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GLOBAL WARMING CAUSES
The climate system varies through
natural, internal processes and in response to variations in external
"forcing" from both human and natural causes. These forcing factors
include solar activity, volcanic emissions, variations in the earth's
orbit (orbital forcing) and greenhouse gases. The detailed causes of the
recent warming remain an active field of research, but the scientific
consensus identifies greenhouse gases as the main influence.
Contrasting with this consensus view, other hypotheses have been
proposed to explain all or most of the observed increase in global
temperatures, including: The warming is within the range of natural
variation; the warming is a consequence of coming out of a prior cool
period, namely the Little Ice Age; and the warming is primarily a result
of variances in solar radiation.
Adding carbon dioxide (CO2) or methane (CH4) to Earth's atmosphere, with
no other changes, will make the planet's surface warmer. Greenhouse
gases create a natural greenhouse effect without which temperatures on
Earth would be an estimated 30 °C (54 °F) lower, so that Earth would be
uninhabitable. It is therefore not correct to say that there is a debate
between those who "believe in" and "oppose" the greenhouse effect as
such. Rather, the debate concerns the net effect of the addition of
greenhouse gases when allowing for compounding or mitigating factors.
One example of an important feedback process is ice-albedo feedback. The
increased CO2 in the atmosphere warms the Earth's surface and leads to
melting of ice near the poles. As the ice melts, land or open water
takes its place. Both land and open water are on average less reflective
than ice, and thus absorb more solar radiation. This causes more
warming, which in turn causes more melting, and this cycle continues.
Due to the thermal inertia of the Earth's oceans and slow responses of
other indirect effects, the Earth's current climate is not in
equilibrium with the forcing imposed by increased greenhouse gases.
Climate commitment studies indicate that, even if greenhouse gases were
stabilized at present day levels, a further warming of about 0.5 °C (0.9
°F) would still occur.
Greenhouse gases in the atmosphere
Recent increases in atmospheric CO2. The monthly CO2
measurements display small seasonal oscillations in an overall yearly
uptrend; each year's maximum is reached during the northern hemisphere's
late spring, and declines during the northern hemisphere growing season
as plants remove some CO2 from the atmosphere.The greenhouse effect,
first discovered by Joseph Fourier in 1824, and first investigated
quantitatively by Svante Arrhenius in 1896, is the process in which the
emission of infrared radiation by atmospheric gasses warms a planet's
surface. On Earth, the major natural greenhouse gases are water vapor,
which causes about 36-70% of the greenhouse effect (not including
clouds); carbon dioxide, which causes 9-26%; methane, which causes 4-9%,
and ozone, which causes 3-7%.
The atmospheric concentrations of carbon dioxide and methane have
increased by 31% and 149% respectively above pre-industrial levels since
1750. This is considerably higher than at any time during the last
650,000 years, the period for which reliable data has been extracted
from ice cores. From less direct geological evidence it is believed that
carbon dioxide values this high were last attained 40 million years
ago.[citation needed] About three-quarters of the anthropogenic
(man-made) emissions of carbon dioxide to the atmosphere during the past
20 years are due to fossil fuel burning. The rest of the anthropogenic
emissions are predominantly due to land-use change, especially
deforestation.
Future carbon dioxide levels are expected to rise due to ongoing burning
of fossil fuels. The rate of rise will depend on uncertain economic,
sociological, technological, natural developments, but may be ultimately
limited by the availability of fossil fuels. The IPCC Special Report on
Emissions Scenarios gives a wide range of future carbon dioxide
scenarios, ranging from 541 to 970 parts per million by the year 2100.
Fossil fuel reserves are sufficient to reach this level and continue
emissions past 2100, if coal, tar sands or Methane clathrates are
extensively used.[citation needed]Carbon sink ecosystems (forests and
oceans) are being degraded by pollutants. Degradation of major carbon
sinks results in higher atmospheric carbon dioxide levels.
Anthropogenic emission of greenhouse gases broken down by sector for the
year 2000.Positive feedback effects such as the expected release of
methane from the melting of permafrost peat bogs in Siberia (possibly up
to 70,000 million tonnes), may lead to significant additional sources of
greenhouse gas emissions not included in IPCC's climate models.[citation
needed]
The measure of the temperature response to increased greenhouse gas
concentrations and other anthropogenic and natural climate forcings is
climate sensitivity. It is found by observational and model studies.
This sensitivity is usually expressed in terms of the temperature
response expected from a doubling of CO2 in the atmosphere. The current
literature estimates sensitivity in the range of 1.5 to 4.5 °C (2.7 to
8.1 °F).
Solar variation
Modeling studies reported in the IPCC Third Assessment Report
(TAR) found that volcanic and solar forcings may account for half of the
temperature variations prior to 1950, but the net effect of such natural
forcings has been roughly neutral since then.[16] The IPCC Fourth
Assessment Report (AR4) gives a best estimate for radiative forcing from
changes in solar activity of +0.12 watts per square metre. This is less
than half of the estimate given in the TAR. For comparison, the combined
effects of all human activity are estimated to be an order of magnitude
greater at +1.6 watts per square meter.
In a review of existing literature, Foukal et al. (2006) determined both
that the variations in solar output were too small to have contributed
appreciably to global warming since the mid-1970s and that there was no
evidence of a net increase in brightness during this period.
Some scientists assert that a warming of the stratosphere, which has not
been observed, would be expected if there were a significant increase in
solar activity.
Some researchers (e.g. Stott et al. 2003) believe that the effect of
solar forcing is being underestimated and propose that solar forcing
accounts for 16% or 36% of recent greenhouse warming. Others (e.g. Marsh
and Svensmark 2000)have proposed that feedback from clouds or other
processes enhance the direct effect of solar variation, which if true
would also suggest that the effect of solar variability was being
underestimated. In general, the IPCC describes the level of scientific
understanding of the contribution of variations in solar irradiance to
historical climate changes as "low."
400 year history of sunspot numbers.The present level of solar activity
is historically high. Solanki et al. (2004) suggest that solar activity
for the last 60 to 70 years may be at its highest level in 8,000 years;
Muscheler et al. disagree, suggesting that other comparably high levels
of activity have occurred several times in the last few thousand years.
Solanki concluded based on their analysis that there is a 92%
probability that solar activity will decrease over the next 50 years. In
addition, researchers at Duke University (2005) have found that 10–30%
of the warming over the last two decades may be due to increased solar
output.
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Terminology
History of warming
Causes
Greenhouse gases in the atmosphere
Solar variation
Attributed and
expected effects
Mitigation
Kyoto Protocol
Climate models
Other related issues
Ocean acidification
Relationship to ozone depletion
Relationship to global dimming
Pre-human global warming
Pre-industrial global warming
References |
