Solar geoengineering rests on a simple idea that it is technically possible to make the Earth a little more reflective so that it absorbs a little less sunlight, which would partly counteract some of the risks that come from accumulating carbon dioxide in the atmosphere. When I say technically possible, it appears that at least doing this in a crude way is actually easy, in the sense that it could be done with commercial off-the-shelf technologies now, and it could be done at a cost that is really trivial, sort of a part in a thousand or a part in ten thousand of global GDP.
