Climate variability
The
Earth's climate has varied considerably over the past hundreds of millions
of years. There have been gradual changes and rapid changes. Gradual changes
have caused the climatic regions to shift over long periods of time and allowed
ecosystems to adapt. Rapid changes have caused extinction of numerous species
over relatively short periods of time. The
earlier climate changes can be explained by variety of theories (e.g.
variations in the Earth's orbital characteristics, volcanic eruptions,
atmospheric carbon dioxide variations, variations in solar output). The present
climate change is predicted to be very rapid, and, for the first time, caused by
humans.
Humans are changing the climate by releasing natural and
man-made greenhouse
gases into the atmosphere. Carbon dioxide (CO2) is
produced when fossil fuels are used to generate energy and when forests are cut
down and burned. Carbon dioxide is currently responsible for over 60 % of the
man-made enhancement of the greenhouse effect. Methane (CH4) and
nitrous oxide (N2O) are emitted from agricultural activities, changes
in land use, and other sources. Artificial chemicals called
halocarbons (CFCs, HFCs, PFCs) and other long-lived gases such as sulphur
hexafluoride (SF6) are released by industrial processes. Ozone in the
lower atmosphere is generated indirectly by automobile exhaust fumes and other
sources. Although the depletion of ozone in the middle atmosphere has a minor
cooling effect, it is by far less important that the adverse effects of the
increased UV-B radiation due to the depletion. This is why it is not reasonable
to deplete middle atmospheric ozone in order to mitigate global warming! Learn
more about the ozone depletion here!
By absorbing
infrared radiation, these gases control the way radiative energy flows through
the climate system. In response to humanity's emissions, the climate has started
to adjust to a 'thicker blanket' of greenhouse gases in order to maintain
the balance between energy arriving from the sun and
energy escaping back into space. Observations show that global temperatures have
risen by about 0.6°C in the 20th century. There is new and stronger
evidence that most of the observed warming over the last 50 years (see
the figure) is attributable to human activities.
A second important human influence on
climate is aerosols. These clouds of microscopic
particles are not a greenhouse gas. In addition to various natural sources, they
are produced from sulphur dioxide emitted mainly by power stations, and by the
smoke from deforestation and the burning of crop wastes. Aerosols settle out of
the air after a few days, but they are emitted in such massive quantities that
they have a substantial impact on climate.
Most aerosols cool the climate locally
by scattering sunlight back to space and by affecting clouds. Aerosol particles
can block sunlight directly and also provide "seeds" for the formation
of clouds, and often these clouds also have a
cooling effect. Over heavily industrialized regions, aerosol cooling may counteract
nearly all of the warming effect of greenhouse gas increases to date.
(Text adapted from the Climate
Change Information Kit.)
Learn more about the science behind climate change for
example from the following Internet resources:
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