[MUSIC] So if we wanna attribute any changes in temperature that have been measured by these various techniques to a human impact on climate, we need to compare the measurements to what the climate models predict. And this si what the comparison looks like. The solid line here is the global average composit temperature that has been measured and then there are two different fields plotted along there. The one in sort of salmon color are the result of climate models that have been driven with human climate forcing and also natural forcing. So the human forcing Includes the greenhouse gases, methane, CO2 and other greenhouse gases and also the cooling impact from the sulfate aerosols, which is actually more uncertain than the greenhouse gas warmings. But put all that together and then the natural forcings are volcano. Volcanic eruptions that put aerosols in the stratosphere and tend to cool things down and any variability in the intensity of the sun can also impact Earth's climate. So you put all that together into drive climate model with it and it can reproduce this warming of temperatures since about 1970. Whereas, if you run the same models and you tell them only about the natural climate forcing, the volcanoes and the intensity of the sun, the models don't get that warming since the 70's. So that's it. That's the smoking gun. That's the smoking gun for human impact on Earth's climate. [SOUND] So one perspective on this was a fairly nasty quote that climate scientists blame CO2 because they can't think of anything else. And it's totally true that there are all kinds of things that could surprise us in the Earth's climate system. It's a very complicated and subtle thing and we certainly don't know absolutely everything. So some examples of processes that could have caused that warming that we might not be able to detect include maybe there was a change in the rate of cosmic rays hitting the atmosphere. Because we're in a change in the shielding of the earth by the magnetic field of the sun, for example. Cosmic rays could act as a cloud seeds, the same way that they used to use cloud chambers to at particle accelerators. A ray would go through and the air is super saturated in water vapor, but it doesn't have anything to condense on and so it just stays in the vapor phase. But then when the cosmic ray goes through there the particle goes through it, it makes a tract, a little cloud particle. So maybe something like that changed the cloudiness of the planet in a way that would be hard to detect. And it's very hard to model clouds as you remember because the processes that control them are taking place on a spatial scale, which is much smaller than the grid resolution of the climate models. Another possible effect is that when the sun gets brighter and dimmer most of the change in the intensity of the sun is not in the visible light that we can see but it's actually an ultraviolet light. Which doesn't just come down to the ground and put its heat on the ground surface like we've been talking about sunlight. It gets absorbed in the upper atmosphere. It produces ozone which is an oxygen molecule with three oxygen atoms. Which is the greenhouse gas and it is very important in determining the way the air in the stratosphere, in particular where this ozone is, circulates. So ozone is a very fundamental player in the atmosphere. So what about these things? Maybe this is what's going on. Well, the way I like to think about this is in terms of sort of a murder mystery. So say you walk in and you catch the butler and he's got a smoking gun in his hand and there's a dead guy on the ground right next to him, and so it looks, to all the world, like the butler did it. And it's a really important case, and so, you check, and you make sure, it's the butler's gun, he's got his prints on the gun, and you know why he did it. And it all sort of checks out and makes perfect sense. You gotta spend all your time writing these silly reports to keep your boss happy, right? But say your partner is kind of a contrary kind of a guy, and he says, no, I think the chauffeur did it. And I think you're just not smart enough to figure out how the chauffeur did it. And you check it out, and it doesn't seem like he did it. He's got an alibi, he wasn't there. But it is very hard to prove a negative. But the point is, if you wanted to convict the chauffeur of that murder, you'd have to unconvict the butler. You'd have to explain what the butler was doing with that smoking gun in his hand. So translating out of this somewhat tortured analogy, If you want to claim that the warming since the 70s was caused by cosmic rays or something like that. And not caused by rising CO2 concentrations, you would have to explain why the rising CO2 concentrations would not lead to the warming that we expect. The climate sensitivity of the Earth, how much the earth should change for changing CO2 is a fairly well constrained parameter. If you wanna throw that out the window, you have to explain why. So there are no climate models that don't predict a significant amount of warming if the CO2 concentration continues to rise. The only sort of climate denier position is to say, well, what about this, what about this, you didn't think about that. There's not a substantive model that can explain the way the Earth works today that doesn't predict that it will warm up significantly with rising CO2 concentrations. So it looks to me like the chauffeur is fairly safe. [MUSIC]
