Earth's 'sunscreen' is thinning, NASA says

The layer of volcanic dust, pollution and other aerosol particles that blocks sunlight and helps counter global warming has thinned since the early 1990s, according to a new NASA study.

The layer of volcanic dust, pollution and other aerosol particles that blocks sunlight and helps counter global warming has thinned since the early 1990s, according to a new NASA study.

The thinning of the layer of reflective particles in the atmosphere could have helped raise global surface temperatures, the authors say in a paper published in Friday's issue of the journal Science.

"When more sunlight can get through the atmosphere and warm Earth's surface, you're going to have an effect on climate and temperature," said lead author Michael Mishchenko in a statement. "Knowing what aerosols are doing globally gives us an important missing piece of the big picture of the forces at work on climate."

The study by NASA's Goddard Institute for Space Studies used records dating back to 1978 from satellites originally designed to observe clouds and weather systems. The results showed large, short-lived spikes in global aerosols caused by major volcanic eruptions in 1982 and 1991 but a gradual decline since 1990.

By 2005 global aerosols had dropped as much as 20 per cent from relatively stable levels from 1986 to 1991.

The new data provides another look at one of the potential causes behind global warming, which a recent UN-led report described as "unequivocal," noting that 11 of the highest average global annual temperatures recorded have come in the past 12 years.

'Dimming' and 'brightening'

The results of the study also coincide with previous observations that showed the amount of sunlight reaching the Earth has increased since 1990 after years of declining. The switch from "global dimming" to "global brightening" happened at the same time as the change in aerosol levels, said Mishchenko.

The Science paper does not conclusively prove the drop in aerosol levels is connected to "global brightening," saying changes in cloud cover cannot be ruled out as a cause.

While increased sunlight from a drop in aerosols could assist global warming, a paper published last week in the American Geophysical Union's Geophysical Research Letters suggests it is only one factor, noting that the seemingly opposing forces of global dimming and warming have occurred at the same time.

The rise in both greenhouse gases and aerosols during this periodmay have trapped some of the heat high in the atmosphere at the same time as direct solar radiation decreased,according to computer models developed byresearchers led by Anastasia Romanou of Columbia University's Department of Applied Physics and Mathematics.

The lower solar radiation may explain why the planet also became dryer: during warmer cycles, sunlight reaching the oceans would normally evaporate more water and increase cloud formation and rainfall.

NASA said current data doesn't tell them whether the aerosols in the atmosphere are naturally occurring or man-made, saying such analysis will have to wait until the 2008 launch of Glory, the space agency's new Earth-observing satellite.