I don't know enough to either endorse or reject the sunscreen idea. At first sight, I find it unpalatable.
But I fear that responding to global warming is going to mean choosing the least ugly of a whole range of unpalatable options. The fact is that what we're experiencing right now is a top-down disaster. -Paul Krugman
It becomes somewhat more palatable, though, if it's a bridge to a better answer. I expect that projections of ongoing, large CO2 emissions will turn out to be pessimistic because falling costs for solar-energy based electric power and fuels will provide an attractive alternative to burning stuff. At that point, political pressure should be very effective at forcing a switchover.
If solar energy gets cheap enough, as I expect, then we can start thinking about removing surplus CO2 from the atmosphere. Extracting it would require a lot of energy, but not completely overwhelming -- a fraction of human power consumption in 2006 over a fraction of a century. Words and ideas I offer here may be used freely and without attribution.
This NREL map of mean annual PV solar radiation indicates that, at equal latitudes across the southern U.S., the energy available for photovoltaic systems varies by something like 40% because of variations in clouds, haze, and so forth. Photosynthesis should see a similar variation.
A rough estimate of the reduction in sunlight needed to offset warming of 1°C can be made by noting that 1°C is about 1/300 of the absolute temperature. Because of the T^4 scaling of thermal radiation with temperature, this is equivalent (keeping albedo and thermal emissivity constant) to a change of about 4/300 in the input and output radiated power -- about 1.3%. From a photovoltaic power perspective, this would be equivalent to reducing PV cells from (for example) 30% to 29.6% efficiency.
From this rough calculation, it seems that weather variability is a much larger effect. Words and ideas I offer here may be used freely and without attribution.
