We may indeed already have the technological fix, though mysteriously it receives little attention. Sulphur dioxide, the cause of acid rain and the poster child for cap and trade--because the cap and trade program for sulphur dioxide has been a big success--is the opposite of a greenhouse gas: it cools the atmosphere by reducing the amount of sunlight that reaches the earth's surface. Injecting relatively small quantities of sulphur dioxide into the atmosphere would offset the effect of atmospheric carbon dioxide in heating the earth's surface. The opposition of environmentalists to using a pollutant to combat global warming and therefore seeming to approve of pollution, and concern with the bad effects of increasing the amount of sulphur dioxide in the atmosphere (effects that might not be limited to a modest increase in the amount of acid rain), have thus far kept this option from serious consideration in political circles.