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This section originally contains an animated flash presentation, Click here to switch to the full version.
Introduction |
Background |
Method |
Results
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One question which is still unanswered in current atmospheric and climate change research is how much radiant energy do clouds trap within Earth's atmosphere and how much do they reflect back out into space? Because there are so many different types of clouds, and because they form, move, change, and dissipate so rapidly over time and space, it is a difficult question to answer. NASA satellite technology has helped to carry cloud research forward in great leaps over the last several decades, and has revealed that as environmental conditions change, so too do clouds and their roles within the climate system. If scientists can't get the cloud physics right in their models, they cannot predict how cloud changes will affect Earth's total energy budget, and therefore they cannot accurately predict future climate change.
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A team of physical scientists, led by Yoram Kaufman and Lorraine Remer,
Have been studying one particular way in which cloud changes influence climate.
When pollution aerosols mingle with certain kinds of clouds they change the clouds'
properties, making them whiter, more reflective, and longer lasting, which enhances their
ability to shade and cool the surface below them. Dubbed the "indirect effect of aerosols," Kaufman and Remer wanted to find out just how much brightening and prolonging clouds enhanced
their ability to reflect sunlight back to space.
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They were using a new instrument called the Moderate Resolution Imaging Spectroradiometer (MODIS) that NASA had launched aboard both its Terra and Aqua satellites. Because it sees almost the entire surface of Earth every day, across a wide portion of the color spectrum, MODIS is an ideal tool for studying clouds and aerosols.
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In order to study the indirect effect, what was needed was examples. Ilan Koren, a scientist on the team, searched for data showing low-level cumulus clouds mingling with smoke plumes.
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The clouds had to sit on the same layer of atmosphere at roughly the same height. Also, they had to be the same in their convective properties. The clouds all had to be the same type, same size, same distance between them, and the same in their meteorological properties, both inside and outside the smoke.
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The Amazon Basin was the perfect place to look. Every day, the skies over the rainforest produce widely scattered cumulus clouds sitting at the top of the boundary layer-the layer of atmosphere from the surface to anywhere from 1 to 3 kilometers high. These cumulus clouds are all about the same size, they are distributed in a very uniform way with good spacing in between, and they cover most of the Amazon jungle during the dry season in the afternoon. Moreover, during the dry season there from August through October, there are typically hundreds of intense fires burning across the region, producing thick plumes of smoke traveling for hundreds of kilometers. Interestingly, the pattern of where there were no clouds often seemed to match the pattern of where there was smoke: where there was heavy smoke, the cumulus cloud cover went from an average of about 40 percent to zero. This raised the question, how come it's impossible to find smoke and clouds together.
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The animated image originally accompaning this page was showing the disapearence of clouds in the presence of smoke.
Satellite images of the Amazon rainforest rarely show smoke and cumulus clouds
together. Smoke, mainly from agricultural fires, displaces the cumulus clouds that normally form
above the forest each afternoon. A uniform layer of scattered cumulus clouds is typically present,
along with some thunderstorms, over the Amazon rainforest. Click on this image of a day with little
smoke, to see the effect heavy smoke makes on cloud formation.
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Searching through the scientific literature, two former recalls of this phenomena where made.
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In a 1997 paper titled "The Missing Climate Forcing," James Hansen, of NASA's Goddard Institute for Space Studies, and co-authors described their efforts to construct a global climate model in which they accounted for various components in the Earth system that exert warming and cooling influences. The authors stated that, on a global scale, aerosols' overall effect is to cool the planet. Even more substantial than aerosols' indirect cooling effect (making clouds more reflective) is the way in which the tiny particles directly scatter and reflect incoming sunlight back to space. Called the "direct effect," aerosols also cool by reducing the amount of sunlight reaching the surface. (See sidebar at right.) In addition to the indirect and direct cooling effects of aerosols, Hansen and his co-authors also predicted a significant third way in which aerosols can influence climate. They called it the "semi-direct aerosol effect on clouds"-the process by which dark-colored aerosol particles (i.e., the soot in smoke) absorb incoming sunlight and warm the atmosphere relative to the temperature of the surface.
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The net effect of this atmospheric warming, they hypothesized, would be to reduce the upward movement of moisture and, in turn, reduce cloud cover. Andy Ackerman, of NASA's Ames Research Center, and co-authors published a paper in 2000, entitled "Reduction of Tropical Cloudiness by Soot." In that study they used a computer model to demonstrate that energy-absorbing aerosols can have a semi-direct affect on cumulus clouds over the ocean. At the time he wrote his paper, Ackerman was unaware of Hansen's paper and so he wasn't familiar with Hansen's term "semi-direct effect." Instead, Ackerman described it as the "cloud-burning effect of soot." But both groups of scientists described the basic underlying physics of the process in pretty much the same way: as the top of the boundary layer becomes filled with dark-colored particles (like soot), the aerosols absorb sunlight and warm the temperature of the air relative to the temperature of the surface. According to Ackerman, this heating at the top of the boundary layer burns away clouds in two ways: (1) by accelerating the process of evaporation of existing clouds, and (2) by suppressing the upward flow of moisture from the surface needed to form new clouds, herefore suggesting aerosols can also amplify the greenhouse forcing. Fewer clouds means more solar energy enters the Earth system, which amplifies the warming effect of carbon dioxide.
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First, the team had to make sure that the absence of clouds in the smoky areas was not due to
differences in meteorology. They accessed weather data-temperature, humidity, wind speed and direction at various
barometric pressure levels, etc.-from the National Centers for Environmental Prediction (NCEP).
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Once they determined what the average weather conditions were like over the Amazon Basin during the dry season in 2002, they then looked for days in which there were anomalous conditions, such as frontal systems passing through or other shifts in meteorology that might have influenced the cloud patterns. They did not analyze Aqua data on any of the days in which they saw anomalous weather. On most of the days, the NCEP weather data proved that the meteorological conditions were exactly the same in the smoky regions as they were in the cloudy regions. So they knew the smoke had to be the reason why average cumulus cloud cover dropped from forty percent to zero in the presence of heavy smoke: During the dry season clouds form in the morning in the east and by early afternoon the whole Amazon Basin-from
the east to the Andes Mountains-is covered by cumulus clouds. The main sources of humidity for forming
these clouds are: windblown moisture carried by strong sea breezes from the Atlantic, and the upward transport
of moisture from the surface. The steady flows of moisture into the air over the rainforest make it
a very cloudy place, even during the dry season.
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In order to determine which pixels in a given scene contained aerosols, Koren had to determine their
"aerosol optical thickness,"
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which is a measure of how much sunlight the smoke prevented from traveling down through
the column of atmosphere.But when measuring aerosol optical thickness, he had to make sure there were no
"cloud-contaminated" pixels, by detecting the clouds in a very precise way in order to determine which pixels were cloudy,
which were smoke, and which clear-air pixels were actually clear and not confused with shadow from the clouds.
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One question which is still unanswered in current atmospheric and climate change research is how much radiant energy do clouds trap within Earth's atmosphere and how much do they reflect back out into space? Because there are so many different types of clouds, and because they form, move, change, and dissipate so rapidly over time and space, it is a difficult question to answer. NASA satellite technology has helped to carry cloud research forward in great leaps over the last several decades, and has revealed that as environmental conditions change, so too do clouds and their roles within the climate system. If scientists can't get the cloud physics right in their models, they cannot predict how cloud changes will affect Earth's total energy budget, and therefore they cannot accurately predict future climate change.
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Given a square patch of Amazon Rainforest 100 km wide by 100 km long filled with an average coverage
of scattered cumulus clouds, but containing no smoke, the clouds in the computer model reflected 36 Watts of sunlight
back up through the top of the atmosphere for every square meter of surface area. On the other hand, when the same
area was filled with heavy smoke and contained no clouds, the smoke reflected 28 Watts per square meter back up to space.
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In short, the model showed that the smoke increased the amount of solar energy added to the climate
system equal to one 8-watt light bulb per square meter of surface area. This finding proved that aerosols don't just
cool the surface of our planet, but through the semi-direct effect they can also contribute to its warming.
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The process which was observed during the Amazon burning season, was when smoke fills the top of the boundary layer, the smoke particles absorb incoming solar radiation and warm the air while reducing the sunlight reaching the surface.
This stabilizes the air near the surface, thus weakening or eliminating the upward movement of warm air (convection)
and choking off the flow of moisture needed to form clouds. Then, because there is less cloud cover, more sunlight passes
through the atmosphere and warms the surface much more than it would have under the cover of widely scattered cumulus clouds.
The smoke, which used to be seen as a reflector, reflecting sunlight back to space, was now shown to, due to absorption,
choke off cloud formation. This is one of aerosols' most important contributions to the global radiant energy budget.
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But how could such a small-scale event-and one that only lasts a matter of weeks-possibly have a significant effect on Earth's
total energy budget?
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Human production of energy-absorbing aerosols is not unique to the Amazon Basin; the problem is much
more widespread and happening year round. Ackerman observed a large, dense pall of the plumes over a large swath of the
Indian subcontinent, exerting the same cloud-burning effect on cumulus clouds over the Bay of Bengal. And the team has
also observed the semi-direct (warming) effect of aerosols over the Canadian boreal forest, during intense wildfires,
as well as over parts of Africa during the burning season there. Still, there is not a lot known about aerosol absorption
on a global scale, therefore impossible to say right now how significant the semi-direct effect is, but it could explain
why we are witnessing a global warming instead of a global cooling, as expected just by accounting aerosol direct
and indirect effects.
Introduction |
Background |
Method |
Results
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