How much sunlight would need to be blocked to cancel out atmospheric CO2? Atmospheric CO2 is trapping too much heat from our star's radiation.  How much light would need to be blocked/reflected before reaching Earth to cancel out the excess heat being trapped by atmospheric CO2?
 A: This may belong to another SE, but based on this, a 2% increase in albedo would roughly offset a doubling of CO2: different models give different numbers and this figure is relatively old in terms of model development & hence quality I suspect.  However it will be in the right ballpark.
Note that you can't fully compensate for CO2: you still get ocean acidification for instance, and you also will get globally weird weather, at least in the engineeringly-plausible approach of dumping a lot of stuff into the stratosphere.  In fact even the 'big mirrors in space' solutions, which are obvious fantasy technically, still do not compensate: the atmosphere with a high CO2 content is just different to that with a lower one.  The best you can do is keep surface temperature under some kind of control.
Surprisingly the stratospheric-aerosol approach is not stupidly expensive: it's expensive but feasible: I think you might need a fleet of 100 planes or something.  You do need to keep doing it though: it's not a once-off fix.
The right search terms to look for are 'Solar Geoengineering' or 'Solar Radiation Management'.
This has traditionally been a slightly taboo area for two reasons.


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*It leads people to think that no other fix is needed: we can just keep pumping out CO2 and fix it by pumping sulphuric acid into the stratosphere.  We can't do that in fact for a bunch of reasons (most of which I don't know: I'm not boing coy).

*It might be weaponizable: if you can mess with the climate over some part of the world where people you don't like live you can perhaps engineer harvest failures & the resulting starvation.
I think the current evidence (based on modelling how stratospheric aerosols behave) is that it's not weaponizable as you can't localise the effects, and I personally think it might be a reasonable approach to deal with the likely 'overshoot' scenario where CO2 emissions get brought under control but there's a very nasty 50-year window before the system responds.
