How much CO2 is too much?
The first question is why are we concerned in particular
about CO2, rather than, say, methane, which is another greenhouse gas?
The answer to that is that CO2, as you know, accumulates in the biosphere,
whereas methane is a transient greenhouse gas
that breaks down in about a decade.
The long-term climate change that we're worried
about is mostly driven by CO2.
I've seen evolution of
peoples' thinking on this over the decades.
In the 1980s and 90s,
people were talking in terms of stabilizing atmospheric CO2 concentration,
trying to come up with how much CO2 we can release
that can make the concentration rise, and then plateau at some level,
like 750 parts per million,
or 550 (doubling preanterpagenic value of about 280),
or 450.
When you release CO2 to the atmosphere,
some of it dissolves in the ocean, and some of it goes into the land biosphere,
and only some of it continues to reside in the atmosphere.
There were efforts to use carbon cycle models to figure out,
given some set of emissions, what the concentration would be.
Figuring out how to follow a
stabilization scenarios was another
inverse problem, like the bore hole temperatures, where you had to use the
model to go backwards from the effect back to the cause.
This was a little dodgy because slight changes in the
trajectory could have very large changes in what they said we
could release this year or next year. It's kind of a confusing situation.