The Earth's a Darker Place

The amount of sunlight reaching the earth's surface has declined over the last two centuries, a phenomenon that scientists call "global dimming". Researchers from the University of Oslo in Norway calculate that the solar energy landing annually on every square metre of land and water has dropped by an average of 2.4 watts.

The main cause of the earth's darkening is our igniting of fuels. Particles of sulphate, soot and organic carbon released from burning fossil fuels, wood and other organic materials block some direct solar light and scatter even more.

These aerosols in the atmosphere also cause clouds to form smaller water droplets that reflect greater amounts of radiation back into space than do normal clouds. Altogether, particles from burning intercept each year around two watts per square metre (W/m²) of solar radiation more now than before 1800.

Other factors contribute as well to dimmer skies. In areas with busy air traffic, including most of the contiguous United States and Europe, cirrus clouds generated by air travel obstruct substantial amounts of direct sunlight. Condensation trails from jets have reduced direct solar radiation in the northeastern United States annually by 4 W/m² and converted another 19 W/m² of direct radiation to diffuse radiation.

Rising concentrations of atmospheric gases that absorb solar radiation are relatively minor influences. Increases in carbon dioxide, methane, water vapour and nitrogen dioxide capture more of the sun's energy before it reaches the ground. Of these, water vapour has the largest impact, particularly over the tropics, where warming temperatures pump extra water into the air. Globally, greater quantities of radiation-absorbing gases stop 0.3 W/m² of solar radiation from reaching the earth's surface each year.

In total, the annual influx of direct solar radiation has declined by 3.3 watts per square metre around the globe. This has been offset by an increase of 0.9 watts in diffuse radiation. The net reduction since the beginning of the industrial era amounts to 2.4 watts of solar energy.

Regionally, however, changes in solar radiation vary considerably. Skies have mainly dimmed over densely populated areas that release large quantities of particulates. US and China have experienced the largest declines in solar energy, of about 10 W/m² a year, or 10% of their total radiation.

Near the poles, reduced ozone has actually allowed more radiation to penetrate the stratosphere. The polar regions have also escaped much of the accumulating gases, aerosols and jet plumes. In contrast to heavily populated areas, both poles have actually brightened in recent years.

Even greater shifts have occurred in the type of radiation reaching the ground. Direct sunlight has declined by 30% over portions of the eastern United States and by 40% over parts of China. Meanwhile, diffuse sunlight, such as what transmits through clouds, has increased, but not to the same degree.