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Intergovernmental
Panel on Climate Change |
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Main
Conclusions of the SPM |
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Summary for Policymakers (SPM) |
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An increasing body of observations gives a
collective picture of a warming world and other changes in the climate
system. |
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Emissions of greenhouse gases and aerosols due
to human activities continue to alter the atmosphere in ways that are
expected to affect the climate. |
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Confidence in the ability of models to project
future climate has increased. |
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There is new and stronger evidence that most of
the warming observed over the last 50 years is attributable to human
activities. |
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Human influences will continue to change
atmospheric composition throughout the 21st century. |
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Global average temperature and sea level are
projected to rise under all IPCC SRES scenarios. |
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Anthropogenic climate change will persist for
many centuries. |
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Further action is required to address remaining
gaps in information and understanding. |
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Temperature |
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Precipitation |
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Snow /
Ice Cover |
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Sea
Level |
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Circulation |
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Extremes |
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The snow cover has decreased by about 10% since
the late 1960s. |
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Northern hemisphere spring and summer sea-ice
extent has decreased by about 10 to 15% since the 1950s. |
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El Nino phenomenon has been more frequent,
persistent, and intense since the mid-1970s. |
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No significant trends of Antarctic sea-ice
extent are apparent since 1978. |
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No systematic changes in the frequency of
tornadoes, thunder days, or hail events are evident in the limited areas
analyzed. |
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Concentrations of atmospheric greenhouse gases
and their radiative forcing have continued to increase as a result of human
activities. |
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Anthropogenic aerosols are short-lived and
mostly produce negative radiative forcing. |
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Radiative forcing is a measure of the influence
a factor has in altering the balance of incoming and outgoing energy in the
Earth-atmosphere system, and is an index of the importance of the factor as
a potential climate change mechanism. It is expressed in Watts per square
metre (Wm-2). |
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The atmospheric concentration of carbon dioxide
(CO2) has increased by 31% since 1750. The present CO2 concentration has
not been exceeded during the past 420,000 years and likely not during the
past 20 million years. The current rate of increase is unprecedented during
at least the past 20,000 years. |
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About three-quarters of the anthropogenic
emissions of CO2 to the atmosphere during the past 20 years is due to
fossil fuel burning. The rest is predominantly due to land-use change,
especially deforestation. |
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Currently the ocean and the land together are
taking up about half of the anthropogenic CO2 emissions. On land, the
uptake of anthropogenic CO2 very likely exceeded the release of CO2 by
deforestation during the 1990s. |
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The atmospheric concentration of nitrous oxide
(N2O) has increased by 46 ppb (17%) since 1750 and continues to increase.
The present N2O concentration has not been exceeded during at least the
past thousand years. |
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About a third of current N2O emissions are
anthropogenic (e.g., agricultural soils, cattle feed lots and chemical
industry). |
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The atmospheric concentration of methane (CH4)
has increased by 151% (1060 ppb9) since 1750 and continues to increase. |
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The present CH4 concentration has not been
exceeded during the past 420,000 years. |
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Slightly more than half of current CH4 emissions
are anthropogenic (e.g., use of fossil fuels, cattle, rice agriculture and
landfills). |
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Since 1995, the atmospheric concentrations of
many of those halocarbon gases that are both ozone-depleting and greenhouse
gases (e.g., CFCl3 and CF2Cl2), are either increasing more slowly or
decreasing, both in response to reduced emissions under the regulations of
the Montreal Protocol and its Amendments. |
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Their substitute compounds (e.g., CHF2Cl and
CF3CH2F) and some other synthetic compounds (e.g., perfluorocarbons (PFCs)
and sulphur hexafluoride (SF6)) are also greenhouse gases, and their
concentrations are currently increasing. |
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The observed depletion of the stratospheric
ozone (O3) layer from 1979 to 2000 is estimated to have caused a negative
radiative forcing (–0.15 Wm-2). |
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The total amount of O3 in the troposphere is
estimated to have increased by 36% since 1750, due primarily to
anthropogenic emissions of several O3-forming gases. This corresponds to a
positive radiative forcing of 0.35 Wm-2. |
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The major sources of anthropogenic aerosols are
fossil fuel and biomass burning. These sources are also linked to
degradation of air quality and acid deposition. |
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In general, the direct radiative forcing of
aerosols is negative (except for black carbon fossil). |
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There is much less confidence in the ability to
quantify the total aerosol direct effect, and its evolution over time. |
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Aerosols also vary considerably by region and
respond quickly to changes in emissions. |
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In addition to their direct radiative forcing,
aerosols have an indirect radiative forcing through their effects on clouds. |
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There is now more evidence for this indirect
effect, which is negative, although of very uncertain magnitude. |
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Since the late 1970s, satellite instruments have
observed small oscillations due to the 11-year solar cycle. Mechanisms for
the amplification of solar effects on climate have been proposed, but
currently lack a rigorous theoretical or observational basis. |
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Stratospheric aerosols from explosive volcanic
eruptions lead to negative forcing, which lasts a few years. Several major
eruptions occurred in the periods 1880 to 1920 and 1960 to 1991. |
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The combined change in radiative forcing of the
two major natural factors (solar variation and volcanic aerosols) is
estimated to be negative for the past two, and possibly the past four,
decades. |
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Detection and attribution studies consistently
find evidence for an anthropogenic signal in the climate record of the last
35 to 50 years. |
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Simulations of the response to natural forcings
alone (i.e., the response to variability in solar irradiance and volcanic
eruptions) do not explain the warming in the second half of the 20th
century. |
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However, they indicate that natural forcings may
have contributed to the observed warming in the first half of the 20th
century. |
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Most of the observed warming over the last 50
years is likely to have been due to the increase in greenhouse gas
concentrations. |
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The globally averaged surface temperature is
projected to increase by 1.4 to 5.8°C over the period 1990 to 2100. |
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Based on global model simulations and for a wide
range of scenarios, global average water vapor concentration and
precipitation are projected to increase during the 21st century. |
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By the second half of the 21st century, it is
likely that precipitation will have increased over northern mid- to high
latitudes and Antarctica in winter. |
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At low latitudes there are both regional
increases and decreases over land areas. |
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Larger year to year variations in precipitation
are very likely over most areas where an increase in mean precipitation is projected. |
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Current projections show little change or a
small increase in amplitude for El Niño events over the next 100 years. |
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Even with little or no change in El Niño
amplitude, global warming is likely to lead to greater extremes of drying
and heavy rainfall and increase the risk of droughts and floods that occur
with El Niño events in many different regions. |
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It is likely that warming associated with
increasing greenhouse gas concentrations will cause an increase of Asian
summer monsoon precipitation variability. |
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Changes in monsoon mean duration and strength
depend on the details of emission scenario. |
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Most models show weakening of the ocean
thermohaline circulation which leads to a reduction of the heat transport
into high latitudes of the Northern Hemisphere. |
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The current projections using climate models do
not exhibit a complete shut-down of the thermohaline circulation by 2100. |
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Beyond 2100, the thermohaline circulation could
completely, and possibly irreversibly, shut-down in either hemisphere if
the change in radiative forcing is large enough and applied long enough.
scenarios. |
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Northern Hemisphere snow cover and sea-ice
extent are projected to decrease further. |
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Glaciers and ice caps are projected to continue
their widespread retreat during the 21st century. |
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The Antarctic ice sheet is likely to gain mass
because of greater precipitation, while the Greenland ice sheet is likely
to lose mass because the increase in runoff will exceed the precipitation
increase. |
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Global mean sea level is projected to rise by
0.09 to 0.88 metres between 1990 and 2100, for the full range of SRES
scenarios. |
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