Models: How to See More by Looking at Less

Hello everybody, and welcome to this Internet course on global change and water resources in mountain regions. We'll be looking at very numerous aspects, and we'll start this course with some basics about climate and climate change. Now climate, we talk about it quite a lot in the media and in scientific circles, but what exactly is climate? Well, the climate is not just the atmosphere, even if the atmosphere is possibly the most obvious element of the climate system. Climate is in fact driven by many different interacting elements, like the oceans, the polar ice fields, sea ice, also the biosphere. That is to say, continental vegetation and life in the oceans are also a part of the climate system. And last but not least, we also have what we call the hydrosphere. That's the water cycle, evaporation at the surface of the oceans, and water coming back in the form of precipitation. So we'll see some of these elements as we go along in this course. Now if we think about the climate in energy terms, we see that climate is in fact a massive planetary thermodynamic engine. We have energy coming from the sun. And part of this energy is reflected back to space according to the texture, the color of the surfaces, as we see on this satellite image of the globe. So what will be available to heat the surface is what is coming in minus what is going out. And then we have the earth that radiates energy back to space in the form of what we call infrared radiation, that is to say, heat radiation, and it obeys the laws of physics. Anybody at a particular temperature will emit radiation at a certain wavelength. Now an obvious feature of this satellite image is that the earth is a sphere in space, which means that the way solar radiation will be intercepted at the surface of the earth will change according to where we are on the surface. If we're close to the equator, the sun's rays are almost vertical, so you have a large amount of energy that is concentrated on fairly small surface areas. Whereas, as you go towards the poles, it's the same quantity of energy, but distributed over larger and larger surface areas, simply because the sun's rays become more and more tangential with respect to the earth's surface. So in other words, as we go towards the poles, the efficiency of solar energy heating declines. So it's obviously colder at the poles compared to the equator. So basically, what the climate system is trying to do is to rebalance the energy differential between the equator and the poles. So it's going to try and transfer energy from the equator, where there's an excess of energy, towards the mid and high latitudes, where there's a deficit of energy. We're now going to see what are the main mechanisms that are capable of transferring this energy from the tropical regions to the polar regions. So this third view graph actually shows us the energy balance at the earth's surface as we go from the South Pole to the left, through the equator in the middle of the graph, to the North Pole to the right. And what we see is that we do have, indeed, an excess of energy in the tropical zone, so roughly between 30 degrees north and 30 degrees south. That is to say, we have more energy being accumulated from the sun than energy being lost by the earth back to space. And as we go beyond 30 degrees north or south compared to the equator, then we have a net deficit of energy, in the sense that we have less solar energy coming in and more terrestrial energy going back to space. So this is basically the earth's energy balance, and this is what the climate system needs to try and balance out. To try and remove the energy from the red areas on this graph and transfer some of this energy to the blue areas on this graph.