Global Dimming & Global Brightening

Unless you've been living in a (deep) hole, you've heard about global warming and the greenhouse effect. However, you might not have heard of the related environmental phenomena known as global dimming and global brightening. What are global dimming and brightening? Basically, they are just what they sound like. Global dimming is when less light reaches the earth's surface, as if the sun was dimmer than before. Global brightening occurs when increased light reaches the earth's surface. However, the activities of the sun aren't the cause of variations in the amount of light so much as changes in the composition of the atmosphere.

Slate Magazine ran a recent feature on global dimming which referred to a BBC documentary proposing a gloom-and-doom scenario of diminished sunlight. What are the take-home messages from the Slate article? First, that the earth may be getting brighter, not dimmer. And second, increased brightness may not be indicative of a 'better' climate.

I mentioned the earth's atmosphere plays a major role in how much light reaches the earth's surface. Aerosols filter out sunlight, particularly soot particles (from volcanoes or from human activities). Water droplets form around these particles, changing the reflectivity of clouds. If you have more aerosol particles, you have many tiny water droplets in clouds as compared with fewer, larger droplets. Multiple tiny droplets have more surface area, or reflective surfaces, than fewer larger droplets, thereby increasing the amount of light bounced back from clouds. Light from the sun is less likely to reach the earth; radiation from a warm earth is better insulated by the same clouds.

The Slate article states that the average amount of light reaching the earth's surface decreased about 10% between 1958 and 1988, however since then, there has been a brightening trend. Was the dimming bad? Is the brightening good? What does it mean? Those are big questions with hotly debated answers. Diminished light can result in less evaporation of water from the earth's surface which in turn leads to less rain. It also results in cooler temperatures.

Though the earth as a whole may be getting brighter again, there are areas which are becoming dimmer. The solar radiation reaching China and India has diminished, possibly due to increased aerosol emissions by those countries. Another thing to consider is that the light bounced back from water droplets has to go somewhere, so while the earth's surface in those regions may be darker, the upper atmosphere is brighter and warmer. It's a complicated situation, where it's difficult to determine cause and effect. Is global dimming and brightening a cause for concern? Not necessarily. It's a natural process. However, it's something to keep in mind when you consider the effects of atmospheric chemistry on climate.

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Photo: NASA/JPL