THE GREENHOUSE EFFECT
& GLOBAL WARMING
The ability of the atmosphere to capture and
recycle energy emitted by the Earth surface is the defining characteristic
of the greenhouse effect. The greenhouse effect is the process in
which the absorption of infrared radiation by an atmosphere warms
a planet. Without these greenhouse gases, the Earth's surface would
be up to 30 °C cooler.
The term "greenhouse effect" may be
used to refer either to the natural greenhouse effect, due to naturally
occurring greenhouse gases, or to the enhanced (anthropogenic) greenhouse
effect, which results from gases emitted as a result of human activities
(see also global warming, scientific opinion on climate change and
attribution of recent climate change).
how the greenhouse effect works
The Earth receives energy from the Sun in the
form of radiation. To the extent that the Earth is in a steady state,
the energy stored in the atmosphere and ocean does not change in
time, so energy equal to the incident solar radiation must be radiated
back to space.
Radiation leaving the Earth takes two forms: reflected
solar radiation and emitted thermal infrared radiation. The Earth
reflects about 30% of the incident solar flux; the remaining 70%
is absorbed, warms the land, atmosphere and oceans, and powers life
on this planet. Eventually Earth, as a warm object, radiates this
energy into space as black-body radiation, which maintains a thermal
balance. This thermal, infrared radiation increases with increasing
temperature.
Solar radiation at top of atmosphere and at Earth's surface.The
key to the greenhouse effect is the fact that the atmosphere is
relatively transparent to visible solar radiation but strongly absorbing
at the wavelengths of the thermal infrared radiation emitted by
the surface and the atmosphere. The visible solar radiation heats
the surface, not the atmosphere. Whereas most of the infrared radiation
escaping to space is being emitted from the upper atmosphere, not
the surface. The infrared photons emitted by the surface are mostly
absorbed by the atmosphere and do not escape directly to space.
The greenhouse gases
The molecules or atoms that constitute the bulk
of the atmosphere; oxygen, nitrogen and argon; do not interact with
the infrared radiation significantly. The dominant infrared absorbing
gases are water vapor, carbon dioxide, and ozone. A substantial
part of the greenhouse effect is due to carbon dioxide as it is
easily excited by infrared radiation. Clouds are also very important
infrared absorbers. Therefore, water has multiple effects on infrared
radiation, through its vapor phase and through its condensed phases.
Other absorbers of significance include methane, nitrous oxide and
the chlorofluorocarbons.
Positive feedback and runaway greenhouse effect
When the concentration of a greenhouse gas (A)
is itself a function of temperature, there is a positive feedback
from the increase in another greenhouse gas (B), whereby increase
in B increases the temperature which, in turn, increases the concentration
of A, which increases temperatures further, and so on. This feedback
is bound to stop, since the overall supply of the gas A must be
finite. If this feedback ends after producing a major temperature
increase, it is called a runaway greenhouse effect.
According to some Earth climate models, such a
runaway greenhouse effect, involving liberation of methane gas from
hydrates by global warming, has caused the Permian-Triassic extinction
event. In this example, the above (A) would be methane gas which
concentration increases as it is liberated from hydrates by temperature,
while (B) would be carbon coming from volcanic eruptions or from
an asteroid impact. Water vapor is thought to provide a positive
feedback of this type in response to increase in carbon dioxide.
human contribution to the greenhouse effect
CO2 production from increased industrial activity
(fossil fuel burning) and other human activities such as cement
production and tropical deforestation has increased the CO2 concentrations
in the atmosphere. Measurements of carbon dioxide amounts from Mauna
Loa observatory show that CO2 has increased from about 313 ppm (parts
per million) in 1960 to about 375 ppm in 2005.
Because it is a greenhouse gas, elevated CO2 levels
will increase global mean temperature. There has been an observed
global average temperature increase of about 0.5 °C since 1960.
Quantitative understanding of climate sensitivity to CO2 concentration
remains elusive due to uncertainties in a variety of feedbacks,
especially those related to clouds, but there is little doubt that
a substantial portion of the warming in the last half century was
caused by the increase in CO2 concentrations in the atmosphere.
Over the past 800,000 years, ice core data shows
unambiguously that carbon dioxide has varied from values as low
as 180 parts per million (ppm) to the pre-industrial level of 270ppm.
Paleoclimatogists consider variations in carbon dioxide to be a
fundamental factor in controlling climate variations over this time
scale.
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