The evidence for global warming is patchy but alarming. By the time we know whether carbon dioxide is the villain it may be too late. Clouds are the natural reflectors of sunlight. Even a small improvement in their reflectivity would save us. An improvement might be made if the top layer of a cloud consisted, not of droplets, but of hollow bubbles about the same size. Each would then have four reflecting surfaces rather than two, and would reflect sunlight much better. A bubble, of course, is lighter than a droplet: so a cloud with both droplets and bubbles should have its bubbles concentrated at the top, just where they are needed. And once a bubble has formed, it won't seal into a solid droplet again. Desorbed air will get into the internal gas-space. Furthermore, says Daedalus, a trace of surface-active solute may form a stabilizing internal monolayer.

One of the sources of CO2 is the flaring of natural gas that cannot be got to market. Daedalus wants to blow this waste into a detergent solution. With well-designed bubblers, this would produce a mass of bubbles each about the size of a cloud droplet. Methane is lighter than air, so these tiny bubbles would rise. With luck, they would join the world's clouds. A natural thermocline restricts the height of clouds, and should control their bubbly top layer too. Fortunately, the initial methane is only 'catalytic'. The bubbles will soon start to lose it by outward emission, at the same sort of rate as they gain air by absorption and water vapour by internal release. Water vapour, of course, is almost as light as methane. So in due course all the clouds in the atmosphere will be topped, not by water droplets, but by tiny bubbles.

This new cloud cover should reflect sunlight more efficiently than the old one. Even a minor programme of methanation will get the process going, and once methane bubbling has started in earnest, we should soon see the bubbly-top cloud layer growing. The speed and efficiency of the scheme will be confirmed by reflectance measurement from satellites.

Daedalus cannot guess the useful life of hollow cloud droplets. Like normal cloud drops, they may evaporate completely. Or they may accrete together and fall as a fizzy sort of rain. But with luck they should be stable enough, and last long enough, to reduce or reverse the menace of global warming. Luckily, little detergent need be introduced into the hydrological cycle. Last time, non-biodegradable detergents gave problems.