Global warming is the rise
in the average temperature of Earth's atmosphere and oceans since the late 19th
century, and its projected continuation. Since the early 20th century, Earth's
mean surface temperature has increased by about 0.8 °C (1.4 °F), with about
two-thirds of the increase occurring since 1980. Warming of the climate system is
unequivocal, and scientists are more than 90% certain that it is primarily
caused by increasing concentrations of greenhouse gases produced by
human activities such as deforestation and the
burning of fossil fuels. These
findings are recognized by the national science academies of all major
industrialized nations.
An increase
in global temperature will cause sea levels to rise and will
change the amount and pattern of precipitation, as well as cause
a probable expansion of subtropical deserts. Warming is expected to be strongest in the Arctic and would
be associated with the continuing retreat of glaciers, permafrost and sea ice. Other likely
effects of the warming include a more frequent occurrence of extreme-weather events
including heat waves, droughts and heavy rainfall, species extinctions due to
shifting temperature regimes, and changes in crop yields. Warming and related changes will vary from region to region
around the globe, with projections being more robust in some areas than others. If global mean temperature increases
to 4 °C (7.2 °F) above preindustrial levels, the
limits for human adaptation are likely to be exceeded in many parts of the
world, while the limits of adaptation for natural systems would largely be
exceeded throughout the world. Hence, the ecosystem services upon which
human livelihoods depend would not be preserved.
An example of the cause of Global
warming is The greenhouse effect. The greenhouse effect is the process by which absorption and emission of infrared radiation by gases in the atmosphere warm a planet's lower atmosphere
and surface. It was proposed by Joseph
Fourier in 1824 and
was first investigated quantitatively by Svante
Arrhenius in 1896.
Naturally
occurring amounts of greenhouse gases have a mean warming effect of about 33 °C (59 °F). The major greenhouse gases arewater vapor,
which causes about 36–70% of the greenhouse effect; carbon
dioxide (CO2),
which causes 9–26%; methane (CH4), which causes 4–9%;
and ozone (O3), which causes 3–7%. Clouds also affect the radiation
balance through cloud
forcings similar to
greenhouse gases. Human activity since the Industrial Revolution has increased the amount of greenhouse
gases in the atmosphere, leading to increased radiative forcing from CO2,
methane, tropospheric ozone, CFCs and nitrous oxide.
The concentrations of
CO2 and methane have
increased by 36% and 148% respectively since 1750. These levels are much higher than at
any time during the last 800,000 years, the period for which reliable data has
been extracted from ice cores. Less direct geological evidence
indicates that CO2 values
higher than this were last seen about 20 million years ago. Fossil fuel burning has produced about
three-quarters of the increase in CO2 from human activity over the past 20
years. The rest of this increase is caused mostly by changes in land-use,
particularly deforestation
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