Friday, 6 February 2015

Global is new local

It turns out that the residents of Beijing and Delhi are not the only ones feeling the effects of their cities’ air pollution - an unwanted by-product of coal-fired economic development.

Researchers looking at how Asian pollution is changing weather and climate around the globe and have found pollution from China affects cloud development in the North Pacific and strengthens extra-tropical cyclones.

These large storms punctuate US winters and springs about once a week, often producing heavy snow and intense cold.

Tainted air is known to cross the Pacific Ocean, adding to homegrown air-quality problems on the US West Coast. But now scientists say the story doesn’t stop there - because pollution doesn't just pollute.

Researchers at NASA's Jet Propulsion Laboratory (JPL) and the California Institute of Technology, both in Pasadena, California, are looking at how Asian pollution is changing weather and climate around the globe.

Scientists call airborne particles of any sort - human-produced or natural - aerosols. And the simplest effect of increasing aerosols is to increase clouds.

To form clouds, airborne water vapour needs particles on which to condense. With more aerosols, there can be more or thicker clouds.

During the last 30 years, clouds over the Pacific Ocean have grown deeper, and storms in the Northwest Pacific have become about 10 percent stronger. This is the same time frame as the economic boom in Asia.

JPL researcher Jonathan Jiang and his postdoctoral fellow, Yuan Wang, designed a series of experiments to see if there was a connection between the two phenomena.

They used a numerical model that included weather factors such as temperature, precipitation and barometric pressure over the Pacific Ocean as well as aerosol transport - the movement of aerosols around the Earth.

They did two sets of simulations. The first used aerosol concentrations thought to have existed before the industrial revolution. The other used current aerosol emissions. The difference between the two sets showed the effects of increased pollution on weather and climate.

"We found that pollution from China affects cloud development in the North Pacific and strengthens extra-tropical cyclones," said Wang.

He explained that increased pollution makes more water condense onto aerosols in these storms. During condensation, energy is released in the form of heat. That heat adds to the roiling upward and downward airflows within a cloud so that it grows deeper and bigger.

"Large, convective weather systems play a very important role in Earth's atmospheric circulation," Jiang said, bringing tropical moisture up to the temperate latitudes. The storms form about once a week between 25 and 50 degrees north latitude and cross the Pacific from the southwest to the northeast, picking up Asia's pollutant outflow along the way.

Wang thinks the cold winter that the US east coast endured in 2013 probably had something to do with these stronger extratropical cyclones - and the intense storms could also have affected the upper-atmosphere wind pattern, called the polar jet stream.

Jiang and Wang are now working on a new experiment to analyse how increased Asian emissions are affecting weather even farther afield than North America. Although their analysis is in a preliminary stage, it suggests that the aerosols are having a measurable effect on climatic conditions around the globe.

Closer to home - a gas-fired power station in Spalding, Lincs, UK.                    Photo: Clive Simpson

Jiang says that Asian emissions have made him and some other climate researchers conceptualise Earth differently.

"Before, we thought about the north-south contrast: the northern hemisphere has more land, the Southern Hemisphere has more ocean. This difference is important to global atmospheric circulation and now, in addition, there's a west-east contrast.

“Europe and North America are reducing emissions - Asia is increasing them. That change also affects the global circulation and perturbs the climate."

Report by Clive Simpson freelance journalist

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