Causes Of Global Warming
The atmosphere is heated
from below'. Explain what is meant by this statement and how changes in the
gaseous composition of the atmosphere may result in climate change. The Earth
is heated primarily by radiation from the Sun, ranging in frequency from
ultra-violet at 2500 nm, but concentrated in the visible light spectrum at
approx. 400-700 nm (Danny Harvey, 2000). Of the incoming solar energy (approx.
340 w/m2) around 30% is reflected back into space by clouds, aerosols, the
atmosphere and the Earth's surface. The remaining energy is absorbed, mostly by
the surface but to a small extent by the atmosphere. It is then reradiated as
long-wave energy in the infrared (IR) frequency; this reradiation is primarily
responsible for the heating of the atmosphere. As a result, writes Cockell (1995:
18), "the atmosphere is heated primarily from below and the overall energy
budget of the Earth system is balanced by the longwave radiation re-emitted
into space". The heating of the atmosphere is augmented by the presence of
so-called greenhouse gases (GHGs). These are atmospheric gases which are not
transparent to the outgoing longwave radiation and therefore trap and reradiate
it - principally water vapour, carbon dioxide (CO2), methane (CH4), nitrous
oxide (N2O) and ozone (O3). (It is notable that this is quite a different
process to that in an actual greenhouse, in which the glass prevents hot air
from rising and escaping as opposed to absorbing and reradiating heat). Because
of the energy conserved as a result of GHGs the average surface temperature of
the Earth is 33° c hotter than it would otherwise be (15° c instead of the
Earth's effective temperature of -18° c) (Danny Harvey, 2000). According to the
IPCC (1995: 75), "a change in the concentration of an atmospheric
constituent can cause a radiative forcing by perturbing the balance between the
net incoming radiation and the outgoing terrestrial radiation". Radiative
forcings can occur in two ways: a change in GHG concentration altering the
magnitude of the greenhouse effect, or a change in atmospheric aerosol levels
altering the amount of solar energy initially reflected. These changes can
occur as a result of both natural and human causes. At present the large
increases in GHG concentrations due to human activity are responsible for an
additional radiative forcing of around 2.4 w/m2 compared with before the
Industrial Revolution, resulting in the overall warming of the climate (IPCC,
1995). Radiative forcings have a direct impact on climate by altering the
energy budget of the atmosphere; however they can also affect the climate
through indirect or feedback effects. Feedbacks may be in the form of chemical
processes which further affect gas concentrations, or effects resulting from
the warming of the climate itself. They can either amplify the initial trend
(positive feedback) or counteract it .
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