SMOKE from forest fires can stop clouds forming raindrops, new satellite
observations have shown. Experts warn that this may lead to unexpected effects
on the world鈥檚 climate.
Climatologists looking at the effects of forest fires鈥攕uch as those
that blackened the skies over Southeast Asia in recent years鈥攈ave till now
focused on the cooling that occurs when smoke blocks sunlight. Smoke particles
can also help water droplets form, increasing cloud cover (New
快猫短视频, 9 July 1994, p 37). But it was unclear how this would affect
rainfall.
Now Daniel Rosenfeld, an atmospheric scientist at the Hebrew University of
Jerusalem, has shown that smoke tends to prevent rainfall. He came to this
conclusion after studying simultaneous visual, infrared and radar observations
from NASA鈥檚 Tropical Rainfall Measuring Mission (TRMM) satellite as it passed
over Borneo in March 1998, when smoke-polluted clouds covered half the
island.
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Rosenfeld found that dense smoke completely turned off normal tropical
rainfall. The smoke filled the clouds with tiny particles that made water vapour
condense. However, the moisture was divided among so many droplets that they
were too small to fall as rain (Geophysical Research Letters, vol 26, p
3105). 鈥淔or certain types of clouds, heavy smoke blocks precipitation
altogether,鈥� Rosenfeld says. At the same time, smoke-free clouds over the other
half of the island produced and ample rainfall.
Smoky tropical clouds can yield some rain, Rosenfeld found, but only if they
rise high enough for water to freeze. He suspects fires are at least partly
responsible for the decline in rainfall in the tropics seen over the past
century (快猫短视频, 4 March 1995, p 18).
Christian Kummerow, a TRMM project scientist at Goddard Space Flight Center
near Washington DC, says similar effects seem to be occurring in other parts of
the world. 鈥淲hen we look, we see them everywhere,鈥� he says. He expects TRMM
observations to show that smoke causes strong regional effects, and possibly
global ones. 鈥淗ow big they are on the global scale is still up for grabs,鈥� he
says.
Hans Graf, a climate researcher at the Max Planck Institute for Meteorology
in Hamburg, adds that anything which changes rainfall is important鈥攏ot
only because of the risks of droughts or floods. Most of the energy that powers
the planet鈥檚 weather systems is released into the atmosphere when water vapour
condenses and rains out. 鈥淎bout two-thirds of the energy deposited in the
atmosphere depends on the formation and fallout of precipitation,鈥� says Graf.
鈥淚t鈥檚 the main effect that drives atmospheric circulation.鈥�
Graf has used the TRMM findings to estimate the effects of the annual burning
of vegetation鈥攂oth accidental and deliberate鈥攐n the climate. It
turns out that the global effects are similar to those produced by the eruption
of Mount Pinatubo in 1991, such as warming at high latitudes in winter
(快猫短视频, 10 August 1996, p 18).
鈥淚 was astonished to see something so
strong,鈥� says Graf. 鈥淚t鈥檚 an effect which has been overlooked for the last 20
测别补谤蝉.鈥�