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.

1.3.2. Climate Scenarios

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:

"Analysis of surface air temperature and precipitation results from regional climate change experiments carried out with AOGCMs indicates that the biases in present-day simulations of regional climate change and the inter-model variability in the simulated regional changes are still too large to yield a high level of confidence in simulated change scenarios. The limited number of experiments available with statistical downscaling techniques and nested regional models has shown that complex topographical features, large lake systems, and narrow land masses not resolved at the resolution of current GCMs significantly affect the simulated regional and local change scenarios, both for precipitation and (to a lesser extent) temperature (IPCC, 1996). This adds a further degree of uncertainty in the use of GCM-produced scenarios for impact assessments. In addition, most climate change experiments have not accounted for human-induced landscape changes and only recently has the effect of aerosols been investigated. Both these factors can further affect projections of regional climate change."

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.

Table 1-1: The global mixed-layer atmosphere-ocean general circulation models (equilibrium 2xCO2 simulations) used for impact assessment studies in this report.

Group Experiment
(# of waves
or lat x long)
Surface Air
Change (C)

GFDL A1 R 15 3.2 Wetherald and Manabe, 1988
GFDL A2 R 15 4.0 Manabe and Wetherald, 1987
GFDL A3 R 30 4.0 Wetherald and Manabe, 1989
OSU B1 4x5 2.8 Schlesinger and Zhao, 1989
MRI C1 4x5 ~4.3 Noda and Tokioka, 1989
NCAR D1 R 15 4.0 Washington and Meehl, 1984; Meehl and Washington, 1990
NCAR D2 R 15 4.6 Washington and Meehl, 1993
CSIRO4 E1 R 21 4.0 Gordon et al., 1992; Gordon and Hunt, 1994
CSIRO9 F1 R 21 4.8 Whetton et al., 1993; Watterson et al., 1995
GISS G1 8x10 4.8 Hansen et al., 1984
UKMO H1 5x7.5 5.2 Wilson and Mitchell, 1987
UKMO H2 5x7.5 3.2 Mitchell and Warrilow, 1987
UKMO H3 2.5x3.75 3.5 Mitchell et al., 1989
CCC J1 T 32 3.5 Boer et al., 1992; McFarlane et al., 1992; Boer, 1993
MPI K1 T 106(a)   Bengtsson et al., 1995, 1996

Note: In general, the findings on impact assessment contained in this report are based on climate change scenarios inferred from the model experiments listed above and cited in IPCC's First Assessment Report (1990) and its supplement (1992).
(a) Time-slice experiments with atmosphere-only ECHAm3 T 106 model.

Table 1-2: A brief description of the global coupled atmosphere-ocean general circulation models (transient simulations) used for impact assessment studies in this report.

Group Model

(# of waves
or lat x long)

Surface Air
Change at CO2
Doubling (C)

BMRC   X1 (a) R 21 1%/yr 1.35 Colman et al., 1995
GFDL   X2 (g) R 15 1%/yr 2.2 Manabe et al., 1991, 1992
MRI   X3 (p) 4x5 1%/yr 1.6 Tokioka et al., 1995
NCAR 5 Ocean X4 (q) R 15 1%/yr 2.3 Meehl et al., 1993
NCAR 1 Ocean X5 (r) R 15 1%/yr 3.8 Meehl, 1996
Washington and Meehl, 1996
UKMO UKTR1 X6 (s) 2.5x3.75 1%/yr 1.7 Murphy, 1995; Murphy and Mitchell,
1995; Senior, 1995
UKMO HADCM2 X7 (z) 2.5x3.75 1%/yr + aerosols ~2.5 Mitchell and Johns, 1997
MPI ECHAM1+LSG X8 (m) T 21 1.3%/yr 1.3 Cubasch et al., 1992
MPI ECHAm3+LSG X9 (y) T 21 1.3%/yr + aerosols na Hasselmann et al., 1995
CSIRO   X10 (d) R 21 1%/yr 2.0 Gordon and O'Farrell, 1997
CCC CGCM1 X11 (b) T 32 1%/yr 2.6 Boer et al., 1997; Flato et al., 1997
GISS   X12 (k) 4x5 1%/yr 1.4 Russell et al., 1995

Note: In general, the climate change scenarios described in this document are based on those inferred from the model experiments listed above and reported in the IPCC Second Assessment Report (1996). The future regional projections for combined greenhouse gases (equivalent CO2) and aerosol forcings (based on experiments X7 and/or X9) also have been discussed for some regions.

na = not available
(a)If different from group name.
(b)Parenthetical refers to experiment listed in Table 6.3 of the SAR Working Group I volume (also see Table B-1 in Annex B).
(c)The GHG scenario refers to the rate of increase of CO2 used in the model experiments; most experiments use 1%/yr, which gives a doubling of CO2 after 70 years (IS92a gives a doubling of equivalent CO2 after 95 years).


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