What Causes Global Warming
Causes Of Global Warming
The average surface temperature
of Earth is
maintained by a balance of various forms of solar and terrestrial
radiation. Solar radiation is often called “shortwave” radiation because the
frequencies of the radiation are relatively high and the wavelengths relatively
short—close to the visible portion of the electromagnetic spectrum. Terrestrial radiation, on the other hand, is often called
“longwave” radiation because the frequencies are relatively low and the
wavelengths relatively long—somewhere in the infrared part
of the spectrum. Downward-moving solar energy is
typically measured in watts per
square metre. The energy of the total incoming solar radiation at the top of Earth’s atmosphere (the
so-called “solar constant”) amounts roughly to 1,366 watts per square metre annually.
Adjusting for the fact that only one-half of the planet’s surface receives
solar radiation at any given time, the average surface insolation is 342 watts
per square metre annually.
greenhouse effectThe
greenhouse effect is caused by the atmospheric accumulation of gases such as
carbon dioxide and methane, which contain some of the heat emitted from Earth's
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The amount of solar radiation
absorbed by Earth’s surface is only a small fraction of the total solar
radiation entering the atmosphere. For every 100 units of incoming solar
radiation, roughly 30 units are reflected back to space by either clouds,
the atmosphere, or reflective regions of Earth’s surface. This reflective
capacity is referred to as Earth’s planetary albedo, and it need not remain fixed over
time, since the spatial extent and distribution of reflective formations, such
as clouds and ice cover,
can change. The 70 units of solar radiation that are not reflected may be
absorbed by the atmosphere, clouds, or the surface. In the absence of further
complications, in order to maintain thermodynamic equilibrium, Earth’s surface and atmosphere must radiate these same 70
units back to space. Earth’s surface temperature (and that of the lower layer
of the atmosphere essentially in contact with the surface) is tied to the
magnitude of this emission of outgoing radiation according to the Stefan-Boltzmann law.
Earth’s energy budget is further
complicated by the greenhouse effect. Trace gases with
certain chemical properties—the so-called greenhouse gases, mainly carbon dioxide (CO2), methane (CH4), and nitrous oxide(N2O)—absorb some of the infrared radiation produced by Earth’s surface. Because of this absorption,
some fraction of the original 70 units does not directly escape to space.
Because greenhouse gases emit the same amount of radiation they absorb and
because this radiation is emitted equally in all directions (that is, as much
downward as upward), the net effect of absorption by greenhouse gases is to
increase the total amount of radiation emitted downward toward Earth’s surface
and lower atmosphere. To maintain equilibrium,
Earth’s surface and lower atmosphere must emit more radiation than the original
70 units. Consequently, the surface temperature must be higher. This process is
not quite the same as that which governs a true greenhouse, but the end effect
is similar. The presence of greenhouse gases in the atmosphere leads to a
warming of the surface and lower part of the atmosphere (and a cooling higher
up in the atmosphere) relative to what would be expected in the absence of
greenhouse gases.
It is essential to distinguish the
“natural,” or background, greenhouse effect from the “enhanced” greenhouse
effect associated with human activity. The natural greenhouse effect is associated
with surface warming properties of natural constituents of
Earth’s atmosphere, especially water vapour, carbon dioxide, and methane. The
existence of this effect is accepted by all scientists. Indeed, in its absence,
Earth’s average temperature would be approximately 33 °C (59 °F) colder than
today, and Earth would be a frozen and likely uninhabitable planet. What has
been subject to controversy is the so-called enhancedgreenhouse effect,
which is associated with increased concentrations of greenhouse gases caused by
human activity. In particular, the burning of fossil fuels raises
the concentrations of the major greenhouse gases in the atmosphere, and these
higher concentrations have the potential to warm the atmosphere by several
degrees.
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