In the years from the 1950s to the early 1990s, scientists noticed something unusual: There was less light from the sun reaching the surface of the Earth. Eventual cloud-top, and even space-based, measurements showed that the Sun itself was fine, and putting out the normal amount of light. The conclusion was that something in the atmosphere was blocking it. The phenomenon was called “Global Dimming”.
Since the 1990s, the trend appears to be reversing, at least in Europe and North America. It continues to be an issue for India and China, however, and places that are downwind from them.
Global Dimming appears to be directly related to human-generated pollution, in particular soot and fine aerosols. These are characteristic of coal-burning power plants. Since the 1970s, legislation in Europe and North America, like the United States’ Clean Air Act, has removed much of the particulate matter and soot from the exhaust stacks of their coal plants. Europe has also moved to reduce its reliance on coal-fired plants, resulting in further declines.
China and India, however, have increased their use of coal-fired plants , and are the biggest consumers of coal, and producers of particulate and aerosol pollution, in the world.
The effect works in a number of ways. High-altitude aerosols can work directly to either absorb or reflect sunlight. Small particulates can also aid cloud formation, resulting in larger clouds with smaller water droplets, which then reflect and absorb sunlight during the day, and radiate heat at night. Aircraft contrails seem to have the same effect, and over central North America aircraft alone account for a 1% drop in sunlight reaching the surface.
Global dimming is not truly global in its effects, and the extent of the dimming is dependent on a number of factors. Wind patterns, air pressure, and air temperature all affect the process. Localized dimming and even surface cooling has been noticed in relation to dimming, while other areas continue to clear and brighten. There is a definite correlation between coal-plant usage and localized dimming.
Since the 1950s, global dimming has caused drops of up to 3% per decade in the amount of visible light reaching the Earth’s surface, at least until 1990. The effect is thought to have masked some of the effects of global warming, and there is an expectation that global warming will get worse, and faster than previously thought, as the masking effect is removed.
High altitude aerosol dispersal is one the methods being researched in geoengineering to help mitigate the effects of global warming. Based on the observation of the last 60 years, it would likely work, but the effects would be intermittent, and it is possible that the project could go too far. If too much light gets reflected into space, the surface will cool, larger clouds will form, and more sunlight will be reflected, in a vicious circle known as a feedback loop.
In the move from fossil-fuels to renewable sources of power, there are likely to be climatological surprises along the way. Even yet, we do not understand the full extent of how we have changed the Earth’s climate over the past 100 years. Global dimming, and global masking, are two unexpected effects. As fewer power plants belch smoke and aerosols into the air, global warming may actually worsen, at least until greenhouse gas levels start to fall. They will eventually do so, drawn out by the trees, oceans and any human sequestration projects, but until then we may be in for a rough ride.