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SOME farmers may be stoking a vicious cycle of desertification, making arid
fields even more parched. Satellite observations suggest that airborne dust from
deserts can stop rain from falling鈥攖he exact opposite of what
meteorologists expected.

The effect may already be influencing the Sahel region of Africa, where herds
of grazing livestock kick up huge amounts of dust, warns Daniel Rosenfeld of the
Hebrew University in Jerusalem. 鈥淵ou would expect that feedback to contribute to
the drying over that part of the world.鈥�

Clouds form when water vapour in rising air condenses on tiny particles or
鈥渃ondensation nuclei鈥�. But it only rains when the droplets grow large enough to
fall. If something stops them from reaching that critical size, clouds can
evaporate without yielding a drop.

Most cloud-forming particles come from desert dust, smoke from burning
vegetation, and aerosols from air pollution. But earlier studies have shown that
the large numbers of tiny particles in smoke and pollution aerosols actually
suppress rainfall: the limited amount of moisture is divided among many
condensation nuclei, producing lots of tiny droplets instead of larger droplets
that fall.

Desert dust particles are larger, however, and computer models had predicted
that these would seed larger droplets, increasing rainfall. This would partly
offset the reductions in rainfall caused by the tiny smoke particles and
aerosols. Yet satellite observations of clouds that formed over dusty and
dust-free regions of the eastern Mediterranean during a heavy dust storm have
now showed this prediction is false.

The clouds that formed in dusty zones mainly contained small droplets, less
than 14 micrometres across, that weren鈥檛 big enough to fall as rain, Rosenfeld
found. Only clouds at high alti-tudes where ice could form contained large
enough droplets. Dust-free clouds, by contrast, produced droplets bigger than
the threshold needed for rain to fall.

When Rosenfeld鈥檚 team analysed particles of desert dust, they found they
contained little or no soluble material. He believes this makes them collect
water less efficiently, although they could still accumulate ice at high
altitudes to produce some precipitation. Earlier computer models, says
Rosenfeld, assumed dust particles were partly soluble, leading to the prediction
of larger droplets.

The observations add to evidence that humans are damaging the planet鈥檚
ability to cycle water, Rosenfeld says. 鈥淚鈥檇 be sceptical that positive
feedbacks of desert dust are the control mechanism for desertification. But it
may be crucially important once land degradation processes begin,鈥� says Lenard
Milich of the UN鈥檚 World Food Programme. Rosenfeld believes such changes will
have a larger impact on people than global warming.