global dimming

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A reduction in solar radiation reaching the earth's surface, caused by fine particulate pollution in the atmosphere, which reflects the sun's radiation back into outer space, especially when water vapor condenses around the particles. Global dimming causes a reduction in the average global temperature and is thought to disrupt weather patterns.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A worldwide decline, during the last few decades, of the intensity of the sunlight reaching the Earth's surface, caused by particulate air pollution. Such dimming has a dampening effect on global warming.