One of our most important tools for understanding how the climate system works, where we could be going and what happened in the past is through the use of climate models. Now, what is a climate model? A climate model is computer code, that's all it is. You can't hold a climate model in your hand. What they are, are physically-based mathematical representations of the climate system. Now they certainly have shortcomings, there are far from perfect. But they still are very key tools for understanding climate and they are very, very useful. What I'm showing here is just kind of a schematic of how a climate model works. Basically what we do is divide the globe up into a series of grid points, usually by latitude, or longitude, or there's other ways to look at it. And also in the vertical, so in a climate model you may have grid points at every degree, latitude and longitude. And let's say 40 levels in the vertical, something like that. And what we're doing in a climate model is treating the exchanges of heat, momentum, and moisture between all the grid cells and the interactions between the atmosphere and the land surface below. And the atmosphere and the ocean surface below, as well as the interactions between the land surface and the ocean. They're really meant to capture all of the physics that are actually at work in the climate system. Now, millions and millions of calculations per second are done in a climate model. But as I say what it is, it's just computer code. What I'm showing here is just a little snippet of what a climate model actually works with. This is just the computer code, this is just one tiny, tiny piece. These climate models have millions of lines of code. This one here, I'm not sure which one it is, but it's written in Fortran, that's what I was brought up myself on Fortran. But they're just computer code, and I think that's the thing we have to emphasize, you can't hold a climate model in your hand. So yes, they live in supercomputers. So here's a supercomputer and I'm not sure which one this is. But yeah, this is where climate models live, they live inside supercomputers. Now, here's a question for you, when were global climate model first developed? The answer is in the 1970s, they've been around for a long time. And as we've gotten more and more and more computing power, climate models have become increasingly complex and handle additional physics in a much more robust way. Now what I'm trying to show here in this image is the increasing climate model components and complexity. If you look in the 1970s, these climate models were dealing the atmosphere. They were dealing the land surface, and the ocean and sea ice with some level of what we call coupling between them. Now, as we move on into the mid 1980s and into FAR which is shown here, which is the first assessment report of the IPCC. We're dealing with the atmosphere and land surface in the oceans and sea has spread a more complex level, more interactions. It's trying to show that by basically the height of those columns. By the second assessment report of the IPCC, we started to include treatment of aerosols in the atmosphere. And we know how important aerosols are, for example through the effects of volcanic eruptions. And then as we move on in time, we start to include carbon cycle, all right? How the carbon cycle works, the interactions between the atmosphere and the ocean, and the land that influence the exchanges of carbon within the system. And then we started to include dynamic vegetation. For example, if we go through time into the future, vegetation actually changes. Of course that has an influence that's all kind up into the carbon cycle. Changing vegetation also does things like change the albedo, it changes evapotranspiration. And then we start to include atmospheric chemistry. And where we are today is that we're coupling the atmosphere, the land surface, the ocean and sea ice. Were including aerosols, the carbon cycle ,dynamic vegetation, atmospheric chemistry, and now land ice. In other words, our climate models now are starting to deal with the interactions of ice sheets. So these are very, very, very complex models. Now there imperfect, right? George P Box, a statistician had this famous saying all models are wrong. Some are useful, it's as correct today as it was back, when he said it. Yeah, these are imperfect representations, because the climate system it really is so incredibly complex. We can capture a lot of it in climate models, but we can't hope to capture all of the interactions at the resolution that we need. But they're still pretty darned useful. Now one of the things we can do with climate models, is try and understand the role of humans in climate change. We can do what are called sensitivity experiments, where we take our climate model, and then what we do is we put in known climate forcings. And that would be like the increase in carbon dioxide, the known aerosol loading of volcanic events, and things like that. Well, and then we only include in another set of runs, just the natural factors. In other words, you don't take the rise in CO2, 'cause that's us, but you just put in the effects of, say, volcanic eruptions and things like that. Solar variability as well. And then we try and see what happens, when we run these models over the historical period of observations. Going from about 1900, and this one goes through about 2010. And you see on the green, that's what's happening with natural factors only. And there's a range there, you see this is spread there, because we're running these models in numerous times. What we call ensemble runs to try, and capture the natural variability in climate, which is always there. So there's that's spread there in the green, and now the one in the blueish that's from natural and human factors. Notably the increase in carbon dioxide levels, and the black line there is the observations themselves. And what we see here, is that in less we include the natural and human factors, notably the increase in CO2. We cannot reproduce the observed changes in temperatures. This is really telling us very, very strong evidence, that the climate change we've seen say, over the past century and more is us. Now, a key thing with climate model projections of the future, is the scenario of future greenhouse gas growth. You can as someone asked you, well, what are temper Arctic temperatures going to look like? Is it going to be the rise in Arctic temperatures by the year 2100? It's a poorly phrased question, because the question should be with a given greenhouse gas scenario or assumption. What will be the temperature change? Because the point is, the future is up to us. So what we've done in looking at the future, is come up with a series of greenhouse gas growth scenarios. And these range from RCP 2.5, and don't worry with RCP, reads representative concentration pathway. It's not really important to know that, but RCP 2.5 is a modus greenhouse growth scenario. That we go up and then we bring on alternative technologies, and we greatly reduce our carbon emissions. All the way up to RCP 8.5, which is basically the bird baby burn scenario, or just going to keep burning up all the fossil fuels. And so the future is really going to depend, on which of these sorts of scenarios we follow. Now these were the two add members, RCP 2.5 and RCP 8.5, maybe we follow RCP 6, which is kind of in the middle. But let's ask ourselves, which emission scenario are we actually currently following at the moment? And the answer is, we're close to RCP 8.5, maybe we'll get a little smarter, and take that down to RCP 6 or 4.5, but right now? But when I understand, we're pretty much on that bird baby burns scenario. Now, we can look at things like the change, or trying to our projections of Arctic sea ice extent. That's a very very active area of modeling, and what we can do here is we can look at hindcasts and projection. So with the hindcast we're taking past, in this case 1900 to the year 2000. We're putting in our best assumption, or best knowledge of what the climate forcings were. And we're running a whole bunch of different models, on often different models. A whole bunch of different times, and we come up with these hindcast and that in the grey. And basically the average of all of hindcast is in that black line, you see this is spread there, because there's a lot of natural variability. And then starting in this case around the year 2000, we start our climate model runs with these different emissions scenarios. The red one being RCP 8.5 for example, in the green one being RCP 2.6. And we've seen it with the 2.6 we end up say by the end of the 21st century with a lot less sea ice than we have today, but still some. Now I should have said this is September ice extent. This is September, ice extent. Now, RCP 8.5 we lose our ice. Now, as I said this a spread here you see that spread in the red. This is saying that some climate models are saying we drop below a million square kilometres of ice in September essentially ice free sometime in the 2040s. Some are saying at as much later. Now in the dark, dark line, the heavy dark line those are the observations. So what we see is that the climate models overall are capturing the observed downward trend in Arctic sea ice extent. Maybe not the magnitude of the trend, but they're all saying we should be going down in terms of sea ice extent and in fact, we are. So this is another illustration that the future really belongs to us. Here's another example, here's arc RCP 2.6 and RCP 8.5 of the change in average surface temperature between the years 1986 and 2005, these are model years, and basically 2081 through 2100. So we're comparing these two periods near the end of this century and 1986 to 2005. Now the one on the left is RCP 2.6, a modest greenhouse gas growth scenario, and basically we level off in those emissions. And what we see is in those yellows, those warm colored, yeah, were warming up. But not too much, you know, what degree, a degree and a half, two degrees? You see some evidence of Arctic amplification, which is something, of course, that we observe. All little dots in there, those are just some indication of how many of the different models that this is based on are agreeing in terms of the sign of the change. And there's a lot of dots there saying they were all agreeing that we're going to warm up. Now the one on the right is the RCP 8.5 that aggressive burn baby burn scenario, and a lot more red, right? But now look at the Arctic. Something like 11 degrees C increase compared to that 1986 to 2005 period. That's pretty remarkable. We do not want to go there. I guarantee you we do not want to go there, but it's another illustration or another example of how the future is really up to us. So with that, I hope we've learned a little bit about climate models and their applications. And in the couple other videos here we'll be looking more closely at some of these projections of things like temperature and precipitation. Thank you.