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If we want to attribute any changes in 
temperature, that have been measured by 

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these various techniques, to a human impact 
on climate, we need to compare 

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the measurements to what the climate 
models predict. 

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And this is what the comparison looks 
like. 

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The solid line here is the global average 
composite temperature 

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that has been measured. 
And then there are two different fields 

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plotted along there. 
The one in salmon color are the 

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result of climate models that have been 
driven with human 

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climate forcing, and also natural forcing. 
The human forcing includes 

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the greenhouse gases, methane, CO2, 

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and other greenhouse gases,

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and also the cooling impact from the 
sulphate aerosols, which is, actually, 

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more uncertain than the greenhouse gas 
warming, but put all that together. 

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The natural forcings are 
volcano, volcanic eruptions 

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that put aerosols in the stratosphere, and 
tend to cool things down,

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and any variability in the intensity of 
the sun can also impact 

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Earth's climate. 

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You put all of that together, and 
drive a climate model with it,

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and it can reproduce this warming of 
temperatures since about 1970. 

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Whereas if you run the same models, and 

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you tell them only about the natural 
climate forcings, 

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the volcanoes and the the intensity of the 
sun, 

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the models don't get that warming since 
the 70s. 

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That's it. 
That's the smoking gun. 

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That's the smoking gun for human impact on 
Earth's climate. 

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One perspective on this was a fairly 
nasty quote, that 

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climate scientists blame CO2 because they 
can't think of anything else. 

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And it's totally true that there are all 
kinds of 

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things that could surprise us in the 
earth's climate system. 

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It's a very complicated and subtle thing. 

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And we certainly don't know absolutely 
everything. 

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Some examples of processes 

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that could have caused that warming that 
we might not be able to detect include 

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(1) Maybe there was a change in the rate of 
cosmic rays hitting the atmosphere,

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because of a change in the 
shielding of 

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the Earth by the magnetic field of the 
sun, for example. 

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Cosmic rays could act as cloud 

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seeds the same way that they used 

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to use cloud chambers at particle 
accelerators -- 

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a ray would go through, and the air is 
super saturated in water vapor, but 

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it doesn't have anything to condense on 
and so it just stays in the vapor phase. 

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But then when the cosmic ray 
goes through there, the 

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particle makes 
a track, actually, of little cloud 
particles. 

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Maybe something like that changed the 
cloudiness of the 

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planet in a way that would be hard to 
detect. 

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And it's very hard to model clouds, as 

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you remember, because the processes that 
control them are 

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taking place on a spatial scale which is 
much 

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smaller than the grid resolution of the 
climate models. 

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Another possible effect is that when the 
sun 

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gets brighter and dimmer, most of the 
change in the intensity of the sun is 

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not in the visible light that we can see, 
but it's actually in ultraviolet light, 

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which doesn't just come down to the 
ground and put its 

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heat on the ground's surface, like 
we've been talking about sunlight. 

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It gets absorbed in the upper atmosphere. 

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It produces ozone, which is an oxygen 
molecule with three oxygen atoms, 

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a green house gas, and it is 
very important for determining the 

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way that the air in the stratosphere, in 
particular where this ozone is, circulats. 

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And so ozone is a very fundamental player 
in the atmosphere. 

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You know, what about these things? 

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Maybe this is what's going on. 

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The way I like to think about this is 
in terms of a murder mystery. 

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Say you walk in and you catch the 
butler and 

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he's got a smoking gun in his hand and 
there's a dead 

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guy on the ground, right next to 
him, 

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so it looks to all the world like the 
butler did it. 

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And it's a really important case and so 

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you check and you make sure 
it's the 

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butler's gun and he's got his prints on 
the 

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gun, and you know why he did it,

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and it all checks out and 
makes perfect sense. 

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You gotta spend all your time writing 
these 

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silly reports to keep your boss happy, 
right? 

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But say your partner is kind of a 
contrary kind of 

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a guy, and he says, no, I think the 
chauffeur did it. 

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And I think you're just not smart enough 
to figure out how the chauffeur did it. 

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And you check it out, and it doesn't 
seem like he did it. 

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He's got an alibi. 

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He wasn't there, but it is very hard to 
prove a negative. 

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But the point is that if you wanted to 
convict 

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the chauffeur of that murder, you'd have 
to unconvict the butler. 

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You'd have to explain what the butler was 
doing with that smoking gun in his hand. 

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Translating out of this somewhat 
tortured analogy, if 

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you want to claim that the warming since 
the 70s 

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was caused by cosmic rays or something 
like that, and 

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not caused by rising CO2 concentrations, 
you would have to 

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explain why this rising CO2 concentrations 
would not lead to the warming that expect. 

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The climate sensitivity of the earth, how 
much the earth 

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should change for changing CO2, is a 
fairly well-constrained parameter. 

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And if you want to throw that out the 
window, you have to explain why. 

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There are no climate models that don't 

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predict a significant amount of warming if 
the CO2 concentration continues 

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to rise. 
The only climate denier position, 

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is to say, well, what about this, what 
about this, you didn't think about that. 

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There's not a substantive model that can 
explain the way the 

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Earth works today, that doesn't predict 
that it will warm up significantly 

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with rising CO2 concentrations. 

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It looks to me like the chauffeur 
is fairly safe.