Lecture 13: IPCC Summary
For PolicyMakers: WG1-Science
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Intergovernmental Panel on Climate Change |
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Main Conclusions of the SPM |
IPCC Web Site
Structure of IPCC 1997 -
2001
Slide 4
Climate Change 2001: The
Scientific Basis
WGI contribution to IPCC Third Assessment Report
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Summary for Policymakers (SPM) |
Slide 6
Understand and Project
Our Climate System
Major Conclusions in 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. |
Evidences of Global
Warming and Other Changes
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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 |
The Land and Oceans Have
Warmed
Global Surface
Temperature
Compared to the Past 1000
Years
Global Sea Level Has
Risen
Precipitation Patterns
Have Changed
Other Climate Changes
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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. |
Slide 16
Slide 17
Emissions of Greenhouse
Gases and Aerosols
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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. |
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). |
Slide 20
Increase of CO2
Concentration
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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. |
Slide 22
Increase of N2O
Concentration
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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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Increase of Methane
Concentration
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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). |
Halocarbon Concentration
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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. |
Ozone Concentration
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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. |
Anthropogenic Aerosols
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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. |
Radiative Forcing
Natural Factors of
Radiative Forcing
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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. |
Slide 30
Slide 31
More Confidence in
Climate Models
Evidence of Global
Warming
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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. |
Slide 34
IPCC Special Report on
Emission Scenarios (SRES)
Projected CO2 Emissions
Projected CO2
Concentration
Projected Aerosol (SO2)
Emissions
Projected Global
Temperature
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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. |
The projected warming is
very likely to be without precedent during at least the last 10,000 years….
Global Distribution of
The Warming
Projected Precipitation
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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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Projected Extreme Events
Projected El Nino
Activity
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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. |
Projected Monsoon
Variability
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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. |
Projected Thermohaline
Circulation
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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. |
Projected Snow/Ice Cover
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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. |
Projected Sea-Level
Change
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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. |
Sea Level Change and
Global Warming
Global Warming and
Sea-Level Change