The basic premise revolves around the emission of aerosol particles, such as sulfur dioxide, into the upper troposphere on which water can condense and form, resulting in clouds with many tiny water droplets rather than fewer larger droplets. Small droplets are better reflectors than large ones. Thus, reflecting more sunlight results in a “dimming” and cooling of the Earth.

It appears, then, that this cooling caused by global dimming likely partially counteracts the warming of the greenhouse effect. How much is not known. But two notable events related to global dimming provide some insight into their impact:

“Scrubbers”:
The cool period during the 1940s-70s is believed to be linked to aerosols, especially sulfur dioxide released in mass quantity from low-quality coal burning. In the early 1970s, the introduction of the use of “scrubbers” inside the coal plants dramatically reduced these emissions. And given the very short lifetime of most aerosols (SO₂ degrades at 1-2% per hour; CO₂ has a 5-150 year atmospheric lifetime), this had an immediate impact on halting the cooling trend.
Jet contrails:
The climate effects of jet contrails—condensation trails sometimes seen in the wake of an airplane—has long been an unsolved mystery simply because of a lack of control, i.e. a day without jets in the sky, for comparison. But following the terrorist attacks of September 11th, 2001, the U.S. closed its entire airspace for three days, and a control was at hand. United States temperatures during this period were compared with the same period averaged over the previous 30 years. The result: a significant increase (~1-2F) in the daily temperature range (the difference between daytime high and nighttime low). It was shown that these contrails act similar to high cirrus clouds, which efficiently reflect incoming sunlight during the day (keeping highs cooler) and block outgoing infrared radiation at night (keeping lows warmer). Thus, the temperature range decreases in the presence of contrails.

Infrared satellite image from 4 Jun 1999 5:48a EDT over the Eastern U.S. Many jet contrails are clearly visible at this time. Beneath some of the wider streaks of contrails, such as in Central Pennsylvania, it likely would have looked mostly cloudy to someone on the ground when in fact no true clouds were in the area.
Source: http://fermi.jhuapl.edu/avhrr/gallery/observations/contrails/1999Jun04/CONTRAILS_ch4.n14.99jun04_0948.gif
So what is the implication for global warming? This isn’t clear, as it appears that the daytime and nighttime effects at least partially cancel out. But what is clear is that a human influence as seemingly minor as jet contrails can have a major impact on our climate.

Global Dimming

This is a concept that is foreign to most people, but it is an important component of the climate change debate.

Do we have other evidence of recent climate change? Yes, see Other Evidence.