Into the great wide open.docVIP

Geoengineering the climate Into the great wide open Scientific studies of techniques for deliberately modifying the climate are getting ready to move out of the laboratory IN 1990 John Latham, a cloud physicist, published a short article in?Nature?under the headline “Control of Global Warming?” It argued that if low-lying maritime clouds were made a bit brighter, the Earth could be cooled enough to make up for the increased warming caused by emissions of greenhouse gases. The brightening was to be achieved by wafting tiny sea-salt particles up into the clouds from below; by acting as “cloud condensation nuclei” (CCN) they would increase the number of water droplets in the clouds, and thus the amount of sunlight they reflect out into space. Latham calculated that a square kilometre of cloud might be kept bright with just 400 grams of spray an hour. And finding out if it was really that easy might be straightforwardly tested. “It seems feasible”, Dr Latham wrote, “to conduct an experiment in which CCN are introduced in a controlled manner into marine stratus.” A quarter of a century on, such a test may soon be on the cards. For more than ten years Dr Latham’s idea was almost entirely ignored. Then it caught the attention of an enterprising engineer, Stephen Salter of the University of Edinburgh, who looked at ways it might be made practicable, and a small number of researchers started to pay attention. But the question of whether anyone could actually produce ship-borne sprayers that would reliably churn out particles a ten-thousandth of a millimetre in diameter at a rate of 1,000 trillion a second remained open. Armand Neukermans, a retired Silicon Valley engineer whose achievements include, among other things, the development of the earliest inkjet printers, has with various colleagues (also mostly retired) looked at a range of possible techniques. One that may be up to the job is “effervescent spray atomisation” in which, rather than trying to make truly tiny dropl