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.