"Parasol" of the earth becomes thinner
Cooling aerosols have been decreasing since 1990Read out
The Earth's climate is the result of many interacting factors. Now NASA researchers have discovered that an important "sunshade" of the planet is getting thinner and thinner. They report in Science that the amount of aerosols floating in the atmosphere has been slowly decreasing since 1990. However, since these are primarily cooling, this trend could have exacerbated global warming.
The tiny suspended particles in the atmosphere swallow some of the sunlight and reflect it back into space. As a result, they reduce the amount of incident radiation and thus also influence the climate. Scientists at the NASA's Goddard Institute for Space Studies (GISS) in New York have now studied how the concentration of aerosols has changed. From the longest existing uninterrupted series of satellite measurements in the lower atmosphere, researchers around Michael Mishchenko extracted the aerosol signal back to 1978.
Decline since 1990
The result: The concentration of aerosols has been decreasing slowly but steadily since 1990. This trend is only interrupted by short-term peaks after large volcanic eruptions, such as Pinatubo's in 1991, when large quantities of dust and gases were released into the atmosphere. By 2005, total values had fallen by 20 percent from the relatively stable level of the period 1986-1991.
The trend that has now been identified also sheds new light on the previously observed but not satisfactorily explainable decrease in solar radiation in recent decades, which then suddenly began to increase again from 1990 onwards. This change from a global "dimming" to a brightening coincides exactly in time with the decrease in aerosols. "When more sunlight penetrates the atmosphere and warms the earth's surface, it has an effect on climate and temperature, " Mishchenko explains. "Knowing what effect aerosols have gives and an important missing puzzle piece in the big picture of forces acting on the environment." Ad
Models support assumptions about the aerosol effect
However, the study by Mishchenko can not prove that really only the aerosols are behind the observed "dimming" and "whitening trend". Clouds could theoretically be responsible for that. Another study published last week, however, supports the aerosol hypothesis: Scientists led by Anastasia Romanou from the University of Columbia fed nine different climate models with aerosol and greenhouse data, proving that aerosols were indeed involved in reducing solar radiation by 1990. However, emissions of greenhouse gases continued to be affected by global warming during this phase.
"A major part of the dimming trend over the northern hemisphere is due to these direct aerosol effects, " explains Romanou. However, aerosols have other, indirect effects on solar radiation. For example, they increase the reflectivity of the clouds and extend their lifespan. These effects turned out to be almost as significant as the direct effects
"Balancing" slows down climate-induced evaporationAerosol level decreases steadily since 1991 (red line) Michael Mishchenko, NASA
The combined effect of global dimming and warming could also explain why one of the main impacts of a warmer climate is the acceleration of the water cycle from evaporation, increased cloud formation, and more rain has not yet occurred. When less sunlight reaches the surface, it counteracts the effects of warmer air temperatures, so evaporation does not change that much, so according to Gavin Schmidt, co-author of the Science study. "Aerosol levels higher by 1990 have likely slowed the expected changes in the water cycle."
Whether, however, the current trend towards the decrease of aerosols continues, is so far open. The measurements and forecasts are complicated by the fact that the suspended particles are not uniformly distributed in the atmosphere globally and, in addition, come from many different sources. Therefore, it is not yet clear whether the observed increases and decreases are attributable to human activities or, of course, caused by them. Greater clarity is expected from the Glory Mission, NASA's new Earth Observation Satellite, which will enter orbit in 2008.
(NASA / Goddard Space Flight Center, 16.03.2007 - NPO)