Synthesis Report - Question 3

Climate Change 2001: Synthesis Report

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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.

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.
WGI TAR Chapters 3, 4, 5, & 6

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