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Global Warming - Dimming the Sun - 03

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  • Description: 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.

    MICHAEL BUERK, Newscaster: Dawn, and as the sun breaks through the piercing chill of night, on the plain outside Korum, it lights up a biblical famine, now, in the 20th Century. This place, say workers here, is the closest thing to Hell, on Earth.

    NARRATOR: The 1984 Ethiopian famine shocked the world. It was partly caused by a decades-long drought right across sub-Saharan Africa, a region known as the Sahel. For year after year, the summer rains failed. There were many factors at work, but now there's evidence that among them was global dimming. The Sahel's lifeblood has always been a seasonal monsoon. For most of the year it is completely dry, but every summer, the heat of the sun warms the oceans north of the Equator. This draws the rain belt that forms over the Equator northward, bringing rain to the Sahel.

    But for 20 years, in the 1970s and 80s, the tropical rain belt consistently failed to shift northward, and the African monsoon failed. For climate scientists like Leon Rotstayn, the disappearance of the rains had long been a puzzle. He could see that pollution from Europe and North America blew right across the Atlantic, but all the climate models suggested it should have little effect on the monsoon.

    But then Rotstayn decided to take the Maldive findings about the impact of pollution on clouds into account.

    DOCTOR LEON ROTSTAYN (Commonwealth Scientific and Industrial Research Organisation, Atmospheric Research): What we found, in our model, was that when we allowed the pollution from Europe and North America to affect the properties of the clouds in the northern hemisphere, the clouds reflected more sunlight back to space, and this cooled the oceans of the northern hemisphere. And to our surprise, the result of this was that the tropical rain bands moved southward, tracking away from the more polluted northern hemisphere towards the southern hemisphere.

    NARRATOR: In Rotstayn's model, polluted clouds kept the heat of the Sun from getting through, the heat that was needed to draw the tropical rains northward. So the life-giving rain belt never made it to the Sahel.

    LEON ROTSTAYN: So, what our model is suggesting is that these droughts in the Sahel, in the 1970s and the 1980s, may have been caused by pollution from Europe and North America affecting the properties of the clouds and cooling the oceans of the northern hemisphere.

    NARRATOR: Other models suggest that global warming was also a factor in the Sahel disaster. But Rotstayn's work shows the potential for air pollution to have far reaching effects on rainfall, perhaps even contributing to a terrible drought that blighted the lives of over 50,000,000 people. And this could be just of taste of what global dimming has in store.( Continued at
  • Tags: , dr_james_hansen, dr_david_travis, dr_gerald_stanhill, dr_beate_liepert, prof_graham_farquhar, dr_michael_roderick, michael_buerk, dr_leon_rotstayn, dr_peter_cox
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