Global dimming

We are all seeing rather less of the Sun. Scientists looking at five decades of sunlight measurements have reached the disturbing conclusion that the amount of solar energy reaching the Earth's surface has been gradually falling. Paradoxically, the decline in sunlight may mean that global warming is a far greater threat to society than previously thought.

The effect was first spotted by Gerry Stanhill, an English scientist working in Israel. Comparing Israeli sunlight records from the 1950s with current ones, Stanhill was astonished to find a large fall in solar radiation. "There was a staggering 22% drop in the sunlight, and that really amazed me," he says.

Intrigued, he searched out records from all around the world, and found the same story almost everywhere he looked, with sunlight falling by 10% over the USA, nearly 30% in parts of the former Soviet Union, and even by 16% in parts of the British Isles. Although the effect varied greatly from place to place, overall the decline amounted to 1-2% globally per decade between the 1950s and the 1990s.

Gerry called the phenomenon global dimming, but his research, published in 2001, met with a sceptical response from other scientists. It was only recently, when his conclusions were confirmed by Australian scientists using a completely different method to estimate solar radiation, that climate scientists at last woke up to the reality of global dimming.

Dimming appears to be caused by air pollution. Burning coal, oil and wood, whether in cars, power stations or cooking fires, produces not only invisible carbon dioxide (the principal greenhouse gas responsible for global warming) but also tiny airborne particles of soot, ash, sulphur compounds and other pollutants.

This visible air pollution reflects sunlight back into space, preventing it reaching the surface. But the pollution also changes the optical properties of clouds. Because the particles seed the formation of water droplets, polluted clouds contain a larger number of droplets than unpolluted clouds. Recent research shows that this makes them more reflective than they would otherwise be, again reflecting the Sun's rays back into space.

Scientists are now worried that dimming, by shielding the oceans from the full power of the Sun, may be disrupting the pattern of the world's rainfall. There are suggestions that dimming was behind the droughts in sub-Saharan Africa which claimed hundreds of thousands of lives in the 1970s and 1980s. There are disturbing hints the same thing may be happening today in Asia, home to half the world's population. "My main concern is global dimming is also having a detrimental impact on the Asian monsoon," says Prof Veerhabhadran Ramanathan, one of the world's leading climate scientists. "We are talking about billions of people."

But perhaps the most alarming aspect of global dimming is that it may have led scientists to underestimate the true power of the greenhouse effect. They know how much extra energy is being trapped in the Earth's atmosphere by the extra carbon dioxide (CO2) we have placed there. What has been surprising is that this extra energy has so far resulted in a temperature rise of just 0.6°C.

This has led many scientists to conclude that the present-day climate is less sensitive to the effects of carbon dioxide than it was, say, during the ice age, when a similar rise in CO2 led to a temperature rise of 6°C. But it now appears the warming from greenhouse gases has been offset by a strong cooling effect from dimming - in effect two of our pollutants have been cancelling each other out. This means that the climate may in fact be more sensitive to the greenhouse effect than thought.

If so, then this is bad news, according to Dr Peter Cox, one of the world's leading climate modellers. As things stand, CO2 levels are projected to rise strongly over coming decades, whereas there are encouraging signs that particle pollution is at last being brought under control. "We're going to be in a situation, unless we act, where the cooling pollutant is dropping off while the warming pollutant is going up. That means we'll get reduced cooling and increased heating at the same time and that's a problem for us," says Cox.

Even the most pessimistic forecasts of global warming may now have to be drastically revised upwards. That means a temperature rise of 10°C by 2100 could be on the cards, giving the UK a climate like that of North Africa, and rendering many parts of the world uninhabitable. That is unless we act urgently to curb our emissions of greenhouse gases.

Horizon producer David Sington on why predictions about the Earth's climate will need to be re-examined. We are all seeing rather less of the Sun. Scientists looking at five decades of sunlight measurements have reached the disturbing conclusion that the amount of solar energy reaching the Earth's surface has been gradually falling. Paradoxically, the decline in sunlight may mean that global warming is a far greater threat to society than previously thought.

The effect was first spotted by Gerry Stanhill, an English scientist working in Israel. Comparing Israeli sunlight records from the 1950s with current ones, Stanhill was astonished to find a large fall in solar radiation. "There was a staggering 22% drop in the sunlight, and that really amazed me," he says.

Intrigued, he searched out records from all around the world, and found the same story almost everywhere he looked, with sunlight falling by 10% over the USA, nearly 30% in parts of the former Soviet Union, and even by 16% in parts of the British Isles. Although the effect varied greatly from place to place, overall the decline amounted to 1-2% globally per decade between the 1950s and the 1990s.

Why would particles in the atmosphere cause global dimming?

It's very simple - they reflect sunlight back into space. This fact has been well-known for many decades (it is why volcanoes, which can throw vast numbers of sulphate particles into the upper atmosphere, can have a strong temporary cooling effect on the Earth).

What came as a surprise was the so-called indirect effect, whereby particles in the atmosphere change the optical properties of clouds. This happens because the presence of man-made particles in the atmosphere increases the number of sites where water droplets can form. The effect is that up to six times as many water droplets form in a polluted air mass as would naturally, but since the amount of water vapour in the atmosphere does not change, the droplets have to be smaller than they would naturally be.

Since the reflectivity of the clouds depends on the surface area of the droplets, these polluted clouds are more reflective than unpolluted ones (since many small droplets have a bigger surface area than fewer big ones). By reflecting more sunlight back into space these more reflective clouds cut down the sunlight reaching the surface - hence contributing to global dimming.

Is global dimming a daytime/daylight phenomenon or a round-the-clock greenhouse/infra-red phenomenon?

Dimming is a daytime effect. At night the sun's radiation is obviously completely blocked by the Earth!

How does anthropogenic particle emission compare with natural sources, for example volcanoes, deserts, sea salt etc?

That is very difficult to say. Emissions from volcanoes can have a big global effect on the climate, but it is transitory - the Earth may go many decades between major eruptions. Sea salt and natural sulphur compounds emitted by plankton are also very important for cloud formation. But the results from the Indian Ocean Experiment suggest that the effects of anthropogenic particles in the atmosphere dominate natural effects, in the Indian Ocean at least.

At the moment the scientific consensus is that global dimming is a man-made phenomenon. Another important aspect of man-made particle pollution is that the evidence is growing that its direct impact on human health is very serious - causing literally hundreds of thousands if not millions of deaths annually. Sea salt doesn't give you cancer!

If the 9/11 contrail evidence suggests warmer nights due to the air travel, isn't that global warming rather than dimming?

The 9/11 study showed that removing contrails resulted in a large increase in the daily temperature range - in other words warmer days and cooler nights. The study does not really provide a clear-cut answer to the question of whether the overall effect of the contrails is a net warming or a net cooling averaged over the whole 24 hours. This question is controversial. But what seems clear is that contrails contribute to a reduction in the amount of daytime solar radiation reaching the surface, and that this has significant effects on temperature.

Given the effect of the air traffic, is there any way to reduce the pollution they cause? For example, changing altitude, fuels and engines or economic measures?

Contrails form at altitude, so in theory flying lower would reduce them - but at the cost of burning even more aviation fuel and therefore making a still greater contribution to global warming.

Why has progress been made on particulate emissions more easily than gaseous emissions?

There are several reasons. Firstly, people can see, smell and taste particle emissions and they have very obvious damaging effects on human health. Also, sulphate emissions cause acid rain, which again has very obvious and immediate effects on vegetation and fish. So the political pressure to reduce particle emissions is strong. Then, the cure for particle emissions is technologically fairly straightforward - it is a question of adding equipment to existing plant rather than replacing plant.

This is a big contrast with what is required to tackle greenhouse gas emissions. Firstly, carbon dioxide is invisible and tasteless, and its catastrophic long-term effects on the environment are not immediately apparent. Secondly, tackling the problem means changing some of our behaviours and radically re-tooling our energy generation systems. These issues have yet to be properly confronted. But the good news is that we already have the technologies needed to solve the problem of greenhouse gases (principally energy conservation measures and nuclear power); what we so far lack is the political will to use them.

Are scientists often sceptical of findings that go against the current orthodoxy?

Yes, for good reason - usually the orthodoxy is correct. A famous physicist once told me that if you doubt every new idea in science you will be right 90% of the time, but you will be wrong the only time it matters.

Is there a possibility that the pan-evaporation method is too crude to be reliable?

Rather the reverse. It is the very simplicity of the pan-evaporation experiments that makes them reliable and comparable between different locales and over long periods of time. That is why many climate scientists regard the pan-evaporation data as the most convincing evidence of solar dimming.

What's the range in the figures on global dimming? Is there a consensus or are they as open to interpretation as global warming figures?

I don't think the figures on global warming are open to interpretation. It is an established fact that global temperatures have risen by 0.6°C over the past century. It is also an established fact that carbon dioxide levels have risen by about 100 parts per million over the same period due to human activity. It is a matter of the basic laws of physics that an increase in carbon dioxide will trap more heat in the Earth's atmosphere, which is why almost no respectable and independent scientist doubts the causal link between these two established facts.

The only surprise is that the warming has not been greater - which is where global dimming comes in. Unfortunately, there is still a very large range in the estimates of the cooling effect of dimming - by up to a factor of four. What seems to have been established already, however, is that the cooling effect of dimming is far larger than previously thought. This may explain why the world has not already warmed more strongly - the cooling effect of particle pollution has been offsetting the warming from carbon dioxide. If so, then we are in for far faster warming in the future as particle emissions are brought under control while greenhouse gas emissions continue to rise.

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