We can't see or taste CO2.
The thing that really bothers us is the temperature.
A natural evolution of this was to graduate to temperature targets.
What does it take to keep earth's temperature at some given level?
some kind of a benchmark? The European Union has taken 2 degrees C
as a target temperature change.
Two 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,
so a more conservative temperature target is advocated by Jim Hanson.
One degree C is within the natural
range of climate variability throughout the
period of time covered by the
ice cores, the last hundreds of thousands of years.
And if we could keep Earth to within one degree C, 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 ocean, changes in ocean chemistry.
All kinds 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 one degree C or less.