Wickham's musings from Chalke

There was an interesting TV programme last night on global dimming. It seems that there are two conflicting environmental effects working at present – global warming and global dimming.

Global warming is due to greenhouse gases and global dimming is due to pollution which reflects sunlight and also causes many smaller raindrops rather than fewer larger ones. This has caused rain to fall in different places during the 1980s and 1990s, for instance causing the decades-long droughts in Ethiopia. Monsoons in Asia have been disrupted.

The cooling effect of reflected sunlight has counteracted the warming effect of global warming so the net increase in global temperature has not been as much as it would have been if the global dimming effect had been absent.

The degree of global dimming was unknown until the September 11th attacks in America provided an opportunity for it to be tested. For three days nearly all air flights over the United States were grounded. A scientist had been trying to measure the effect of aircraft vapour trails on global temperature. He realised that the atmosphere was much clearer on the day after the attacks so he got all the weather data from about four hundred US weather stations.

Since daily temperature varies due to weather conditions he used the temperature range between the day and night temperatures which is more stable. He plotted the ranges for three days prior and immediately after the attacks and for three days after flights recommenced and compared the results with the annual average.

He found that for the three days before the attacks and three days after the recommencement the temperature range was slightly below the annual average but while the flights were grounded the range was ten per cent higher than average, a huge difference.

Since the 1990s the world has started reducing pollution by cleaning power station gases, car exhaust filters and so on and already the rainfall in Ethiopia and the Far East is more normal. However, for the last few years daily the temperatures have been greater than usual in many areas of the world. England had the first daily temperature over 100 degrees Fahrenheit.

Scientists are now concerned that the effect of greenhouse gases will be much greater than before and that a run-away temperature increase could result if greenhouse gases are not controlled drastically very soon. There is only an opportunity during the next two decades otherwise the effect will be unstoppable. If pollution is increased again to counteract the effect as before other problems will arise such as respiratory diseases so this is not an option.

If the Greenland ice cap melts then the sea level may rise two metres and swamp a large number of cities.

There is a huge store of methane in the sea bed, especially in the Arctic Ocean, which will be released as sea temperatures rise; this will add massively to the greenhouse gases and we will be unable to stop the release once it starts except by lowering the sea temperature – quite beyond our present technological powers.

Postscript 16/01/05

One scientist said that he thought most of the effects assumed to be global dimming were in fact due to sunspot activity. I always thought sunspot activity was in bursts, usually over a few days but also over a few years. Climate change has been a steady change over at least three decades so I’m not sure if he is correct.