Smoke from Amazon fires reduces local rainfall
Smoke from Amazon fires reduces local rainfall
mongabay.com
August 14, 2008
Smoke released by fires set to clear the Amazon rainforest inhibit the formation of clouds, thereby reducing rainfall, report researchers writing in the journal Science. The study provides clues on how aerosols from human activity influence cloud cover and ultimately affect climate.
While the link between Amazon fires and reduced rainfall is well-documented, the new research sought to identify a link human-caused aerosols and cloud structure.
“Scientists have observed instances where increases or decreases in the amount of these tiny particles have increased and decreased cloud cover in different places and times,” said study co-author Lorraine Remer of NASA’s Goddard Space Flight Center. “We saw an example of this ourselves: increased aerosols over the Amazon produced less cloud cover. Over the Atlantic Ocean, however, increased aerosols actually produced more cloud cover. We wanted to know what the link was between these different outcomes from varying amounts and types of aerosols.”
The team developed a model based on “combined knowledge of cloud development, satellite observations and mathematical calculations of aerosol concentration and cloud properties in an effort to explain how the two opposing effects of aerosols on clouds can influence cloud coverage and life cycle,” according to a statement from NASA.
 Peruvian rainforest canopy. Photo by Rhett A. Butler |
The model was then tested using aerosol and cloud observations from NASA’s Terra satellite during the 2005 dry season in the Amazon. Aerosols are the tiny particles that make up smoke, dust, and ocean vapor, and affect cloud formation and rainfall depending on their density
“As we’d expected in applying our model, increased smoke from the fires created clouds rife with a more pronounced radiative effect — rich with human-caused aerosols that absorbed sunlight, warmed the local atmosphere, and blocked evaporation. This led to reduced cloud cover over the Amazon,” said co-author J. Vanderlei Martins of the University of Maryland Baltimore County. “And it’s encouraging to know the science behind our model should stand no matter the region.”
In other words, aerosol-rich clouds tend to create more cloud cover but generate less rainfall. However aerosols also affect cloud formation by absorbing heat from the sun, which in turn can make conditions less hospitable for cloud growth. So it’s a complex relationship but NASA says the research helps clarify the picture.
“This result helps us understand aerosols’ effect on a cloud’s mass and lifetime — how long it will provide cloud cover, how deep the clouds will be, and when and where it will rain,” added Remer. “This improved understanding leads to prediction and prediction can help us plan and perhaps prevent some of the potential consequences of putting aerosols from human activity into the atmosphere.”
