OK, I concede the mylar point. I guess I've gotten lazy working with political scientists, compared to whom my physics instincts are usually good enough for rough calculations, and who certainly can't check up on my wild-ass guesses....

On the broader CO2 point, I agree the planet isn't particularly fragile, and if our carbon emissions stopped tomorrow I wouldn't worry about emissions to date messing things up much. Project emissions out over a century or so, where you talk about doubling or tripling CO2 concentrations, and some kind of climate pertubation seems plausible. Even if mechanisms like increased cloud cover prevent much change in mean _temperature_, that still would be a change in _weather_ that we might not like. Ditto, increased plant growth that soaks up CO2 could mean changes in ecosystems that we'd prefer to avoid.