the CO2 was higher or lower it was because of changes in volcanism and so that's kind of like what we're doing except slower. So you can use that and when we try to put together when we try to put together what the sensitivity looks like from that information it's kind of similar to our climate models giving us numbers like two or three or maybe four degrees for a doubling of CO2. The third way that you can do it is you can try to get it from recent observational record just the 20th century. In other words look at the time period where humans actually started doing something. And over this time period we have much better information but so far we've got a pretty puny climate change. It's still been less than one degree. So it's also and there have been a lot of different kinds of human influence most importantly the fact that we're putting a lot of air pollution into the atmosphere carbon and soot and sulfate and it's really hard to tell how much of an impact that's having. So all of this makes it kind of hard to answer the question this way either. So we have three imperfect ways of doing it. They all kind of tell us that the sensitivity is significant but they don't any of them tell us exactly what it is. The warming pattern that you see in any model in the tropics is you see some warming at the surface and then as you go up in the atmosphere you see the warming getting stronger and stronger until you start getting close to the top of the troposphere layer and then it starts to go away. So there's this kind of maximum that exists about 10 or 12 kilometers above the surface in the tropics. And a lot of people have been looking at observations over the last 10, 20 years to try to find this and many of them don't find it and it has become a big controversy because every time someone doesn't find it they write a paper and then a bunch of people that want to be skeptical find it and feel vindicated. The problem is that we don't really have any good observation systems that measure atmospheric temperatures accurately enough over a long period of time to give us a really believable picture of what this pattern of warming looks like. We even have trouble at the surface. We've got thousands. There's around 10,000 surface observing stations on Earth and despite that there are arguments about how much the surface has warmed over a period of several decades in the tropics where we actually have relatively few stations. But the situation surface is great compared to what it is as you go up in the atmosphere where we have only a few dozen radio sun stations in the tropics. And you can turn to satellites to try to get this but they have their own problems that are not well calibrated and there's new satellites every few years and they also don't have very good... They can't really tell the difference between what's happening at 12 kilometers and what's happening at 6 kilometers in the atmosphere. So in my view, none of these observing systems is definitive and there are some that do show this heating maximum in the tropical upper troposphere. So it continues to be a subject of research and debate as to whether there is or isn't evidence of something different from what our models are saying. A tropical hot spot occurs because of basic tropical meteorology, thermodynamics of gases. We've got a gas near the surface that's got a lot of water vapor in it and when it mixes with the upper troposphere that water vapor condenses and it releases heat and so for that reason any temperature change at the surface is multiplied in the upper troposphere and that should happen no matter what causes the change. So we really have to distinguish between two different questions here. One is, is the tropical surface warming and the other is, is there something about the ratio of warming at one level versus another level that's different from what we think and if there is, that's telling us something really interesting about atmospheric mixing and thermodynamics. It's not necessarily telling us anything about global warming. If the surface isn't warming and we think it is, should be, then that would be telling us something about global warming potentially. So we need to keep these two things separate, I would say. Probably the most well-known fingerprint of carbon dioxide induced warming is the cooling of the stratosphere that you get when you add CO2 to the atmosphere and we certainly see that. There's no question that the stratosphere has been cooling. A complication there is that ozone depletion also contributes to it so it's again hard to say exactly how much of that is from carbon dioxide from ozone depletion, especially since the ozone depletion rate isn't very well measured. Otherwise, it's not easy to find fingerprints of human-caused warming versus, say, from the sun. I think the stratosphere is probably the main one. Yeah, so every once in a while some new scientific understanding comes along which has been called a paradigm shift where scientists tell people something that doesn't really fit with their worldview or really overturn some basic assumptions that we have. So the classic one where the term came from is the Copernican Revolution where Nicholas Copernicus realized that it made more sense to think of the Earth as being going around the sun rather than the sun going around the Earth and he couldn't prove it, but it made sense. It was a simpler way of explaining the data and it took along a lot of convincing and of course there were famous battles between him and the church before people would accept this. And a lot of the same arguments that people used against it for decades are kind of similar to the arguments people use against climate change. You know, I think there's some other way of explaining the observations that you've got and the real problem was that it was uncomfortable to think that the Earth wasn't the center of the universe because that's kind of what everybody believed. And so today we have this lifestyle we've built up for decades, more than a century, that's based on fossil fuels and we all know that this is how we've kind of gotten where we are and it's pretty uncomfortable to think that they're causing this problem for our kids and our grandkids. So I think there's some similarity there in terms of why it is that it's difficult to get people to accept this. And another example is Einstein's theory of relativity believe it or not was also not well accepted outside the scientific community at the time it was proposed. And the more the data confirmed it, the more a lot of people in the science community but outside the people who are really familiar with it turned against it. And they didn't want to think that this guy Einstein had figured this out and they didn't want to think that space and time weren't the sort of simple thing that they thought. And so we've seen this happen a few times in the history of science. In the case of Einstein and his theory, he himself remarked in his letters that you could tell whether somebody was for against his theory by what political party they belong to, which is a bit reminiscent of the situation today. And it so happens that when one of these confronting ideas comes along, sometimes people's responses to it tend to orient along pre-existing political faults in the society. Particularly in teaching of physics, it has become clear that to teach someone physics, you often have to get rid of their pre-existing incorrect notions before the better ideas will really take hold. So a typical example is we're so used to dealing with things that are damped by friction that students have really hard time understanding the physics of a planet orbiting a sun where the force is actually perpendicular to the motion. And you can explain to them and say, yeah, yeah, yeah. And then you give them a test question a month later and they go right back to the dumb answer they would have given before they took the course. So what you really have to do is show how their reasoning is wrong first. And I'm trying to use, I'm trying to figure out how to use that in my own teaching more than I do right now to get students into groups and give them problems to think about and then identify where they're thinking might have gone wrong so that we can then come in and say, okay, here's a different way to think about it. Yeah, one of the most prevalent misconceptions about climate change is that scientists saw temperature readings going up around the world and then started scratching their heads and thinking, well, why would this happen? Oh, I know, it's our fault. When what really happened was that this was anticipated more than 100 years ago just by scientists going all the way back to Joseph Fourier who's famous among anyone who studies math and physics for his ubiquitously used mathematical technique and they realized right away that if you increase the infrared opacity of the atmosphere the temperature would go up. And what we're seeing now is really the playing out of something that was understood a long time ago. When I started teaching this subject I would always ask my students every year at the beginning before we learned anything whether they thought which of two statements they agreed with more. Basically one was we observed this first and then figured out why it was happening or the other way around and consistently, and this is students at Yale who are reasonably well informed, almost every student for year after year would express the same misconception that they thought we had observed the warming first before we knew what was causing it and I think that's a big problem because the community isn't getting credit for the fact that they actually did understand this before it happened. The number of papers considering this problem increased exponentially so going back to the 1970s you would see a few going back farther even fewer and fewer and fewer but if you look through all of that time at those people who are actually considering the global average climate they've always been saying well the carbon dioxide is rising so we expect it to warm with a few people raising other possibilities but by and large that's what the literature has always said. How do you distinguish between what might be the vanguard of a new scientific revolution or people who are just rejecting a legitimate scientific consensus? Yeah, so one of the things about past cases where somebody like Galileo has come along and been sort of dismissed by a lot of his colleagues at first and the same with plate tectonics. There was actually a meteorologist who came along and figured out plate tectonics and at first none of the geologists wanted to have anything to do with this stupid idea and in each case there was a theory that the person had that explained a lot of different observations but it really went against what people were assuming and over time the power of the theory sort of won people over so if you're trying to figure out whether somebody is a Galileo or not one of the most important questions asked is do they have a theory that seems to have predictive power seems to be able to explain things in the natural world and the climate system is very complicated so I wouldn't claim that we can predict everything that you see happening but our models are able to do a lot of things they can explain why there's a seasonal variation of temperature or rainfall and why there's an El Nino and a La Nino and a lot of other things and the skeptics don't have a model they never do there's no basis for their predictions that there won't be any warming other than wishful thinking I would say there are many different people who reject the science of climate change in different ways or for different reasons so there are a lot of people out there who are just naturally skeptical and they want to be convinced and maybe they haven't looked into it that much and I think those people you can engage with particularly one on one or in a small setting and point some things out that they didn't know there's also a lot of people that have already made up their mind as almost part of their identity that they're not going to accept it and those people you kind of can't do anything with I think in communicating to the public about climate it's almost impossible to simplify things too much it's helpful to come up with analogies so I think one of the best ones around is a kind of medical analogy and it kind of goes back to what I was saying about whether climate change was observed first or whether it was predicted first we're kind of in a situation when the doctor told us quite a while ago that we had this condition and then we started to develop symptoms and then we started questioning the doctor and now we do a lot of asking well is this pain in my toe is that my cancer or is that my Ebola that you say I have and these are not the right questions to be asking the question you should be asking is what's the prognosis of this disease based on what we know so I think an analogy like that helps and there's others that are out there my research is on clouds and how they're connected to climate and clouds are a very important feedback mechanism on climate they're also really important for determining how weather systems evolve in our atmosphere and they may even be important for how circulations in the atmosphere are affected by things like climate change so one of the things we see happening right now is that the tropics which has a pretty well-defined edge according to how the circulations go in the atmosphere that edge has been moving outward for the last 30 years and nobody really knows exactly why and none of our models are able to predict a rate of expansion that is any more than maybe about a third of what we observe so there's something funny going on there and some of us think that clouds might have part of the responsibility for making this happen so much faster than what a model says should happen and there's a lot of other things going on in the world that are happening faster than the models say we see rainfall patterns in the tropics enhancing at a very rapid rate so places that get some rain over the oceans are getting a lot more and places that got less rain over the oceans are getting a lot less and that's happening faster than we expected and again clouds may be involved in that and that's just to name two examples there are a lot of other things as well so one of the problems that I'm really interested in right now and there's a kind of growing international movement as exemplified by a new grand challenge that's been organized by the World Climate Research Program to really look at what role clouds play in all of these various global scale phenomena not just feedbacks on global warming but a lot of other stuff I started out my academic career in physics, I liked it I got a degree, worked for a couple years in the defense industry in the US and decided I didn't want to keep doing that forever and in graduate school didn't quite know what I wanted to do but I landed in the laboratory of a fellow named Ramanathan who is one of the most famous climate scientists in the world and an expert on radiation and who at the time was becoming very interested in clouds and so I kind of got drawn into the subject that way it wasn't something that I planned to do as an undergraduate I didn't take a single course in this area as an undergraduate but I think it's a really interesting field and it's one that could definitely use more people who like physics because that's really what we're doing The humans are influencing the climate system and we're doing it mainly by burning fossil fuels and the sooner we come to terms with that the better off we're going to be and unfortunately every kilogram of CO2 that we put into the atmosphere is pretty much going to stay there forever at least for practical purposes so it's not a problem that we can sort of think about and solve later when it becomes painful it's a problem that we have to, if we're going to do anything about it it has to be done fairly soon