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Last Updated: Friday, 6 May 2005, 10:27 GMT 11:27 UK
Cleaner air makes brighter skies
By Richard Black
BBC News environment correspondent

Smoke stack (BBC)
Human activities put tiny particles into the atmosphere
The amount of sunlight reaching the Earth's surface is increasing, two new studies in Science magazine suggest.

Using different methods, they find that solar radiation at the surface has risen for at least the last decade.

Previous work had found the opposite trend, leading to a popular theory known as "global dimming".

But the latest Swiss and US research indicates the dimming in the past has now been reversed, possibly because of reduced atmospheric pollution.

Hard-won data

The idea of global dimming holds that tiny particles - aerosols - in the atmosphere are reflecting sunlight back into space, and the effect is to cool the Earth's surface.

The aerosols - a large proportion of which come from human activities - are therefore acting against any human-induced greenhouse effect. And only when societies clean up the production of aerosols will the true extent of global warming become apparent.

That is the theory, but global dimming has been hard to study definitively.

The other thing that happened in 1990 was the breakdown of the Soviet Union and its bloc of countries, which led to less output from their highly polluting industries
Dr Martin Wild
As with many other issues relevant to climate science, the answers researchers seek are not easily obtained, because previous generations did not build the instruments and set up the experiments that present-day investigators now suddenly need.

With the growing realisation that climate change may be a major hazard for the planet and for human society, the gaps which are left in this area are quickly being plugged.

Martin Wild's team, from the Swiss Federal Institute of Technology in Zurich - one of the groups reporting in this week's edition of the journal Science - has taken advantage of recent developments.

"We needed a system which allowed high-quality measurements of solar radiation," he told BBC News.

"Since 1990, many stations giving high-quality data have been built - and overall, they show an increase in radiation reaching the Earth's surface."

Looking down

Between about 1960 and 1990, however, Dr Wild believes that global dimming did occur, as found in records from an earlier generation of monitoring stations, and from experiments measuring the rate of evaporation of water - an indirect measure of the Sun's energy at the Earth's surface.

The second paper in the magazine uses data gathered by satellites observing the Earth, rather than monitoring stations on its surface.

"We have satellite observations that are global, which can look at the global picture starting from 1983 until 2001," principal investigator Dr Rachel Pinker, from the University of Maryland, US, told BBC News.

Cars on freeway (Getty)
Legislation in the US and Europe has sought to control aerosol emissions
"We have found in many places that indeed we do agree with observations that have been made at the ground, that there was a decrease up to the '90s and then there was an increase.

"When we analyse the entire record for the 20 years, we don't see a dimming; we see a slight increase in the amount of radiation when averaged over global scales."

So are other scientists convinced - have the lights unequivocally gone out on the global dimming hypothesis?

"These studies do add important information to what we knew previously," commented Dr Eleanor Highwood, a climatologist at the UK's Reading University, "and there is some consistency between the papers."

The really crucial question, she said, was why things changed around 1990 - why light which had slowly been fading started to regain its strength.

Effective legislation

The most obvious theory of all - that the Sun has simply increased its output - was swiftly demolished by Dr Wild.

"The size of the changes we are seeing are just too big to be explained that way," he said.

So whatever the explanation is, it must lie in the atmosphere - and Dr Wild believes that regional differences in his data may give a clue.

There is a third possibility here, which is changes in cloud cover
Dr Eleanor Highwood
"In India, we see no upturn in radiation, nor in China," he explained. "But over most of the rest of the world, notably Europe, we do.

"Since 1990, the atmosphere has become much cleaner with the introduction in many places of clean air legislation.

"The other thing that happened in 1990 was the breakdown of the Soviet Union and its bloc of countries, which led to less output from their highly polluting industries."

So it could simply be that over many areas of land, the atmosphere is cleaner - more sunlight is penetrating to the surface; whereas Asia is still pumping out aerosol particles and, as a consequence, no brightening is observed there.

Heated debate

If global dimming was masking the true scale of the human-induced greenhouse effect, and that dimming has now been superseded by global brightening, what does it all mean for the climate?

"We need to understand the basis of this," observed Dr Highwood, "and see what types of aerosol particles might be involved.

Smog in Asia (AP)
Asia has yet to make the same gains in cleaner air
"If it's due to changes in soot-type aerosols, which absorb radiation, it might not affect things very much; but if it's changes in particles which scatter sunlight, such as sulphate aerosols, then we might not have the computer models of climate change right.

"But there is a third possibility here, which is changes in cloud cover. Until we can sort this out, we're going to struggle."

Once again, then, a conclusion arrived at by decades of painstaking observation raises a host of further questions, which can only be answered by yet more decades of painstaking observation and the expensive instruments needed to make them.

One thing which shows absolutely no sign at all of dimming is the heat of scientific debate between climate change scientists.

Hear how aerosols can hold back warming

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