Long-term aerosol pollution affects cloud formation and rainfall.
Small particles play a big role in cloud formation, influencing the occurance of droughts and floods.
Aerosols increase the frequency of precipitation in clouds with higher water content, but decrease the frequency of precipitation in clouds with low water content.
Long-term aerosol pollution affects cloud formation and rainfall, exacerbating extreme weather conditions, suggests a new study.
The study, led by Professor Zhanqing Li, from the University of Maryland, indicates that aerosols affect cloud height and thickness, reducing rainfall in dry times and increasing storm activity in wet regions.
The findings, which appear in this week's Nature Geoscience, are based on a 10-year dataset of atmosphere measurements from a climate research facility on the Southern Great Plains of Oklahoma.
The data was supported by model simulations of cloud formation.
Scientists have known for a long time that aerosols such as soot, dust and other small particulate matter, alter cloud density and the radiative balance of the atmosphere.
Aerosols also affect cloud microphysics because they act as nuclei around which water droplets or ice forms.
What scientists haven't been able to determine, until now, is the net effect of these factors.
"We have uncovered for the first time, the long-term, net impact on cloud height and thickness and resultant changes in precipitation frequency and intensity," says Li.
The researchers found that aerosols increase the size of certain clouds, which contain water and ice and also have low, warm bases. They attribute this effect, which is most significant in summer, to an aerosol induced invigoration of upward winds. In contrast, aerosols did not increase the thickness or precipitation intensity of clouds which had cold bases and no ice.
The study also shows that aerosols increase the frequency of precipitation in clouds with higher water content, but decrease the frequency of precipitation in clouds with low water content.
The researchers say their study has important implications for climate studies and the management, availability and use of water resources around the world.
"The findings presented here imply a potentially adverse impact on sustainable development over regions vulnerable to extreme meteorological events such as drought or flooding," the researchers write.
Study's co-author, Dr Daniel Rosenfeld from the Hebrew University in Jerusalem says Australian scientists suspected smoke from burn-offs of harvested sugar cane fields was suppressing rainfall as far back as 1968.
However, Rosenfeld says, the same scientists began to question their own findings years later after realizing that Queensland regularly undergoes large swings in climate.
In 2000, a study by Rosenfeld using satellite measurements found air pollution was suppressing rain and snow over south-eastern Australia.
"This was followed by considerable scientific debate in Australia, because no net decrease of precipitation was clearly evident," he says.
"Now, with the new findings it might be that the suppressed rain from the shallower clouds is compensated by more intense rain from the deeper clouds."
"In Australia, it might well be that during times of drought the clouds are shallower so that pollution aerosols suppress the rain even further, whereas during floods the clouds are deep, and hence the air pollution would make them rain even more," says Rosenfield.
"Resolving this question requires long-term measurements."