by Kenneth Chang
New York Times
May 6, 2005
Reversing a decades-long trend toward "global dimming," Earth's surface has become brighter since 1990, scientists are reporting today.
The brightening means that more sunlight - and thus more heat - is reaching the ground. That could partly explain the record-high global temperatures reported in the late 1990's, and it could accelerate the planet's warming trend.
"We see the dimming is no longer there," said Dr. Martin Wild, a climatologist at the Swiss Federal Institute of Technology in Zurich and the lead author of one of three papers analyzing sunlight that appear in today's issue of the journal Science. "If anything, there is a brightening."
Some scientists have reported that from 1960 to 1990, the amount of sunshine reaching the ground decreased at a rate of 2 percent to 3 percent per decade.
In some places, the brightening of the 1990's has more than offset the dimming, Dr. Wild said. In other places, like Hong Kong, which lost more than a third of its sunlight, the dimming has leveled off, but skies remain darker than in the past. In a few places, like India, the dimming trend continues, he said.
The new papers also call attention to a major gap in the understanding of climate. Scientists do not exactly know what caused the dimming and the brightening, or how they affect the rest of the climate system.
Earth reflects about 30 percent of the incoming sunlight back into space. Slight changes in the reflectivity, possibly caused by changes in cloud cover and air pollution, can have as much impact on the climate as heat-trapping greenhouse gases like carbon dioxide.
Some scientists say that the dimming and the brightening might explain why for many years temperatures on Earth lagged what was predicted by many climate models and then shot upward more recently.
"I think what could have happened is the dimming between the 60's and 80's counteracted the greenhouse effect," Dr. Wild said. "When the dimming faded, the effects of the greenhouse gases became more evident. There is no masking by the dimming anymore."
But Dr. Rachel T. Pinker, a professor of meteorology at the University of Maryland who led the team that wrote one of the other papers, said the picture might not be so simple. More sunlight should increase evaporation rates, leading to more clouds, and the additional cloud cover could then increase Earth's reflectivity, limiting the warming effect.
"I think that's a complex issue," Dr. Pinker said. "There are many feedbacks involved."
The findings of Dr. Wild and his colleagues are based on data through 2001 from a network of ground-based sensors that directly measure the sunlight hitting the ground. But the sensors are not evenly distributed, with the greatest number in Europe, few in Africa and South America, and none covering the 70 percent of Earth's surface that is water.
Dr. Pinker's team analyzed satellite data from 1983 to 2001 that covered the globe. Its findings about brightening, which basically agree with Dr. Wild's, rely on computer models to estimate how much sunlight reaches the surface.
Finally, a team led by Dr. Bruce A. Wielicki of NASA's Langley Research Center in Virginia reports that measurements from the agency's Aqua satellite show a slight decrease in the amount of light reflected off Earth since 2000, which corresponds to a brightening on the surface.
The NASA findings conflict with measurements, reported last year, suggesting that Earth had resumed dimming since 2000. Those measurements looked at the illumination of the dark side of the Moon by light reflected off Earth.
Dr. Philip R. Goode, a professor of physics at the New Jersey Institute of Technology who was one of the researchers behind last year's report, said it was not clear why the findings differed so markedly. "We've been working with them to understand the origins of the differences," Dr. Goode said of the Wielicki group.
Dr. Wielicki said his data supported a report last month by a team led by Dr. James E. Hansen of the Goddard Institute for Space Studies in New York. In a paper published on Science's Web site, Dr. Hansen and his colleagues said much of the excess heat generated by global warming has been stored in the oceans. Even if no more greenhouse gases are added to the atmosphere, they said, Earth will continue to warm by 1 degree Fahrenheit over the coming decades, as the heat in the oceans is released into the air.
Dr. Wielicki said the amount of energy coming from the Sun matched the gain in heat in the oceans reported by Dr. Hansen. "It is consistent with the ocean heat storage that the oceanographers are seeing," Dr. Wielicki said, "and it is consistent with the climate models' predictions of what the heat storage should be."
Dr. Robert J. Charlson, a professor of atmospheric sciences at the University of Washington and an author of a commentary that accompanied the three papers, said, "This set of papers, taken together, calls attention for more emphasis on research in these topics."
But he added, "Unfortunately, impediments have come up." Four years' worth of data from the Earth Radiation Budget Satellite is unanalyzed, he said, because there is no money for scientists to work with it.
Another satellite, the Deep Space Climate Observatory, which was scheduled to be launched on a space shuttle, awaits in storage. Proposed budget cuts in earth science research at NASA could limit the analysis of data from other satellites, Dr. Charlson said.