|The Regional Impacts of Climate Change|
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1.3. Baseline Data and Climate Scenarios
1.3.1. Climate Observations
Current trends in regional variations of temperature and precipitation also are important parts of the baseline against which the potential effects of climate change should be assessed. IPCC (1996, WG I, Chapter 3) provided time series plots and global maps depicting trends for temperature and precipitation. This information was extended and updated by one of the lead authors of the WG I assessment (T. Karl, USA). The information was provided to the regional assessment lead authors and is contained in Annex A of this special report, which describes the data sets used for depicting these trends. Additional information based on regional analyses has been added to several of the regional chapters by the lead authors.
GCM-based scenarios are the most credible and frequently used projections of climate change. Other types of climate projections include synthetic scenarios and analogue scenarios. These approaches and their limitations are described in IPCC (1994b).
In the IPCC's second assessment (1996, WG I, Chapter 6), seven regions were identified for regional analysis of climate simulations. That analysis was based on transient runs with atmosphere-ocean general circulation models (AOGCMs) suitable for construction of regional climate scenarios, using additional regionalization techniques to improve the simulation of regional climate change. The team of lead authors that conducted that analysis, led by F. Giorgi and G. Meehl, prepared information on the simulation of regional climate change with global coupled climate models and regional modeling techniques for use by the regional assessment teams. That information, which is presented in Annex B of this report, is based entirely on the information included in the WG I contribution to the SAR. No new information has been added to the previous analysis.
The wide range of changes in temperature and precipitation indicated at the time of doubled CO2 concentrations for each region is illustrated in Figures B-1 and B-2, which show large model-to-model differences. Annex B provides the following conclusion regarding the confidence that can be placed in regional climate projections:
The wide range of projected changes in temperature and precipitation suggest that caution is required in treating any impact assessments based on GCM results as firm predictions. This uncertainty is why the term "climate scenarios" has been adopted in most impact assessments. Such scenarios should be regarded as internally consistent patterns of plausible future climates, not as predictions. Decisionmakers need to be aware of the uncertainties associated with climate projections so that they can weigh them in formulation of strategies to cope with the risks of climate change.
The review chapters in this report summarize impact studies based on a range of climate scenarios where they were available. Most studies were based on the older, mixed-layer GCM climate scenarios; results from coupled transient models have only recently become available, and studies using these scenarios are only beginning to be conducted. The older GCM runs estimate stable equilibrium conditions for 1xCO2 and 2xCO2 climates and generally show more global mean warming than recent transient model runs (see Table 1-1 for a list of equilibrium scenarios used in studies assessed in this special report). In the transient model runs (see Table 1-2 for a listing of transient scenarios cited), in which trace gases increase slowly over a period of years, the full effects of changes in temperature and precipitation lag the effects of changes in atmospheric composition by a number of decades. Thus, in impact studies using transient scenarios (e.g., model studies of potential climate change impacts on vegetation distribution), the positive effects of CO2 on plant growth and other variables dependent upon plant production precede the full effects of changes in climate.
This complication does not mean that impact assessments based on older equilibrium GCM projections are of no value. Rather, it suggests that their results should be carefully interpreted. Where possible, the actual projected changes in temperature, precipitation, and so forth have been stated in the text, and climate scenarios representing the range of potential changes in temperature and precipitation have been used for regions where a range of scenarios is available. Space limitations prevent the presentation of fine detail, but the original source papers and reports are listed. Unfortunately, even some of the original material does not give as much precise information as might be desired.
At the very least, impact assessments based on older climate scenarios can
be used to estimate the sensitivity of the various sectors to climate change.
New transient GCMs based on improved coupling to the oceans; better scenarios
of greenhouse gas and sulfate aerosol emissions; and better representation of
processes of cloud formation, water vapor transport, ice/snow formation, vegetation
feedbacks, and ocean circulation will produce quantitatively different results.
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