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Of course, we can't see or taste CO2.
The thing that really bothers us is the temperature.
So a natural evolution of this was to graduate to temperature targets.
So what does it take to keep Earth's temperature at some given level?
And sort of a benchmark.
So, the European Union has taken 2 degrees C as sort of a target temperature change.
Now, 2 degrees C is warmer than the earth has been in millions of years.
It's warmer than it has been the whole time that civilized
humanity has existed with agriculture, and all that sort of thing.
So a more conservative temperature target is advocated by Jim Hansen,
1 degree C, is sort of, within the natural range of climate variability
over the last, throughout the time period of time covered by the ice cores,
actually the last hundreds of thousands of years.
And if we could keep Earth to within 1 degree C,
that it decreases the odds of provoking some kind of
a slow Earth system feedback like the collapse of ice sheets,
the release of extra carbon from melting permafrost,
and methane hydrates in the oceans, changes in ocean chemistry.
All kinds of sort of scary things that are outside what's
happened in the last half a million years.
We could avoid if we could keep the temperature to 1 degree C or less.
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