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IPCC Working Group I 1995 Summary for Policymakers

a "low" value of climate sensitivity and including the effects of future changes in aerosol concentrations leads to a projected increase of about 1°C by 2100. The corresponding projection for the highest IPCC scenario (IS92e) combined with a "high" value of climate sensitivity gives a warming of about 3.5°C. In all cases the average rate of warming would probably be greater than any seen in the last 10,000 years, but the actual annual to decadal changes would include considerable natural variability. Regional temperature changes could differ substantially from the global mean value. Because of the thermal inertia of the oceans, only 50-90% of the eventual equilibrium temperature change would have been realised by 2100 and temperature would continue to increase beyond 2100,

even if concentrations of greenhouse gases were stabilised by that time. • Average sea level is expected to rise as a result of thermal expansion of the oceans and

melting of glaciers and ice sheets. For the iS92a scenario, assuming the "best estimate" values of climate sensitivity and of ice melt sensitivity to warming, and including the effects of future changes in aerosol, models project an increase in sea level of about 50 cm from the present to 2100. This estimate is approximately 25% lower than the "best estimate" in 1990 due to the lower temperature projection, but also reflecting improvements in the climate and ice melt models. Combining the lowest emission scenario (IS92c) with the "low" climate and ice melt sensitivities and including aerosol effects gives a projected sea level rise of about 15 cm from the present to 2100. The corresponding projection for the highest emission scenario (IS92e) combined with "high" climate and ice-melt sensitivities gives a sea level rise of about 95 cm from the present to 2100. Sea level would continue to rise at a similar rate in future centuries beyond 2100, even if concentrations of greenhouse gases were stabilised by that time, and would continue to do so even beyond the time of stabilisation of global mean temperature. Regional sea level changes may differ from the global mean value owing to land movement and ocean

current changes. • Confidence is higher in the hemispheric-to-continental scale projections of coupled

atmosphere-ocean climate models than in the regional projections, where confidence remains low. There is more confidence in temperature projections than hydrological changes. All model simulations, whether they were forced with increased concentrations of greenhouse gases and aerosols or with increased concentrations of greenhouse gases alone, show the following features: greater surface warming of the land than of the sea in winter, a maximum surface warming in high northern latitudes in winter, little surface warming over the Arctic in summer, an enhanced global mean hydrological cycle, and increased precipitation and soil moisture in high latitudes in winter. All these changes are associated with identifiable physical mechanisms. In addition, most simulations show a reduction in the strength of the north Atlantic thermohaline circulation and a widespread reduction in diurnal range of temperature. These features too can be explained in terms of identifiable physical mechanisms. The direct and indirect effects of anthropogenic aerosols have an important effect on the projections. Generally, the magnitudes of the temperature and precipitation changes are smaller when aerosol effects are represented, especially in northern mid-latitudes. Note that the cooling effect of aerosols is not a simple offset to the warming effect of greenhouse gases, but significantly affects some of the continental scale patterns of climate change, most noticeably in the summer hemisphere. For example, models that consider only the effects of greenhouse gases generally project an increase in precipitation and soil moisture in the Asian summer monsoon region, whereas models that include, in addition,

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IPCC Working Group I 1995 Summary for Policymakers

some of the effects of aerosols suggest that monsoon precipitation may decrease. The spatial and temporal distribution of aerosols greatly influence regional projections, which

are therefore more uncertain. • A general warming is expected to lead to an increase in the occurrence of extremely hot

days and a decrease in the occurrence of extremely cold days. • Warmer temperatures will lead to a more vigorous hydrological cycle; this translates into

prospects for more severe droughts and/or floods in some places and less severe droughts and/or floods in other places. Several models indicate an increase in precipitation intensity, suggesting a possibility for more extreme rainfall events. Knowledge is currently insufficient to say whether there will be any changes' in the occurrence or

geographical distribution of severe storms, e.g., tropical cyclones. • Sustained rapid climate change could shift the competitive balance among species and even

lead to forest dieback, altering the terrestrial uptake and release of carbon. The magnitude is uncertain, but could be between zero and 200 GTC over the next one to two centuries,

depending on the rate of climate change. There are still many uncertainties

Many factors currently limit our ability to project and detect future climate change. In particular, to reduce uncertainties further work is needed on the following priority topics: • estimation of future emissions and biogeochemical cycling (including sources and sinks) of

greenhouse gases, aerosols and aerosol precursors and projections of future

concentrations and radiative properties. • representation of climate processes in models, especially feedbacks associated with clouds,

oceans, sea ice and vegetation, in order to improve projections of rates and regional
patterns of climate change.
systematic collection of long-term instrumental and proxy observations of climate system
variables (e.g., solar output, atmospheric energy balance components, hydrological
cycles, ocean characteristics and ecosystem changes) for the purposes of model testing,
assessment of temporal and regional variability and for detection and attribution studies.

Future unexpected, large and rapid climate system changes (as have occurred in the past) are, by their nature difficult to predict. This implies that future climate changes may also involve "surprises". In particular these arise from the non-linear nature of the climate system. When rapidly forced, non-linear systems are especially subject to unexpected behaviour. Progress can be made by investigating non-linear processes and sub-components of the climatic system. Examples of such non-linear behaviour include rapid circulation changes in the North Atlantic and feedbacks associated with terrestrial ecosystem changes.

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Summary for Policymakers:

Scientific-Technical Analyses of Impacts, Adaptations, and Mitigation of Climate Change

A Report of Working Group II
of the Intergovernmental Panel on Climate Change

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Scientific-Technical Analyses of Impacts, Adaptations, and Mitigation of Climate Change

3 1. Scope of the Assessment

Box 1. Ultimate Objective of the UNFCCC (Article 2) The charge to Working Group II of the Intergovernmental Panel on Climate Change (IPCC) was to review the state of "...stabilization of greenhouse gas concentrations in the knowledge concerning the impacts of climate change on phys- atmosphere at a level that would prevent dangerous ical and ecological systems, human health, and socioeconomic anthropogenic interference with the climate system. sectors. Working Group I also was charged with reviewing Such a level should be achieved within a time frame available information on the technical and economic feasibili- sufficient to allow ecosystems to adapt naturally to ty of a range of potential adaptation and mitigation strategies. climate change, to ensure that food production is not This assessment provides scientific, technical, and economic threatened, and to enable economic development to information that can be used, inter alia, in evaluating whether proceed in a sustainable manner." the projected range of plausible impacts constitutes “dangerous anthropogenic interference with the climate system." as referred to in Article 2 of the United Nations Framework precipitation, soil moisture, and sea level. Based on the range Convention on Climate Change (UNFCCC), and in evaluat- of sensitivities of climate to increases in greenhouse gas coning adaptation and mitigation options that could be used in centrations reported by IPCC Working Group I and plausible progressing towards the ultimate objective of the UNFCCC ranges of emissions (IPCC IS92; see Table 1), climate models, (see Box 1).

taking into account greenhouse gases and aerosols, project an increase in global mean surface temperature of about 1-3.5°C

by 2100, and an associated increase in sea level of about 15-95 2. Nature of the Issue

cm.' The reliability of regional-scale predictions is still low,

and the degree to which climate variability may change is Human activities are increasing the atmospheric concentrations uncertain. However, potentially serious changes have been of greenhouse gases—which tend to warm the atmosphere identified, including an increase in some regions in the inciand, in some regions, aerosols—which tend to cool the atmos- dence of extreme high-temperature events, floods, and phere. These changes in greenhouse gases and aerosols, taken droughts, with resultant consequences for fires, pest outbreaks, together, are projected to lead to regional and global changes in and ecosystem composition, structure, and functioning, includclimate and climate-related parameters such as temperature, ing primary productivity.

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*Approximate conversion factor: I barrel = 6 GJ. Source: IPCC, 1992: Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment. Section A3. prepared by IPCC Working Group I (J.T. Houghton, B.A. Callander, and S.K. Varney (eds.)) and WMO/UNEP. Cambridge

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