Well, first off this energy would be initially replacing energy that was previously coming from fossil fuels, so on account of that not much. But, as Stross po

Well, first off this energy would be initially replacing energy that was previously coming from fossil fuels, so on account of that not much. But, as Stross points out there is a 70% loss in transmission, and presumably most of that ends up as heat in the atmosphere. Now as to the question of how significant is the waste heat from that energy? I once did a ball-park calculation in which I determined that something like 4% of global heating comes not from greenhouse effect but from waste heat[1]. If that's right, then if we replace all of our energy usage with space-based solar of which only 1/3 makes it to earth and the rest is turned to waste heat we'd be be down to about 12% of current (combined green-house and waste heat) global heating. That's seems like a huge win, and we'd have enough energy to spare to start extracting CO2 from the atmosphere until we even compensate for that. [1] I can't find my calculations right now, and I'd love for someone to independently check that.