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article imageThe secret life of clouds: How pollution affects climate change

By Karen Graham     Nov 26, 2013 in Science
There has been a long-standing debate on how pollution affects climate warming. Climatologists have always thought that pollution, through a process called convection caused bigger thunderclouds. Instead, they have learned it's just the opposite.
There has been an ongoing discussion by climatologists and researchers for a number of years on how pollution affects global warming. But a recent study conducted by atmospheric scientist Jiwen Fan and her colleagues at the Department of Energy's Pacific Northwest National Laboratory has resolved that question, and in the process provided a gauge that makes weather and climate models easier to understand.
It has been long assumed that thunderclouds became draftier, allowing pollution to create larger and longer-lived storm clouds through convection. This whole process was nipped in-the-bud when researchers discovered just the opposite to be true.
Fan and her team found that pollution makes clouds hang around longer by decreasing their size and increasing the lifespan of cloud and ice particles. This difference was found to affect how scientists represented clouds in computer models.
The research also showed that real-life observations coincided with computer models showing taller and larger anvil-shaped clouds forming in storm systems with pollution, even though stronger convection wasn't always present.
The research showed that pollution can decrease the overall daytime temperatures because of these higher clouds. After a thunderstorm, the clouds usually spread out across the sky, becoming anvil-shaped. While they help cool the earth by day with their shadows, they tend to trap heat at night.
A question began to form based on these observations. If pollution caused the clouds to linger longer than they would in clear skies, then why was this possible?
The scientists went back to studying cloud and ice particles. There are two types of particles, or aerosols, natural and man-made. They serve as seeds for cloud droplets to form around. If pollution is present, it stands to reason there will be more particles, or aerosols, than if the sky is clear.
Aerosols  particularly black carbon  can alter reflectivity by depositing a layer of dark residue on...
Aerosols, particularly black carbon, can alter reflectivity by depositing a layer of dark residue on ice and other bright surfaces. Black ash covered the summit of New Zealand’s Mount Ruapehu in 2007.
NASA Earth Observatory
This means that if there is more pollution, then there is less water available to form around each seed. So pollution ends up making more, but smaller seeds. These smaller, lighter droplets don't end up raining down, but instead, they rise higher, eventually freezing.
The freezing squeezes the heat out of the droplets, making the thundercloud draftier. A chain-reaction ensues, as more and stronger convection lifts more droplets higher, creating bigger and bigger clouds. Using observations in three different areas around the globe, it was found that convection didn't always become a determining factor in size or intensity of thunder clouds.
This one observation could determine the accuracy of future computer models in determining the ultimate affects of pollution on global temperature changes in the next several decades. Accuracy in representing storm clouds in computer models is also essential to the overall predictability of gauging temperature changes.
The research team used real-time visual observations of summer thunderstorms and computer models that could zoom into simulated clouds. The computer models had to include cloud particles as well as the ability to see convection. While most models take days or weeks, this particular model took nearly six months.
More about Pollution, Climate warming, Clouds, particles, Thunderstorms
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