While researchers argue whether Earth is getting warmer and if humans are contributing, a heated debate over the global effect of sunlight boiled to the surface today.
And in this debate there is little data to go on.
A confusing array of new and recent studies reveals that scientists know very little about how much sunlight is absorbed by Earth versus how much the planet reflects, how all this alters temperatures, and why any of it changes from one decade to the next.
Determining Earth's reflectance is crucial to understanding climate change, scientists agree.
Reports in the late 1980s found the amount of sunlight reaching the planet's surface had declined by 4 to 6 percent since 1960. Suddenly, around 1990, that appears to have reversed.
"When we looked at the more recent data, lo and behold, the trend went the other way," said Charles Long, senior scientist at the Department of Energy's Pacific Northwest National Laboratory.
Long participated in one of two studies that uncovered this recent trend using satellite data and ground-based monitoring. Both studies are detailed in the May 6 issue of the journal Science.
Thing is, nobody knows what caused the apparent shift. Could be changes in cloud cover, they say, or maybe reduced effects of volcanic activity, or a reduction in pollutants.
This lack of understanding runs deeper.
A third study in the journal this week, tackling a related aspect of all this, finds that Earth has reflected more sunlight back into space from 2000 to 2004 than in years prior. However, a similar investigation last year found just the opposite. A lack of data suggests it's impossible to know which study is right.
The bottom line, according to a group of experts not involved in any of these studies: Scientists don't know much about how sunlight interacts with our planet, and until they understand it, they can't accurately predict any possible effects of human activity on climate change.
Reflecting on the problem
The percentage of sunlight reflected by back into space by Earth is called albedo. The planet's albedo, around 30 percent, is governed by cloud cover and the quantity of atmospheric particles called aerosols.
Amazingly, one of the best techniques for measuring Earth's albedo is to watch the Moon, which acts like a giant mirror. Sunlight that reflects of Earth in turn reflects off the Moon and can be measured from here. The phenomenon, called earthshine, was first noted by Leonardo da Vinci.
Albedo is a crucial factor in any climate change equation. But it is one of Earth's least-understood properties, says Robert Charlson, a University of Washington atmospheric scientist. "If we don't understand the albedo-related effects," Charlson said today, "then we can't understand the effects of greenhouse gases."
Charlson's co-authors in the analysis paper are Francisco Valero at the Scripps Institution of Oceanography and John Seinfeld at the California Institute of Technology.
Plans and missions designed to study the effects of clouds and aerosols have been delayed or cancelled, Charlson and his colleagues write.
To properly study albedo, scientists want to put a craft about 1 million miles out in space at a point were it would orbit the Sun while constantly monitoring Earth.
The satellite, called Deep Space Climate Observatory, was once scheduled for launch from a space shuttle in 2000 but has never gotten off the ground. Two other Earth-orbiting satellites that would study the albedo have been built but don't have launch dates. And recent budget shifts at NASA and other agencies have meant some data that's available is not being analyzed, Charlson and his colleagues contend.
While some scientists contend the global climate may not be warming or that there is no clear human contribution, most leading experts agree change is underway.
Grasping the situation is crucial, because if the climate warms as many expect, seas could rise enough to swamp many coastal communities by the end of this century.
Charlson says scientists understand to within 10 percent the impact of human activity on the production of greenhouse gases, things like carbon dioxide and methane that act like blanket to trap heat and, in theory, contribute to global warming. Yet their grasp of the human impact on albedo could be off by as much as 100 percent, he fears.
One theory is that if humans pump out more aerosols, the small particles will work to reflect sunlight and offset global warming. Charlson calls that "a spurious argument, a red herring."
Greenhouse gases are at work trapping heat 24 hours a day, he notes, while sunlight reflection is only at work on the day side of the planet. Further, he said, greenhouse gases can stay in the atmosphere for centuries, while aerosols last only a week or so.
"There is no simplistic balance between these two effects," Charlson said. "It isn't heating versus cooling. It's scientific understanding versus not understanding."