African Science



side titles1. Powell's african journey3. Dusty skies don't rain2. Cancer compound4. Tales of the linguist5. One grand myth


Could a billion tons of airborne dust, smoke and pollution change rainfall?
Powerplants, factories and vehicles on the U.S. East Coast spew soot and other pollutant particles into the atmosphere. The polluted clouds (yellowish-green) contain small water droplets. The cleaner clouds (pink) over Canada have larger droplets that make more rain.
Courtesy Daniel Rosenfeld, The Hebrew University, Israel.

Dry as dust
Between taxes and country club fees, between drought in the Middle East and desertification in Africa, who needs more worries?

We do, especially since Israeli scientists showed how the latter woes are joined in a nightmarish feedback loop.

Satellite view of dust blowing from North Africa across the Mediterranean, July 8, 2000. Courtesy NASA

Tawny cloud drifts across the Mediterranean to Italy and Greece.They say dust storms in the Sahara Desert are interfering with rainfall, which would increase drought, reducing ground cover and causing more dust -- which would further interfere with rainfall.

The impact of dust is not academic, since the stuff blows in the wind for thousands of miles. These satellite images show African dust reaching the Western Hemisphere. Dust originating in China can block the view at California's Death Valley National Monument.

Long dusty road
A lot of dust enters the atmosphere each year: 900 to 1,500 million metric tons just from natural soil dust, not to mention immense amounts of particles from farming, air pollution and smoke.

Evidence that dust inhibits rainfall comes from new research by Daniel Rosenfeld of The Hebrew University in Jerusalem. While dust helps rain droplets to form, he thinks Africa's deserts are supplying too much of a good thing. Due to thermodynamic equations beyond the ken of Why Filers (but doubtless within the ken of math whizzes Ken and Barbie), water vapor molecules can condense and form cloud droplets only on existing particles, primarily dust and smoke.

Dusty but trusty
Rosenfeld and colleagues used satellite data on the size of droplets to compare conditions between dusty clouds and similar, nearby clouds that lacked dust.

With so many condensation nuclei present in the dusty clouds, cloud droplets remained too small to merge into heavy drops that can fall as rain. Predictably, the dusty clouds produced less rain than the comparison clouds.

The chemistry of the particles also played a role, Rosenfeld told us by e-mail. "The size of the drops depends on the amount of water absorbing material in the dust particles." Although many dust particles were large, he adds, "the very small amount of water-absorbing substance in the dust particles made them conducive to creation of only small cloud droplets."

Dust or bust
New England, New York and Pennsylvania show greenish-yellow clouds, north of Lake Erie, pink clouds reveal cleaner air.For perspective, Rosenfeld says smoke from burning vegetation, and urban and industrial air pollution inhibit more precipitation on the global scale. However, in dry regions like the Middle East and the Sahel, deserts may supply the majority of atmospheric dust.

Because global rainfall is fixed and must balance global evaporation, Rosenfeld wrote us, "The reduction of rainfall in the polluted or dusty areas may cause excessive rainfall ... up to many thousands of kilometers away. Therefore, the dust, smoke and air pollution can cause more extremes, more droughts in some areas and more floods in other areas."

Here's another hazard: According to recent speculation, dust-borne pathogens could be whacking coral reefs in the Caribbean.

Barren deserts will continue to emit dust no matter what we do, Rosenfeld says. But air pollution, overgrazing and other destructive agricultural practices are all controllable, at least in theory. However, with the world economy booming, and Africa's surging population placing more pressure on fragile landscapes, we wouldn't bet on a reduction of dust and pollution any time soon.

What does language tell us about African history?



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