GLOBAL DIMMING
The crux of this BBC Horizon show was this: less sunshine is reaching the surface of the earth. Two simple tests have shown this to be a fact. I’ve forgotten the first one, but the second was a straightforward Water Evaporating From A Saucepan experiment, conducted in Australia over decades.
Not surprisingly, the reason for this is pollution and the way it has changed the nature of clouds. Moist air condenses around a tiny particle (aerosol). Formerly, this would be a speck of pollen or sea salt. Now, though, it tends to be a piece of soot, ash or sulphur. Man-made particles are smaller and more numerous than naturally occurring ones, which means ‘dirty air’ contains many more microscopic surfaces onto which moisture can condense. Thus, there are more water droplets in polluted clouds than in non-polluted clouds, and these droplets are smaller than they would normally be.
The greater surface area of the more numerous yet smaller raindrops means more light is reflected back out to space. The photons (and heat) of the sun are not able to penetrate. The polluted clouds behave like giant mirrors. This is what caused the Ethiopian drought and famine. The summer rains did not arrive in northern parts of Africa, because the oceans were cooler than they should have been. Hence, clouds did not bubble up over the oceans (which would then have formed seasonal rain belts).
Only trouble is, reducing these sunshine-blocking particles won’t resolve climatic problems. Here’s why. In the aftermath of September 11th, all commercial aircraft in the US were grounded for three days. This meant no jet emissions. Scientists realised that during this three day period, temperatures were colder at night and hotter by day. There was a much greater diurnal range of temperature. Removing only one environmental pollutant had a massive effect. Removing many such environmental pollutants would probably cause a massive increase in global warming. Reducing global dimming (minimising clouds that behave like mirrors) would heat up the surface of the earth considerably. In fact, it’s already happening. In the West, we have catalytic converters, etc which cut down particle pollution, but in 2003 we had the hottest summer on record.
It seems that we have underestimated the rate of climatic change. Greenland’s icecaps are likely to melt in our lifetime, raising sea levels by eight metres, which will be the real beginning of the end.
It was heartening that this show was broadcast during the same week that Airbus released the launch of its new, extra capacity Very Big Plane. So maybe aircraft pollution will maintain a balance and save us after all.
The crux of this BBC Horizon show was this: less sunshine is reaching the surface of the earth. Two simple tests have shown this to be a fact. I’ve forgotten the first one, but the second was a straightforward Water Evaporating From A Saucepan experiment, conducted in Australia over decades.
Not surprisingly, the reason for this is pollution and the way it has changed the nature of clouds. Moist air condenses around a tiny particle (aerosol). Formerly, this would be a speck of pollen or sea salt. Now, though, it tends to be a piece of soot, ash or sulphur. Man-made particles are smaller and more numerous than naturally occurring ones, which means ‘dirty air’ contains many more microscopic surfaces onto which moisture can condense. Thus, there are more water droplets in polluted clouds than in non-polluted clouds, and these droplets are smaller than they would normally be.
The greater surface area of the more numerous yet smaller raindrops means more light is reflected back out to space. The photons (and heat) of the sun are not able to penetrate. The polluted clouds behave like giant mirrors. This is what caused the Ethiopian drought and famine. The summer rains did not arrive in northern parts of Africa, because the oceans were cooler than they should have been. Hence, clouds did not bubble up over the oceans (which would then have formed seasonal rain belts).
Only trouble is, reducing these sunshine-blocking particles won’t resolve climatic problems. Here’s why. In the aftermath of September 11th, all commercial aircraft in the US were grounded for three days. This meant no jet emissions. Scientists realised that during this three day period, temperatures were colder at night and hotter by day. There was a much greater diurnal range of temperature. Removing only one environmental pollutant had a massive effect. Removing many such environmental pollutants would probably cause a massive increase in global warming. Reducing global dimming (minimising clouds that behave like mirrors) would heat up the surface of the earth considerably. In fact, it’s already happening. In the West, we have catalytic converters, etc which cut down particle pollution, but in 2003 we had the hottest summer on record.
It seems that we have underestimated the rate of climatic change. Greenland’s icecaps are likely to melt in our lifetime, raising sea levels by eight metres, which will be the real beginning of the end.
It was heartening that this show was broadcast during the same week that Airbus released the launch of its new, extra capacity Very Big Plane. So maybe aircraft pollution will maintain a balance and save us after all.
