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Greenhouse Gases

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The Earth's Energy Balance

The earth's major energy source is the sun.  About half of the radiant energy that enters our atmosphere is either reflected or absorbed by the molecules that make up our atmosphere.  While, oxygen and ozone intercept much of the UV radiation, the rays that are not intercepted are infrared rays.  This radiation is absorbed by the greenhouse gases in the earth's atmosphere, and as a result, the entire earth is warmed.  The current average temperature is approximately 16°C (60°F), which is much higher than the -270°C of space.

Almost 85% of the heat the earth radiates is absorbed by gases in the atmosphere and then reradiated back to the earth's surface.  As a result of this exchange, the total energy input from the sun balances the energy output from the earth.  A steady state is established, with more or less average temperatures

The atmospheric "greenhouse" is made up of molecules that absorb infrared rays.  Carbon dioxide, water vapor, methane and several other atmospheric compounds all are capable of doing this and are called Greenhouse Gases.  They permit the radiation coming from the sun to pass through, and absorb much of the heat reradiated by the earth, as seen in the above figure.

Obviously, the Greenhouse Effect is essential in keeping out planet habitable, but some carbon dioxide in the atmosphere does not mean that more is better.  An increase in the concentration of these Greenhouse Gases will very likely mean that more than 85% of the radiated energy will be returned to the earth's surface, resulting in an significant increase in the earth's average temperature.  The key to assessing the current and future status of the Greenhouse Effect are the recent trends in atmospheric carbon dioxide and average global temperature.  There is evidence that carbon dioxide concentrations have risen by about 25% in the last century.  Other measurements indicate that during the past 120 years, the average temperature of the planet has increased by somewhere between .4 and .8°C.  Most scientists agree that there is some sort of trend.  Evidence indicates that carbon dioxide from human related sources are a cause of recent global warming.  Current estimates state that doubling the carbon dioxide concentration in the atmosphere will result in a temperature increase between 1 and 4°C.  If and when that doubling will occur depends on the actions of the human beings who inhabit the planet.  While, we are a long way from destroying the earth, we face difficult decisions about how to prevent irreversible devastating destruction from occurring due to the Greenhouse Effect.