Now the problem is global brightening: Earth has become brighter, but no one is sure why CNET News.com: "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."