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The clouds are alive

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"The field collapsed," says David Sands, a bacteriologist now at Montana State University in Bozeman, who began working with syringae in the 1960s. "The people who were in it couldn't get grants anymore." Vali shifted his focus back to cloud physics. Sands continued to study syringae but concentrated on crop diseases, where he could still get funding. The idea that a microscopic, floating biomass was influencing the world's weather was just too weird for most atmospheric scientists.

In 2005 Sands decided the time was ripe to reopen the investigation and find out whether bacteria really do manipulate the weather. Genetic techniques had advanced considerably in the last two decades, so that it was possible to identify even a few ice nucleators like syringae out of thousands of microbes that might inhabit a cloud. Sands contacted Brent Christner, a postdoc at Montana State who had cut his teeth searching for traces of life in 100,000-year-old Antarctic ice, and invited him to expand his search for life into the clouds.

Christner, now at Louisiana State University, started with a simple experiment. He collected snow from mountains around the world and repeated Vali's frozen-drop experiment to look for invisible particles that spawned ice. Once Christner found the drops of water that froze above 20ºF, he treated them with an enzyme that kills bacteria but leaves other particles alone. When he cooled those droplets back down, up to 85 per cent of them no longer froze (pdf).

It was indirect but tantalizing evidence that bacteria were helping the water freeze. But it left an open question: Were airborne microbes forming ice in the clouds, or were they merely getting mopped up by snowflakes as they fell to Earth? The only way to find out was to fly into a cloud to see what was happening from moment to moment.

Prather unwittingly answered the question in late 2007, soon after Christner finished his experiment but before he published it. Prather had spent 15 years developing a device that could suck in microscopic particles from the air and analyze their chemical composition in a few thousandths of a second. She intended the device to study air pollution. But that year she teamed up with Paul DeMott, a cloud physicist from Colorado State University, to see if they could use the machine to identify the particles that form ice in clouds.

Prather and DeMott made a dozen flights through clouds as high as 7,600 metres over the mountains of Wyoming, Colorado, and Montana. The results caught the researchers off guard. They saw the expected soot, sea salt, and plenty of local dust inside the liquid droplets, but those things rarely had ice associated with them. No, the icy patches in clouds contained something else: 50 per cent of the particles were high-titanium dust from the deserts of Asia or possibly Africa, and another 30 per cent seemed to be biological based on their carbon, nitrogen, and phosphorus content.

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