Definition of Role of the Atmosphere in Global Climate

Interactions between the atmosphere, hydrosphere, biosphere,

and lithosphere control global climate. 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, and hence

global temperature. 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, greenhouse gases allow incoming short-wavelength

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).

Plants, which release O2 to the atmosphere, now take up

CO2, by photosynthesis. In the early part of Earth’s history

(in the Precambrian before plants covered the land surface),

photosynthesis did not remove CO2 from the atmosphere,

with the result that CO2 levels were much higher than at present.

Atmospheric CO2 is also presently taken up by marine

organisms that 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), which is 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 is 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

that 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 dry periods, hot wet periods,

or cold 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 AIR PRESSURE; AURORA; CLIMATE; GREENHOUSE

EFFECT; WEATHERING.

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