Zeev's Turnips Talk Politics (ZTTP)

[tlc]

http://www.pbs.org/wgbh/nova/transcripts/3310_sun.html

A 10-percent fall in sunlight meant that particle pollution was having a far bigger effect than anyone had thought possible.

VEERABHADRAN RAMANATHAN: Our models led us to believe the human impact on the dimming was close to half to one percent, so what we discovered was 10-fold.

NARRATOR: INDOEX showed that the particles of pollution were blocking some sunlight themselves. Even more significant was what they were doing to the clouds. They were turning them into giant mirrors.

Clouds are made of droplets of water. These form when water vapor in the atmosphere starts to condense on the surface of naturally occurring airborne particles, typically pollen or sea salt. As they grow, the water droplets eventually become so heavy they fall as rain.

But Ramanathan found that polluted air contained far more particles than the unpolluted air, particles of ash, soot and sulfur.

VEERABHADRAN RAMANATHAN: We saw 10 times more particles in the polluted air mass north of the Maldives compared with what we saw south of the Maldives, which was a pristine air mass.

NARRATOR: In the polluted air, billions of manmade particles provided 10 times as many sites around which water droplets could form. So, polluted clouds contained many more water droplets, each one far smaller than it would be naturally. Many small droplets reflect more light than fewer big ones, so the polluted clouds were reflecting more light back into space, preventing the heat of the sun from getting through. This was the main cause of global dimming over the Indian Ocean.

VEERABHADRAN RAMANATHAN: Basically, the global dimming we saw in the north Indian Ocean was contributed, on the one hand, by the particles themselves shielding the ocean from the sunlight, on the other hand, making the clouds brighter. So this insidious soup, consisting of soot, sulfates, nitrates, ash and what have you, was having a double whammy on the global dimming.

NARRATOR: And when he looked at satellite images, Ramanathan found the same thing was happening all over the world: over India; over China, and extending into the Pacific; over Western Europe extending into Africa; over the British Isles. But it was when scientists started to investigate the effects of global dimming that they made the most disturbing discovery of all. Those more reflective clouds could alter the pattern of the world's rainfall, with tragic consequences.