21 May 2004
Global Warming Could Mean Less Sunshine
by Kate Melville
For forty years scientists have observed a 1.3% per decade decline in the amount of sun reaching the Earth's surface. This phenomenon - "solar dimming" - is due to changes in clouds and air pollution that are impeding the suns ability to penetrate. Researchers believe that the combination of growing quantities of man-made aerosol particles in the atmosphere and increased moisture are causing the cloud cover to thicken.
Despite this decline in solar radiation, the Earth's surface continues to warm. Now, researchers led by Dr. Beate Liepert of Columbia University suggest an explanation for this paradox, as well as new findings that a warmer world may mean a dryer and dimmer world.
Published in Geophysical Research Letters, Liepert and fellow researchers' findings suggest that solar radiation is being both reflected and trapped in the clouds and aerosol layer, thereby decreasing the amount of radiation that would ordinarily hit the Earth's surface. It is widely agreed that greenhouse gas trapping is causing the Earth's surface temperatures to rise. What has not been understood until now is that temperatures would be rising faster or higher if the aerosol layer and cloud cover were not reflecting some of the radiation away. Further, the researchers conclude that the imbalance of less solar radiation with warming surface temperatures will lead to weaker turbulent heat fluxes resulting in a reduction in evaporation and precipitation, which will lead to a dryer world.
Although rising temperatures should moisten the atmosphere, the research shows that man-made airborne aerosols will condense the water to form smaller cloud droplets. This process is contributing to the observed thickening of the Earth's cloud cover. Smaller droplets are not heavy enough to sink through the air as rain. As a result, the cloud cover lasts longer and there is less rain.
"Water has a characteristic residence time in the atmosphere before it gets rained out. In a warmer world, this residence time is longer because a warmer atmosphere can hold more water. Aerosols affect clouds by suppressing rain and increasing its residence time. The overall effect is that rainwater is about half a day older," said Liepert.
Examples of data supporting this new hypothesis include studies indicating a steady decline of water evaporation in the Northern Hemisphere over the past 50 years. Over the last 60 years, large regions of Eurasia have seen soil moisture increase by more than one centimeter per decade, yet no significant changes in precipitation are being seen.
Solar dimming has also resulted in an observable difference in the lightness of daytime. The atmosphere is more polluted and therefore darker, even in remote areas. The fog we see today is about 3% thicker than it was 40 years ago.
Liepert conducted model simulations for this research that included pre-industrial aerosol and greenhouse gas conditions and present day conditions. Their models incorporated aerosol absorption of sunlight, heating of the aerosol layer, aerosol particles acting as cloud condensation nuclei and increasing reflective power, and cloud lifetime by the suppression of drizzle over oceans.
"Aerosols are highly variable in space and time, which is why aerosol forcing of climate has generally not been taken into account in climate studies. Furthermore, aerosols are found near the Earth's surface and affect mainly the fluxes of energy and water at the surface. Because good surface observations are sparse, validating models is a very difficult task. Carbon dioxide concentrations are much more homogeneous and easier to measure than aerosol concentrations," said Liepert. "These new ideas on the affects of aerosols might open up many avenues and solve more discrepancies in the climate change debate."