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.

More here.