Global Dimming

Cyril examines the intracisies of global warning

The major importance of world climate change was reinforced recently by a BBC2 Horizon programme on January 15th. It was entitled 'Global Dimming', which the narrator referred to as "a deadly new phenomenon" and one which "demands action". Was this just Horizon doing its usual sensationalist stuff? It certainly was a programme to give you a jolt - but justifiably so in my opinion. For those who didn't see it, the following is my brief summary of its contents.

Global Dimming is a process that until recently many scientists were sceptical about - it seemed a contradiction in terms. How could the globe be dimming, when they knew it was heating up? At first sight the two processes appear to conflict. But despite that it now appears that both things have been going on at one and the same time.

Although Global Dimming was recognised only recently, there has nevertheless been quite a trail, over a number of years, leading up to its discovery. Forty years ago a biologist, Dr Gerald Stanhill, was working for the Agricultural Research Organisation in Israel. He was helping to design a national irrigation scheme. As solar radiation is the factor that decides how much water crops need, his job was to measure the strength of the sun over Israel. Over a year he used a network of light meters to collect the data for use in the scheme.

Some years later, in the 1980s, Stanhill decided to repeat his measurements to check they were still valid. The results amazed him: over the years there had been a "staggering" 22% drop in the sunlight. This seemed so amazing to the scientific community that they ignored his published results. However, a German climatologist, Beate Liepert, found that the same thing was happening over the Bavarian Alps.

Working independantly Liepert and Stanhill searched through publications, journals and meteorological records from around the world. This showed the same story: between the 1950s and the early 1990s solar energy reaching earth's surface had dropped as follows:- Antarctica 9%, USA 10%, Russia about 30%, part of British Isles 16%. It was a truly global phenomenon, so Stanhill called it 'Global Dimming'. But, because of the fact of Global Warming, others were sceptical.

However, Liepert and Stanhill's was not the only work that didn't seem to fit in with Global Warming. Two Australian biologists, Michael Roderick and Graham Farquhar, found another paradoxical result: a world-wide decline in 'pan evaporation rate'. (The term simply denotes the evaporation of water contained in a pan; each morning a check is made to see how much water is needed to bring it back to the level it was at the same time the day before).

This checking is done all over the world. In some places this daily task has been performed for over a hundred years. It is the long-term measurements which give it its value. For decades little notice was taken of the measurements until, in the 1990s, it was spotted that the rate of pan evaporation was falling. An apparent paradox - global temperature increasing, but evaporation going down!

Naturally one would reason that, like heating water on a stove, increasing global temperature should increase evaporation rate. But Roderick and Farquhar's calculations showed that temperature was not the most important factor in pan evaporation. In fact the key things are sunlight, humidity and wind - with the dominant factor being the sunlight. "It was the energy of the photons hitting the surface, the actual sunlight, that kicks the water molecules out of the pan and into the atmosphere."

This suggested that if the pan was going down, then maybe the sunlight was also going down. Searching in the library, Roderick found an article called 'Evaporation Losing Its Strength'. This supported his own finding on evaporation, reporting an evaporation decline for Russia, United States and Eastern Europe - on average about a hundred millimetres less of water in the last thirty years.

Roderick knew the amount of sunlight needed to evaporate a hundred millimetres = 2 1/2 mega joules. Multiply this by a hundred and you get 250 mega joules which, he found, was in fact what the Russians had measured for the decline in sunlight in the last thirty years. Results were the same for Europe and the USA; Liepert and Stanhill's figures for the drop in sunlight matched exactly the drop in evaporation rate. The two independent sets of observations came to the same conclusion - Global Dimming could not now be doubted.

But what was causing it? Prof. Veerabhadran Ramanathan (University of California), one of the world's leading climatic scientists, had noticed declining sunlight over much of the Pacific Ocean in the mid-1990s and set about finding the cause. One obvious suspect was atmospheric pollution. When we burn fuel, we not only produce the invisible greenhouse gases that cause global warming. We also produce much visible pollution: an "insidious soup, consisting of soot, sulphates, nitrates, ash and what have you". These produce the haze that surrounds our cities.

The long island chain of the Maldives was chosen as the suitable place for Ramanathan's study. This multi-national project was called INDOEX, lasted for four years and cost twenty-five million dollars. The northern islands, which sit in a stream of dirty air coming from India, were compared with those at the extreme tip at the southern end which enjoy relatively clean air coming from Antarctica. "Every possible technique was used to sample and monitor the atmosphere over the Maldives."

Ramanathan was stunned to find that the polluted air mass north of the Maldives, three kilometres thick, was cutting down the sunlight reaching the ocean by more than 10 %. The pollution particles were not only blocking out sunlight themselves, they were having an even more significant effect on the clouds by turning them into giant mirrors. These were reflecting light back into space, preventing the sun's heat getting through. This was the cause of Global Dimming, and satellite images showed that it was happening over countries all around the world.

When investigations into the effects of G. Dimming were started, a most disturbing discovery was made. The reflective clouds could alter the pattern of the world's rainfall, and there is now evidence that this was the culprit for the shocking Ethiopian famine of 1984. Dr Leon Rotstayn (CSIRO Atmosphere Research): "What our model is suggesting is that the droughts in the Sahel in the 1970s and 1980s may have been caused by pollution from Europe and North America."

The lifeblood of sub-Saharan Africa has always been a seasonal monsoon. Every summer the heat of the sun has warmed the oceans north of the equator. This drew the rain belt over the equator northwards, bringing rain to the Sahal. But, in the 70s and 80s the tropical rain belt consistently failed to shift northwards - and the monsoon failed.

Polluted clouds had stopped the heat of the sun getting through. That heat was needed to draw the tropical rains northwards. So, the life-giving rain belt never made it to the Sahel. It seems that emissions from our vehicles and power stations contributed to the deaths of a million people - and made life hell on earth for 50 millions more!

This effect of air pollution - quite apart from others such as all sorts of respiratory diseases - make it essential to significantly reduce it. In Europe we have in fact made a start, by using scrubbers in power stations, catalytic converters in cars and low sulphur fuels. These have already begun to cut down visible air pollution. Thankfully this should be good news for the Sahel, where recently the droughts have not been as severe.

The bad news is, that as we reduce Global Dimming we may be faced with something even worse. We produce two kinds of pollution: a.) the visible particulates which cause bad air quality and dimming, and b.) the invisible greenhouse gases which cause Global Warming. Both things are happening and it is useless to tackle one without dealing with the other. Global Dimming has in fact been masking the full effect of Global Warming, which could be much worse that previously thought.

Dr David Travis (University of Wisconsin) caught a glimpse of what the world could be like without Global Dimming. He had been studying aircraft vapour trails, or contrails, for fifteen years. His problem was that, in order to establish how big an effect the contrails were having, he needed a period when, although conditions were right for them to form, there were no flights. But of course, normally this never happened - until the tragedy of 9/11. Then, for the following three days virtually all commercial aircraft in the US were grounded.

The opportunity could not be missed. Travis collected data from over 5,000 weather stations across the US. He did not look only at temperature, but focused on what normally changes only slowly: temperature range. The result surprised him; it was larger than expected. With the 3 day drop in Global Dimming contributed from airplanes, the temperature range jumped by over a degree Celsius - the largest temperature swing for the last thirty years.

Travis commented: "The study showed that if you remove a contributor to Global Dimming, jet contrails, for just 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."

In Western Europe the steps taken to cut air pollution have made a noticeable improvement in air quality and a slight reduction in Global Dimming, recently. But, after decades of holding steady, temperatures have risen rapidly, culminating in the 'savage summer' of 2003. "Forest fires devastated Portugal. Glaciers melted in the Alps. In France people died by the thousand. Could this be the penalty of reducing Global Dimming, without tackling the root causes of Global Warming?

Dr Peter Cox (Hadley Centre, Met Office): "If we don't do anything, by about twenty thirty we could have a global warming of exceeding two degrees, and at that point it's believed the Greenland ice sheet would start to melt in a way that you wouldn't be able to stop once it started.....ultimately it would lead to a sea level rise of seven or eight metres." Dr Cox goes on to list a catastrophic chain of events if we fail to take action.

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So, I ask, what should the politicians' priorities be for the coming election campaign - health, education, tax, etc. etc., or Global Warming?

Cyril Marsters