I am a climate scientist, and I hate weather. I have spent too much time in California, and I strongly feel that weather should be optional. (Laughter) So I don't want to experience clouds, let alone study them. But clouds seem to follow me wherever I go. The thing is, clouds are a real challenge for climate science. We don't know how they're going to react as the planet heats up, and hidden in that uncertainty might be hope. Maybe, just maybe, clouds could slow down global warming, and buy us a little bit more time to get our act together, which would be very convenient right now. I mean, even I could put up with a few more cloudy days if clouds saved the planet. Now, we are sure about some things. Carbon dioxide is a greenhouse gas, we're emitting a lot of it, and the planet is heating up. Case closed. But I still go to work every day. It turns out that there is a lot that we don't understand about climate change. In particular, we haven't answered what seems to be a very fundamental question. We know it's going to get hot, but we don't know exactly how hot it's going to get. Now, this is a really easy question to answer if one of you would like to give me a time machine. But I'm going to be honest with you: if I had a time machine, I would not be hanging out at this particular point in history. So in order to see the future, we have to rely on the output of computer simulations -- climate models, like this one. Now, in my line of work, I encounter many very charming people on the internet who like to tell me that climate models are all wrong. And I would just like to say: no kidding! Seriously? I get paid to complain about climate models. But we don't want models to be perfect. We want them to be useful. I mean, think about it: a computer simulation that's able to exactly reproduce all of reality. That's not a climate model; That's "The Matrix." So, models are not crystal balls. They're research tools, and the ways in which they're wrong can actually teach us a lot. For example: different climate models are largely able to capture the warming that we've seen so far. But fast-forward to the end of the century under a business-as-usual scenario, and climate models don't really agree anymore. Yeah, they're all warming; that's just basic physics. But some of them project catastrophe -- more than five times the warming we've seen already. And others are literally more chill. So why don't climate models agree on how warm it's going to get? Well, to a large extent, it's because they don't agree on what clouds will do in the future. And that is because, just like me, computers hate clouds. Computers hate clouds because they're simultaneously very large and very small. Clouds are formed when microscopic water droplets or ice crystals coalesce around tiny particles. But at the same time, they cover two-thirds of the earth's surface. In order to really accurately model clouds, we'd need to track the behavior of every water droplet and dust grain in the entire atmosphere, and there's no computer powerful enough to do that. So instead, we have to make a trade-off: we can zoom in and get the details right, but have no idea what's going on worldwide; or, we could sacrifice realism at small scales in order to see the bigger picture. Now, there's no one right answer, no perfect way to do this, and different climate models make different choices. Now, it is unfortunate that computers struggle with clouds, because clouds are crucially important in regulating the temperature of the planet. In fact, if all the clouds went away, we would experience profound climate changes. But without clouds, would it be warmer or colder? The answer is both. So I'm going to be honest with you, I am not a cloud spotter. My favorite type of cloud is none. But even I know that clouds come in all shapes and sizes. Low, thick clouds like these are really good at blocking out the sun and ruining your barbecue, and high, wispy clouds like these cirrus largely let that sunlight stream through. Every sunny day is the same, but every cloudy day is cloudy in its own way. And it's this diversity that can make the global impact of clouds very hard to understand. So to see this global effect of clouds, it really helps to take a selfie. It will never cease to blow my mind that we can see our planet from outer space, but we can't see all of it. Clouds are blocking the view. That's what they do. These low, thick clouds are extremely effective sunshades. They turn back about 20 percent of everything the sun sends us. That is a lot of wasted solar power. So, low clouds are powerful sunshades, making the planet cooler. But that's not the only effect of clouds. Our planet has a temperature, and like anything with a temperature, it's giving off heat. We are radiating thermal energy away into space, and we can see this in the infrared. Once again, clouds are blocking the view. That's because high clouds live in the upper reaches of the atmosphere, where it's very cold. And this means that they lose very little heat to space themselves. But at the same time, they block the heat coming up from the planet below. The earth is trying to cool itself off, and high clouds are getting in the way. The result is a very powerful greenhouse effect. So, clouds play this very large and dual role in the climate system. We've got low clouds that act like a sunshade, cooling the planet, and high clouds which act like a greenhouse, warming the planet. Right now, these two effects -- they don't cancel out. That sunshade -- it's a little bit more powerful. So if we got rid of all the clouds tomorrow, which, for the record, I am not advocating, our planet would get warmer. So clearly, all of the clouds are not going away. But climate change is change. So we can ask: How will global warming change clouds? But remember, clouds are so important in regulating the earth's temperature, and they both warm and cool the planet. So even small changes to cloud cover could have profound consequences. So we might also ask: How will clouds change global warming? And that is where there might be space for hope. If global warming triggers cloud changes that make for a less powerful greenhouse or a more effective sunshade, then that would enhance the cooling power of clouds. It would act in opposition to global warming, and that's what's happening in those climate models that project relatively muted warming. But climate models struggle with clouds, and this uncertainty -- it goes both ways. Clouds could help us out with global warming. They could also make it worse. Now, we know that climate change is happening because we can see it: rising temperatures, melting icecaps, shifts in rainfall patterns. And you might think that we could also see it in the clouds. But here's something else unfortunate: clouds are really hard to see. I see everybody from the Pacific Northwest is like, "I have some suggestions for you." (Laughter) And you guys, we have tried looking up. (Laughter) But in order to do climate science, we need to see all of the clouds, everywhere, for a very long time. And that's what makes it hard. Now, nothing sees more clouds than a satellite -- not even a British person. (Laughter) And fortunately, we do have satellite observations of clouds that, like me, date back to the 1980s. But these satellites were designed for weather, not climate. They weren't in it for the long haul. So to get that long-term trend information, we need to do climate science. We have to stitch together the output of multiple satellites with different viewing angles and orbits and carrying different camera equipment. And as a result, there are gaps in our knowledge. But even from this very cloudy picture, we're starting to get hints of a possible future. When we looked at the observations, one thing jumped out at us: the clouds are moving. As the planet's temperature increases, high clouds rise up. They move to the colder upper reaches of the atmosphere, and this means that even as the planet heats up, high clouds don't. They remain at roughly the same temperature. So they are not losing more heat to space. But at the same time, they're trapping more heat from the warming planet below. This intensifies the greenhouse effect. High clouds are making global warming worse. Clouds are moving in other dimensions, too. The atmospheric circulation, that large-scale motion of air and water in the atmosphere, is changing, and clouds are going with it. On large scales, clouds seem to be moving from the tropics toward the poles. It's kind of like your grandparents in reverse. And this matters, because if your job is to block incoming sunlight, you are going to be much more effective in the tropics under that intense tropical sun than you are in higher latitudes. So if this keeps up, this will also make global warming worse. And what we have not found, despite years of looking, is any indication of the opposite. There is no observational evidence that clouds will substantially slow down global warming. The earth is not going to break its own fever. Now, there are still uncertainties here. We don't know for sure what the future holds. But we are sending our kids there, and they are never coming back. I want them to be prepared for what they'll face, and that is why it is so important to keep our earth-observing satellites up there and to hire diverse and smart and talented people who do not hate clouds to improve the climate models. But uncertainty is not ignorance. We don't know everything, but we don't know nothing, and we know what carbon dioxide does. I started my career as an astrophysicist, so you can believe me when I say that this is the greatest place in the universe. Other planets might have liquid water. On earth, we have whiskey. (Laughter) (Applause) We are so lucky to live here, but let's not push our luck. I don't think that clouds will save the planet. I think that's probably up to us. Thank you. (Applause)