VEERABHADRAN RAMANATHAN: The stunning part of the experiment was this pollutant layer, which was three kilometers thick, cut down the sunlight reaching the ocean by more than 10 percent.
NARRATOR: 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. 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. VEERABHADRAN RAMANATHAN: The Sahel is just one example of the monsoon system. Let me take you to anther part of the world, Asia, where the same monsoon brings rainfall to 3.6 billion people, roughly half the world's population. My main concern is this air pollution and the global dimming will also have a detrimental impact on this Asian monsoon. We are not talking about few millions of people, we are talking about few billions of people. NARRATOR: For Ramanathan the implications are clear. VEERABHADRAN RAMANATHAN: There is no choice here; we have to cut down air pollution, if not eliminate it altogether. NARRATOR: In Europe and North America, air pollution is already in decline. Scrubbers in power stations, catalytic converters in cars, and low sulfur fuels, though they do nothing to reduce greenhouse gases, have already led to a marked reduction in visible air pollution. Coincidence or not, this should be good news for the Sahel, and in recent years the droughts have not been nearly so bad. But in developing countries, like China and India, air quality has been getting worse. The health effects are terrible. It's estimated that respiratory diseases kill a million Indians each year. As a result, it's likely that India and China will follow the same path as the developed world and strive to bring air pollution under control. But there's a terrible catch. While global dimming is a major threat, it now appears it has been protecting us from an even greater threat: accelerated global warming. It was David Travis who first caught a glimpse of what the world could be like without global dimming. It happened in those chaotic days following the tragedy of 9/11. For 15 years, Travis had been studying the vapor trails, or contrails, left behind by high-flying aircraft. As a jet passes through the air, the pollution particles it emits can trigger the condensation of water droplets. These manmade clouds seem small, but when they all spread out, they can blanket the sky. DAVID TRAVIS: Here are some examples of what we call "outbreaks" of contrails. These are large clusters of contrails. And here's a particularly good one from Southern California. Here's the west coast of the United States. And you can see, here, this lacing network of contrails, covering at least 50 percent, if not 75 percent or more of the sky in that area. It doesn't take an expert to realize that if, if you look at the satellite picture and see this kind of contrail coverage that they've got to be having an effect on temperature at the surface. NARRATOR: But the problem Travis faced was to establish exactly how big an effect the contrails were actually having. The only way to do that was to find a period of time when conditions were right for contrails to form, but there were no flights. And, of course, that never happened-until September, 2001. Then, for three days after the 11th, virtually all commercial aircraft were grounded, so Travis set about gathering temperatures from all over the U.S.A., and comparing them to records from the last 30 years. DAVID TRAVIS: ...initially, data from over 5,000 weather stations across the 48 United States, the area that was most dominantly affected by the grounding. NARRATOR: Travis was not looking just at temperature, which varies a lot from day to day. Instead he focused on something that normally changes quite slowly: the temperature range, the difference between the highest temperature during the day and the lowest at night. Had this changed at all during the three days of the grounding? DAVID TRAVIS: As we began to look at the climate data and the evidence began to grow, I got more and more excited. The actual results were much larger than I expected. So here we see, for the three-day period preceding September 11th, a slightly negative value of temperature range with lots of contrails, as normal. Then we have this sudden spike right here of the three-day period. This reflects lack of clouds, lack of contrails, warmer days cooler nights, exactly what we expected, but even larger than we expected. NARRATOR: During the three-day grounding, the nights had gotten colder and the days, warmer. Averaged over the whole continental U.S., the temperature difference between day and night had suddenly increased by over a degree Celsius or two degrees Fahrenheit. Travis had never seen anything like it before. DAVID TRAVIS: This was the largest temperature swing of this magnitude in the last 30 years. NARRATOR: Manmade clouds from aircraft are a minor contributor to global dimming. If removing them had such a dramatic effect, what would happen if air pollution were to be reduced all over the world? DAVID TRAVIS: The 9/11 study showed that if you remove a contributor to global dimming, jet contrails, just for a three-day period, we see an immediate response of the surface temperature. Do the same thing globally, we might see a large-scale increase in global warming. NARRATOR: This is the crux of the problem. While the greenhouse effect has been warming the planet, it now seems global dimming has been cooling it down. This new understanding is something that climate modelers like Peter Cox have to contend with. DOCTOR PETER COX (University of Exeter): Climate change, to the current date, appears to have been a tug of war, really, between two manmade pollutants. On the one side, we've got greenhouse gases that are pulling the system towards a warmer state, on the other hand, we've got particles from pollution that are cooling it down. And there's a kind of tug of war going on between the two, in which the middle of the rope, if you like, determines where the climate system is going in terms of warming or cooling. NARRATOR: So which is stronger in the tug of war? Something powerful enough to push and pull on the global temperature has what scientists call a "climate forcing" effect. moq_discuss mailing list Listinfo, Unsubscribing etc. http://lists.moqtalk.org/listinfo.cgi/moq_discuss-moqtalk.org Archives: http://lists.moqtalk.org/pipermail/moq_discuss-moqtalk.org/ http://moq.org.uk/pipermail/moq_discuss_archive/
