Research into the possibility of engineering a better climate is progressing at an impressive rate-and meeting strong opposition. Geoengineering is shorthand for the idea of fixing the problem of man-made climate change once the greenhouse gases that cause it have already been emitted into the atmosphere, rather than trying to stop those emissions happening in the first place. Many people think such ideas immoral ... and certain to provoke unintended consequences, to boot. Over the past few years research in the field has boomed. What is sometimes called Plan B seems to be taking shape on the laboratory bench-and seeking to escape outside. Polluting the stratosphere. Liming the oceans. Locking Greenland's glaciers to its icy mountains. It is easy to see why sceptics balk at geoengineering. And if viewed as a substitute for curbing greenhouse-gas emissions, a cover for business-as-usual into the indefinite future, then it might indeed prove a Faustian bargain. Fertilising the ocean with iron has already been tried-admittedly without much success, but also without perceptible harm being done. The harm done by stopping geoengineering experiments is that the good which might come from them will never be known. Yet even some enthusiastic researchers worry about undue haste. Geoengineering's growth spurt will need to be matched by some grown-up questioning. Who benefits? Who decides? Who faces the risk?