DEFINITION OF GREENHOUSE EFFECT

The greenhouse effect refers to time

periods when the Earth is abnormally warm in response to

the atmosphere trapping incoming solar radiation. Global climate

represents a balance between the amount of solar radiation

received and the amount of this energy that is retained in

a given area. The planet receives about 2.4 times as much

heat in the equatorial regions as in the polar regions. The

atmosphere and oceans respond to this unequal heating by

setting up currents and circulation systems that redistribute

the heat more equally. These circulation patterns are in turn

affected by the ever-changing pattern of the distribution of

continents, oceans, and mountain ranges.

The amounts and types of gases in the atmosphere can

modify the amount of incoming solar radiation. For instance,

cloud cover can cause much of the incoming solar radiation

to be reflected back to space before being trapped by the

lower atmosphere. On the other hand, certain types of gases

(known as greenhouse gases) allow incoming short-wave

length solar radiation to enter the atmosphere but trap this

radiation when it tries to escape in its longer wavelength

reflected form. This causes a buildup of heat in the atmosphere

and can lead to a global warming known as the greenhouse

effect.

The amount of heat trapped in the atmosphere by greenhouse

gases has varied greatly over Earth’s history. One of the

most important greenhouse gases is carbon dioxide (CO2)

that plants (which release O2 to the atmosphere) now take up

by photosynthesis. In the early part of Earth history (in the

Precambrian), before plants covered the land surface, photosynthesis

did not remove CO2 from the atmosphere, so CO2

levels were much higher than at present. Atmospheric CO2 is

also presently taken up by marine organisms, which remove

it from the ocean surface water (which is in equilibrium with

the atmosphere) and use the CO2 along with calcium to form

their shells and mineralized tissue. These organisms make

CaCO3 (calcite), the main component of limestone, a rock

composed largely of the dead remains of marine organisms.

Approximately 99 percent of the planet’s CO2 is presently

removed from the atmosphere/ocean system, because it has

been locked up in rock deposits of limestone on the continents

and on the seafloor. If this amount of CO2 were

released back into the atmosphere, the global temperature

would increase dramatically. In the early Precambrian, when

this CO2 was free in the atmosphere, global temperatures

averaged about 550°F (290°C).

The atmosphere redistributes heat quickly by forming

and redistributing clouds and uncondensed water vapor

around the planet along atmospheric circulation cells. Oceans

are able to hold and redistribute more heat because of the

greater amount of water in the oceans, but they redistribute

this heat more slowly than the atmosphere. Surface currents

are formed in response to wind patterns, but deep ocean currents

(which move more of the planet’s heat) follow courses

that are more related to the bathymetry (topography of the

seafloor) and the spinning of the Earth than they are related

to surface winds.

The balance of incoming and outgoing heat from the

Earth has determined the overall temperature of the planet

through time. Examination of the geological record has

enabled paleoclimatologists to reconstruct periods when the

Earth had glacial periods, hot and dry periods, hot and wet

periods, or cold and dry periods. In most cases the Earth has

responded to these changes by expanding and contracting its

climate belts. Warm periods see an expansion of the warm

subtropical belts to high latitudes, and cold periods see an

expansion of the cold climates of the poles to low latitudes.

See also ATMOSPHERE.

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