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Last Updated: Tuesday, 6 March 2007, 09:00 GMT
Global impact of Asia's pollution

Smoggy street. Image: AFP/Getty
Smog is having a global impact on weather and climate, scientists say
Industrial pollution coming from Asia is having a wider effect on global weather and climate than previously realised, research suggests.

The "Asian haze" of soot is boosting storms in the Pacific, scientists find.

It is also enhancing the growth of large clouds, which play a key role in regulating climate globally.

Writing in the journal Proceedings of the National Academy of Sciences (PNAS), the researchers say impacts may be felt as far away as the Arctic.

"It's a complex picture," observed study leader Renyi Zhang from Texas A&M University in College Station, US.

"But the bottom line is that the aerosols actually enhance convection and increase precipitation over a large domain," he told the BBC News website

Hot water

While clean air legislation has reduced production of industrial aerosols - fine particles of dust, soot and sulphur - in Europe and North America, the opposite trend is seen in Asia.

Smog shrouds Malaysian prime minister's office building in Putrajaya, near Kuala Lumpur
Rapid industrialisation is driving increases in the "Asian haze"
Here, rapid industrialisation has led to the formation of a pollution haze which is especially marked in winter as coal burning increases.

Sulphur emissions have increased by more than one-third over the last decade.

These aerosols drive cloud formation, as water droplets coalesce around the tiny particles.

When aerosols are abundant, the droplets stay too small to form rain. Under these conditions, clouds may grow bigger and last for longer.

When the clouds are of the type known as deep convective clouds, this means they also transmit more heat from the Earth's surface into the higher atmosphere.

Deep convective clouds play a key role in regulating the global climate; and the role of aerosols in cloud development remains the major uncertainty in forecasting climate change.

Clouding up

In the latest research, Professor Zhang and his colleagues used satellite records to show that the amount of deep convective clouds over the north Pacific has increased.

Coverage for the period 1994-2005 was between 20% and 50% higher than in the preceding decade.

With increased clouds and increased convection came a growth in storminess - the "storm track" - over the ocean.

Computer models suggest that the trends are being driven by Asian aerosol production, rather than by other factors such as changes in ocean temperature.

"The storm track regulates the jet stream," commented Professor Zhang. "And if more heat is being transported from lower to higher latitudes, that is going to have a large effect on the global circulation."

But the link between clouds and aerosols works in the opposite direction too. Clouds transport the tiny particles, and more abundant and persistent clouds will transport them further - even to polar regions, Professor Zhang suggests.

Some studies have suggested that accumulation of these particles is changing the properties of Arctic ice, making it absorb more of the Sun's energy.

This would mean the ice is more prone to melting, as well as reducing the Earth's capacity to reflect solar energy back into space.

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