The Earth is “constantly” bathed in solar radiation. On average, the Earth receives 1368 W/m² (1.96 ly/min) of solar radiation at the outer edge of the atmosphere, called the “solar constant“. However, the actual amount received at the edge of the atmosphere and the Earth’s surface varies from place to place and day to day on account of the orientation of the Earth to the Sun. The solar radiation that makes its way through the atmosphere and to the surface is called insolation. The amount of insolation received at the surface depends on 1) the sun angle, 2) day length, 3) ground slope, 4) path length, and 5) the state of the atmosphere.

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Heat Budget of Earth

Global Dimming

Human activities from agriculture to industry release particulates into the air. Studies in many parts of the world indicate that insolation has been decreasing as particulate concentrations have been increasing, a phenomenon called global dimming. Research conducted in the Maldives demonstrated that dirty air descending from India produced a three kilometers thick pollutant layer that hung over the northern islands. The pollutant layer cut down the sunlight reaching the ocean by more than 10 percent. Climate scientists fear that the reduction of insolation by atmospheric particulates are masking the true magnitude of global warming. A reduction of particulate pollution to protect human health may cause an increase in insolation and thus accelerate global warming.