There is geoengineering to remove CO2, which is a dubious proposition
energetically but which would be absolutely fantastic news if someone
could somehow get it to work.
And there is geoengineering to deflect solar radiation. My impression
is that it's almost certainly a bad idea. If someone could get that
to work, it could prevent the mean surface temperature of the earth
from rising, but it could not balance out the local forcings.
Anthropogenic global change is the problem, and global warming is only
a symptom.
The idea of crudely cancelling out a growing perturbation with another
one seems to miss the point, as well as being fraught with peril, like
treating alcoholism with aspirin.
As symptomatic relief, the cost of solar umbrellas of various sorts
should not be compared with the cost of reducing emissions or the cost
of increasing sequestration. If a case can be made that such an action
would protect the ice caps preferentially, for instance, its cost
might be compared against the cost of sea level rise. But stabilizing
greenhouse gas concentrations stabilizes the system. Adding a contrary
perturbation does not.
We'd still be kicking the climate system with increasing
perturbations, and we'd be moving farther away from any paleoclimate
analogs, so we'd be increasingly less certain of what we were doing.
And if people took this as a license to keep emitting greenhouse
gases, the solar perturbation would have to keep increasing apace,
continually increasing the risk.
Global mean temperature is a symptom, albeit an important one. Nobody
lives in a global mean climate, so stabilizing global mean temperature
isn't in itself a useful goal; it in no way guarantees an end to
anthropogenic climate change.
mt
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