If we want to 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 is what the comparison looks like.
The solid line here is the global average composite temperature
that has been measured. And then there are two different fields
plotted along there. The one in salmon color are the
result of climate models that have been driven with human
climate forcing, and also natural forcing. The human forcing includes
the greenhouse gases, methane, CO2,
and other greenhouse gases,
and also the cooling impact from the sulphate aerosols, which is, actually,
more uncertain than the greenhouse gas warming, but put all that together.
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.
You put all of that together, and drive a 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 forcings,
the volcanoes and the the intensity of the sun,
the models don't get that warming since the 70s.
That's it. That's the smoking gun.
That's the smoking gun for human impact on Earth's climate.
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.
Some examples of processes
that could have caused that warming that we might not be able to detect include
(1) Maybe there was a change in the rate of cosmic rays hitting the atmosphere,
because of a change in the shielding of
the Earth by the magnetic field of the sun, for example.
Cosmic rays could act as cloud
seeds the same way that they used
to use cloud chambers 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 makes a track, actually, of little cloud particles.
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 in ultraviolet light,
which doesn't just come down to the ground and put its
heat on the ground's 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,
a green house gas, and it is very important for determining the
way that the air in the stratosphere, in particular where this ozone is, circulats.
And so ozone is a very fundamental player in the atmosphere.
You know, what about these things?
Maybe this is what's going on.
The way I like to think about this is in terms of a murder mystery.
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,
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 and he's got his prints on the
gun, and you know why he did it,
and it all 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 that 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.
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 this rising CO2 concentrations would not lead to the warming that expect.
The climate sensitivity of the earth, how much the earth
should change for changing CO2, is a fairly well-constrained parameter.
And if you want to throw that out the window, you have to explain why.
There are no climate models that don't
predict a significant amount of warming if the CO2 concentration continues
to rise. The only 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.
It looks to me like the chauffeur is fairly safe.