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REPORTS - ASSESSMENT REPORTS |
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Synthesis Report - Question 3
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Climate Change 2001: Synthesis Report |
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| Question 3
What is known about the regional and global
climatic, environmental, and socio-economic consequences in the
next 25, 50, and 100 years associated with a range of greenhouse
gas emissions arising from scenarios used in the TAR (projections
which involve no climate policy intervention)?
To the extent
possible evaluate the:
- Projected changes in atmospheric concentrations,
climate, and sea level
- Impacts and economic costs and benefits
of changes in climate and atmospheric composition on human health,
diversity and productivity of tecological systems, and socio-economic
sectors (particularly agriculture and water)
- The range of options for adaptation,
including the costs, benefits, and challenges
- Development, sustainability, and equity
issues associated with impacts and adaptation at a regional and
global level
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3.1
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The greenhouse gas emissions scenarios used as the basis for the climate
projections in the TAR are those contained in the IPCC Special Report
on Emissions Scenarios (see Box 3-1). Because the SRES scenarios had only
been available for a very short time prior to production of the TAR, it
was not possible to include impact assessments based on these scenarios.
Hence, the impacts assessments in the TAR use climate model results that
tend to be based on equilibrium climate change scenarios (e.g., 2xCO2),
a relatively small number of experiments using a 1% per year CO2
increase transient scenario, or the scenarios used in the Second Assessment
Report (i.e., the IS92 series). The challenge in answering this question
therefore is to try and map these impact results onto the climate change
results, which have used the SRES
scenarios. This, by necessity, requires various approximations to be made
and in many cases only qualitative conclusions can be drawn. Projections
of changes in climate variability, extreme events, and abrupt/non-linear
changes are covered in Question 4.
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| Box 3-1: Future emissions of greenhouse
gases and aerosols due to human activities will alter the atmosphere
in ways that are expected to affect the climate. |
| Changes in climate occur as a result of internal
variability of the climate system and external factors (both natural
and as a result of human activities). Emissions of greenhouse gases
and aerosols due to human activities change the composition of the
atmosphere. Future emissions of greenhouse gases and aerosols are
determined by driving forces such as population, socio-economic development,
and technological change, and hence are highly uncertain. Scenarios
are alternative images of how the future might unfold and are an appropriate
tool with which to analyze how driving forces may influence future
emission outcomes and to assess the associated uncertainties. The
SRES scenarios, developed to update the IS92 series, consist of six
scenario groups, based on narrative storylines, which span a wide
range of these driving forces (see Figure
3-1). They are all plausible and internally consistent, and no
probabilities of occurrence are assigned. They encompass four combinations
of demographic change, social and economic development, and broad
technological developments (A1B, A2, B1, B2). Two further scenario
groups, A1FI and A1T, explicitly explore alternative energy technology
developments to A1B (see Figure
3-1a). The resulting emissions of the greenhouse gases CO2,
CH4, and N2O, along with SO2 which
leads to the production of sulfate aerosols, are shown in Figures
3-1b to 3-1e; other gases and particles are also important. These
emissions cause changes in the concentrations of these gases and aerosols
in the atmosphere. The changes in the concentrations for the SRES
scenarios are shown in Figures 3-1f to 3-1i. Note that for gases which
stay in the atmosphere for a long period, such as CO2 shown
in panel (f), the atmospheric concentration responds to changes in
emissions relatively slowly (e.g., see Figure
5-3); whereas for short-lived gases and aerosols, such as sulfate
aerosols shown in panel (i), the atmospheric concentration responds
much more quickly. The influence of changes in the concentrations
of greenhouse gases and aerosols in the atmosphere on the climate
system can broadly be compared using the concept of radiative forcing,
which is a measure of the influence a factor has in altering the balance
of incoming and outgoing energy in the Earth-atmosphere system. A
positive radiative forcing, such as that produced by increasing concentrations
of greenhouse gases, tends to warm the surface; conversely a negative
radiative forcing, which can arise from an increase in some types
of aerosols such as sulfate aerosols, tends to cool the surface. The
radiative forcing resulting from the increasing concentrations in
panels (f) to (i) is shown in panel (j). Note that, as with the IS92
scenarios, all combinations of emissions of greenhouse gases and aerosols
in the SRES scenarios result in increased radiative forcing. |
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WGI TAR Chapters 3, 4,
5, & 6 |
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