I think having a constraint in mind will be very helpful in driving policy.
At this point, the gap between what is necessary and what is possible
appears narrow enough that we should choose the tightest constraint
that might be feasible.
Logically, I think it makes most sense to target cumulative emissions,
rather than concentrations or global mean temperature change, both of
which have some associated uncertainty (though the focus on the latter
these days seems misguided; the carbon cycle uncertainties are a much
bigger deal at this point).
However, that's a hard concept to get across in public discussions. It
brings in all sorts of subtleties that obviously can be used to
confuse the issue and that aren't really well handled in public
discourse. So, it may be better to work backward from the tightest
constraints to what we hope those constraints might achieve; that
might well be a global mean temperature change because it's an easy
concept to understand.
The consequences of any of these constraints taken seriously would be similar.
The point is that the time for chitchat is over, and we need to get to
work. Picking a target that is probably going to be missed (say,
emissions probably leading to 2C) is the only way to motivate
sufficient effort to limit the actual outcome to not much more than
3C, which is already in risky territory.
mt
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