INTRODUCTION

To maintain a steady global temperature, Earth must emit the same amount of energy back into space as it receives from solar radiation. Earth's radiant energy balance is the accounting of the average annual energy gain from solar radiation versus the average annual loss from infrared radiation that leaves the Earth system. When the energy gain and energy loss are in balance, Earth neither heats up nor cools down.

Many physical factors of the Earth and its atmosphere play roles in Earth's radiant energy balance. Without the atmosphere, for instance, the average surface temperature on Earth would be about 35°C colder than it is at present. The warming of Earth that results from retention of heat in its atmosphere is called the greenhouse effect. Changes to the atmosphere, therefore, can change the temperature of Earth.

ANIMATION SCRIPT

The energy that drives global climate patterns ultimately originates from the sun. Of the incoming solar radiation, about 30% is reflected by clouds and atmospheric gases and by the surface of Earth. The reflected solar radiation makes Earth visible from space. The rest of the solar radiation is absorbed by gases in the atmosphere and absorbed by the surface of Earth.

When an object absorbs electromagnetic radiation, it heats up and, in turn, re-emits electromagnetic radiation, but in much longer, less energetic, infrared wavelengths. In this way, a relatively large amount of energy is emitted by Earth's surface.

Some of the emitted infrared radiation is transmitted straight through the atmosphere and lost to space. However, the rest is absorbed by gases in the atmosphere. These molecules are warmed and then re-radiate photons, many of them back to Earth's surface and to clouds, where they are again absorbed. This continual process of re-absorption and re-radiation keeps energy within the Earth's system, heating both the Earth's surface and the lower atmosphere. Radiant exchange between the surface and the atmosphere explains why Earth's surface is warmer than it would be if there were no atmosphere at all.

Ultimately, however, the radiant energy emitted into space balances the amount of incoming solar radiation that is absorbed by Earth's surface and atmosphere. The warming of Earth's surface that results from the retention of energy by the atmosphere is called the greenhouse effect. A subset of gases in the atmosphere strongly absorb infrared radiation. These so-called greenhouse gases include carbon dioxide, methane, and nitrous oxide, as well as water vapor.

The greenhouse effect is a natural effect—but also one that human activities are influencing. By burning fossil fuels and through other activities, humans are increasing the atmospheric concentrations of greenhouse gases, especially carbon dioxide and methane. This is increasing the radiant exchange between Earth's surface and lower atmosphere, raising the temperature of both. With its greenhouse gases, Earth's average surface temperature is 17°C, rather than -18°C, which it would be without an atmosphere.

CONCLUSION

Human activities are influencing Earth's radiation balance in the following ways:

• Burning fossil fuels and clearing forests add CO₂ (a greenhouse gas) to the atmosphere, which means that more radiation is re-radiated back to Earth.
• Expansion of livestock and wetland crop production adds CH₄ and N₂O (also greenhouse gases).
• Deposition of dust and dark-colored soot particles ("black carbon") energy absorbed by snow and ice, thereby increasing the melting of persistent ice fields such as the Greenland ice sheet and Arctic sea ice, as well as advancing the timing of spring snowmelt in some temperate regions.