Too hot to handle

Our success in cutting air pollution could be actually speeding up global warming. Steve Connor reports on an unexpected consequence of trying to save the planet

The Earth is getting brighter. More sunlight is reaching the land surface of the planet than at any time for the past 30 years, and scientists are only now beginning to understand why. They are also learning about how this relatively sudden and unexpected increase in sunshine could make global warming more obvious - and potentially more dangerous.

The Earth is getting brighter. More sunlight is reaching the land surface of the planet than at any time for the past 30 years, and scientists are only now beginning to understand why. They are also learning about how this relatively sudden and unexpected increase in sunshine could make global warming more obvious - and potentially more dangerous.

Measurements of the sunlight reaching the Earth's surface show that between about 1960 and 1990 the days were in fact getting dimmer. Indeed, scientists estimated that during this period the Earth experienced a decline of between 4 and 6 per cent in the amount of sunlight reaching the ground. It became known as "global dimming", yet few people knew of its existence let alone understood its cause.

Then something happened at the end of the Eighties. Another network of sunlight monitors around the world detected an apparent reversal of the trend. Last week this was confirmed in a study showing that global dimming has indeed come to an end and that we have now entered a period of "global brightening". Over the past decade the days have brightened by about 4 per cent.

The most obvious explanation is that the Sun is going through a cycle of higher activity which is throwing more solar radiation our way. But the levels of increased sunlight being measured on the ground are far higher than anything that could result from increased solar activity. Others thought that clouds could provide the answer, while some suggested an increase in pollutant particles in the atmosphere could account for the dimming. But to many experts, the ups and downs in the amount of sunlight reaching the ground is a mystery.

So what's going on? Is global brightening something we should worry about, or should we just enjoy the extra rays?

The answer is, yes, we should be concerned because more sunlight means more heat and that could make global warming worse. But on the other hand, the brighter days could also be a sign that some of the anti-pollution measures designed to clean up the air and rid it of smoke and sooty particles are working. Global brightening could be an indication that the air is getting cleaner in terms of visible pollutants, although not in terms of greenhouse gas pollutants - the fossil-fuel emissions that exacerbate global warming.

Global dimming was first noticed in 1985 when scientists at the Swiss Federal Institute of Technology in Zurich analysed a database of sunlight measurements from around the world, which later became the International Baseline Surface Radiation Network. To their surprise the data showed a significant fall in levels of sunlight reaching the land - however, no detectors were positioned out to sea, which covers 70 per cent of the Earth's surface area. Yet the findings did not receive much publicity. In fact the Intergovernmental Panel on Climate Change (IPCC) - the body set up to oversee research into global warming - appears to have ignored them.

This is unfortunate because if the IPCC had been aware of the data then the findings could explain one of the major mysteries of global warming. This says that we know carbon dioxide levels in the atmosphere have increased consistently over the past half century, yet global average temperature changes over the land have only begun to increase since about the end of the Eighties. Why has it taken so long for the land temperatures to rise?

According to Martin Wild, a scientist at the Swiss institute, one answer could be that global dimming has been masking the effect of increased greenhouse gases in the atmosphere. Pollutants that cut out some of the sunlight before it reaches the ground - global dimming - have counteracted the effect of other pollutants that trap radiating heat reflected from the ground - the greenhouse effect. In effect, global dimming from 1960 to 1990 has counterbalanced the tendency for global warming caused by a worsening of the greenhouse effect.

"Sunlight at the land surface declined between the Sixties and Eighties, and this was possibly related to increased air pollution which may have masked global warming," Wild says. "The dimming ended during the Eighties in many locations, and the atmosphere has since become more transparent for sunlight, possibly favoured by air-pollution control. The absence of solar dimming may have no longer masked the greenhouse effect in the past two decades, with the result that the full dimension of the greenhouse effect has become only evident in the past two decades after dimming has ceased," he says.

The importance of sunlight reaching the ground has been likened to the physics of a pot of water heating on a stove - it is only heated from the pot's bottom. Sunlight warms the atmosphere from the ground up and it is only when the radiation reaches the ground that significant warming takes place, explains Charles Long from the US Department of Energy's Pacific Northwest National Laboratory in Richland, Washington, a co-author with Wild on the global-brightening study.

"The more solar energy at the surface means we might finally see the increase in temperature that we expected to see with global greenhouse warming," Long says. "Sunlight or solar radiation is by far the major source of energy for the Earth-atmosphere system. The majority of solar energy is deposited at the surface, which in turn heats the atmosphere from the bottom. This is the reason that, in general, temperature decreases with height in the atmosphere. Thus a decrease in solar energy at the surface decreases the energy available to heat the atmosphere above," he explains.

This could now all be ending with the advent of global brightening. "Thus, in the coming years, a greenhouse warming signal might become more apparent," says Long.

Although Wild has linked global brightening to cleaner air, Long is not so sure. "Presently, scientists can only speculate as to what phenomena might be responsible for the solar dimming and brightening trends," says Long.

Wild, however, is more convinced that the answer lies in the amount of pollutant particles in the atmosphere caused by heavy industry. The particles cut out the visible sunlight reaching the ground to cause the dimming effect, and clear-air legislation in the Western world over recent decades has begun to have an effect. "Air pollution has reduced as the atmosphere has got cleaner. However, in developing countries that are building an industry based on the burning of fossil fuels, sunlight dimming is significant," he says.

To live in a brighter world, with less air pollution, should be good news. Yet there is an unexpected irony in a cleaner world, which has shone a spotlight on the more sinister, long-term problem of climate change. Greenhouse gas emissions will continue to rise and global warming is going to get worse. Global brightening has now begun to unmask a problem that had been largely hidden from view - with the help of a murky atmosphere.