CLIMATE
The Earth System exists in a state of dynamic equilibrium.
The net result of all the interactions between the planet's subsystems, when averaged over periods of decades to centuries, is the Earth System's mean condition, or the "global climate," for the period considered.
Earth's climate -- as reflected in local, regional, and global distributions of temperature and precipitation -- is determined by the latitudinal distribution of the energy received from the Sun, the absorption and transport of that energy within the Earth System, and its re-emission to space.
While Earth's climate has varied, the actual range of variation over geologic time has been surprisingly narrow.
Astronomical effects set the stage for Earth System's climate:
Earth's rapid rotation about its axis (compared to the time for one orbit around the Sun) determines day and night, spreads the incoming solar radiation more or less uniformly around circles of latitude, and strongly affects circulation patterns in the oceans and atmosphere.
The eccentricity of Earth's orbit modulates the intensity of the incoming solar radiation through the course of the year.
The tilt of Earth's axis with respect to the ecliptic plane results in the seasons.
The eccentricity of Earth's orbit and the orientation of the planet's axis of
rotation vary over time; these variations are termed the Milankovitch cycles
Shortly after it formed, the Sun was only about 70% as bright as it is today.
This gives rise to the "Paradox of the Faint Young Sun": even though the Sun was less bright in the distant past and has warmed by about 25%, Earth's global temperature has remained the relatively narrow range +5°C to -10°C from the present
15° C.
Fluctuations in the Sun's output (apparently with a period of 500 to 1000 years) can produce small changes in climate over intervals of 100 to 200 years (e.g., the "Little Ice Age") this is controversial.
The movement of the continents across Earth's surface modulates the global climate on very long time scales by changing wind patterns, ocean currents, and the ease with which ice can accumulate.
The major oceanic and atmospheric circulations are driven by latitudinal differences in heating of Earth's surface, and continuously transport energy poleward from the warm equatorial region.
The global circulations of the atmosphere and oceans currents are strongly influenced by Earth's rotation, its seasonal cycle, and the current arrangement of its continents and ocean basins.
Earth's two fluid envelopes are closely coupled, e.g., changes in the
pattern of sea surface temperatures are reflected in changes in wind patterns
around the globe (for specifics, see discussion of El
Nino, La Nina, and the Southern Oscillation.
The ice component of hydrosphere -- the
cryosphere -- has varying global and seasonal distribution, and is sensitive
to small changes in Earth's climate.
Since ice and snow have relatively low albedoes, they tend to reflect more sunlight than land or ocean surfaces. This provides a positive ice-albedo feedback that tends to further
... cool the planet as ice- and snow-covered areas expand,
... warm the planet as these areas contract.
The mean global temperature of about 15°C is maintained
by the Greenhouse effect.
The current states of Earth's subsystems, indeed, the very existence of the current subsystems, is the result of biogeochemical cycles operating over long time.
Over geologic time, the biogeochemical cycle of carbon is one of the main regulators of Earth's climate.
The global average amount of atmospheric water vapor appears to be controlled by the amounts of carbon dioxide and other trace gases present in the atmosphere.
Clouds contribute both to cooling the Earth System by reflecting sunlight back to space and to warming the Earth System by absorbing and re-emitting terrestrial radiation back to surface.
The oceans, with their large thermal inertia, play a key role in stabilizing Earth's climate.
Earth's current climate has been in an ice
age for the last 2.4 M years.
Overall, the current Pleistocene age can be considered a period of cold, dry conditions with extensive snow and ice cover and lowered sea levels, punctuated by brief interludes of warm, wet conditions with reduced snow and ice cover and elevated sea levels.
At present we are in the Holocene interglacial, a period relatively warm, wet conditions notable for its stable climate.
Geologic evidence establishes that the extent of coverage of Earth's surface by ice has varied widely during the planet's history.
The first global ice age appears to have occurred about 2.7B YBP, in the Precambrian. Large ice sheets also appear to have been present in the Permian.
For details on today's snow and ice cover, see Cryosphere.
Glaciers and ice sheets modify components of the Earth System, affecting hemispheric weather, supporting landforms, and the amount of energy received from the Sun. They are important reservoirs in the hydrologic cycle, storing water for thousands of years.
Eruption of large tropical volcanoes
(one every 100 to 200 years) can produce small changes in the climate over intervals
of 1 to 10 years.
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