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How Does Air Pollution Affect Clouds?

The answer could help clarify how warm climate change might get
cloud formation due to pollution


Few clouds used to populate our skies before the Industrial Revolution, and pollutants spewed by factories since then have vastly increased the cloudiness of our atmosphere. 
Credit: Thinkstock

Deadening calm fills the Horse Latitudes, where there's ocean, sky and little else. A satellite peers down, capturing wisps of cloud, counting particles suspended in the air, measuring rainfall and monitoring weather.

There is little wind. These latitudes, between 30 and 35 degrees away from the equator, are so calm that Spanish sailors in the 17th century could not move their heavily laden ships, or so the legend goes. So, the sailors dumped their cargo—horses—into the subtropical ocean and heaved on. But they left the place with a name: Horse Latitudes.

These windless tracts have yielded a new hypothesis relevant to climate science: Few clouds may have populated our skies before the Industrial Revolution, and pollutants spewed by factories since then may have vastly increased the cloudiness of our atmosphere. The results were published yesterday in the journal Science.

The finding cuts to the heart of uncertainty contained in climate models today. Most scientists agree that humans are releasing massive quantities of carbon dioxide into the atmosphere and causing global temperatures to rise. But they disagree on the rate of warming. A doubling of CO2 concentrations could warm the planet by between 2 and 4.5 degrees Celsius, according to the Intergovernmental Panel on Climate Change (IPCC).

Part of the uncertainty is due to clouds. They come in various shapes and types, as most people know—puffy popcorns (cumulus); loose brush strokes of mostly ice (cirrus); towering, dark monsters of thunderstorms (cumulonimbus) and many others.

Clouds can either reflect the sun's incoming rays back into space, cooling the Earth. Or they can act as a sheath and trap heat close to the Earth's surface, warming the planet. Often, they do a little of both. And they do it incredibly well. Clouds have the ability to heat the planet much more than CO2, depending on the type of cloud, its geography and its altitude. And to make things more complicated, cloud particles can have various sizes, shapes and various traits. Translating these into predictions about the overall effect of clouds on the climate can be quite difficult.

Replacing a simplistic view
Today's climate models do include clouds, but some types are better represented than others.

"Unfortunately, the climate system is very sensitive to little changes in the cloud," said Andreas Muhlbauer, research scientist at the Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington. He was not involved in the Science study. "So being off by just a bit in a climate model can have a significant impact on the ability to predict."

Lumpy clouds (marine stratocumulus clouds) off the west coast of continents, but no others. The Science study unravels a different cloud type—the cumulus—which is not represented in climate models.

These clouds are so complicated that it can take a couple of days to explain them, said Ilan Koren, a planetary scientist at the Weizmann Institute of Science in Israel and lead author of the study, reached while en route to a Rolling Stones concert.

"I'm sorry, but there are no simple answers here," he said.

Koren and his colleague, Orit Altaratz, also a scientist at Weizmann, base their findings on a well-accepted theory—that clouds grow rapidly in the presence of microscopic particles called aerosols. In the past, aerosols used to be microscopic salt particles from the ocean, debris from volcanoes, organic material or but of soil carried by the wind. Since the Industrial Revolution, black carbon and soot from cars, factories and cookstoves constitute most of the cloud-forming aerosols.

Aerosols are key in whipping up a cloud, a process that begins with the sun. As the sun's rays hit the ocean, water evaporates into the gas phase. Water vapor attaches itself to aerosol particles floating in the air and condenses into a seed of water and dust that blooms into a full-fledged cloud that climbs up the sky.

In the absence of aerosols, there can be no cloud. Scientists on an icebreaker in the Arctic demonstrated this in a video of a cup of hot tea that does not fume despite the below-zero temperatures. Then, someone flicks on a lighter and water vapor from the tea grabs aerosol particles emitted by the lighter (inefficient combustion) and a tiny storm appears, above the teacup.

Adding just a little bit of pollution goes a long way toward cloud formation in a very pristine environment, said Andreas Muhlbauer, research scientist at the Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington. 

"Ultimately, it [aerosols] affects the amount of clouds that are out there, and also the properties of the clouds—the area, for example, they cover over the globe. And all that affects the radiation that can actually hit the [Earth's] surface," Muhlbauer said.

Computer models must wait for better data
To demonstrate the aerosol effect, Koren and his colleagues observed clouds forming in the Horse Latitudes of the Southern Hemisphere. This region of the global oceans has little wind, which means pollutants are not easily carried over from continents. A few clouds may exist, but not too many given the aerosol-starved nature of the region.

The scientists used data from four different satellites to observe the clouds, the aerosol content, temperature, meteorology and rainfall over 92 days in the winter of 2007. They found that the skies became more overcast as the aerosol levels in the air increased naturally. And the effect did not cease; there was no point of saturation beyond which aerosols stopped affecting the clouds.

As the cloud cover doubled, they reflected more incoming solar rays back to space. Thus, the clouds had a cooling effect.

Koren and Altaratz hypothesized that the last time the skies were this clean of aerosols—other than in the Horse Latitudes, that is—was before the Industrial Revolution. The skies then must have been much less cloudy than today, Koren said.

An implication of this theory is that as these cumuls clouds became more widespread at the beginning of the Industrial Revolution could have cooled the Earth. Including these clouds in climate models could alter results significantly.

But there's no way to know for sure. No one maintained records of aerosol levels in preindustrial times, and for now, Koran's suggestion is mere speculation.

Muhlbauer said this was an "interesting" scenario but cautioned that the findings are extremely preliminary. Adding cumulus clouds into climate models is a "long way off," he said.

"That's going to be another 10 years at least."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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