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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
Acronym |
Horizontal
Resolution
(# of waves
or lat x long) |
Global
Surface Air
Temperature
Change (°C) |
Reference(s) |
|
| 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 |
4°x5° |
2.8 |
Schlesinger and Zhao, 1989 |
| MRI |
C1 |
4°x5° |
~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 |
8°x10° |
4.8 |
Hansen et al., 1984 |
| UKMO |
H1 |
5°x7.5° |
5.2 |
Wilson and Mitchell, 1987 |
| UKMO |
H2 |
5°x7.5° |
3.2 |
Mitchell and Warrilow, 1987 |
| UKMO |
H3 |
2.5°x3.75° |
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
Name(a) |
Experiment
Acronym(b) |
Horizontal
Resolution
(# of waves
or lat x long)
|
GHG
Scenario(c) |
Global
Surface Air
Temperature
Change at CO2
Doubling (°C) |
Reference(s) |
|
| 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) |
4°x5° |
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) |
2.5°x3.75° |
1%/yr |
1.7 |
Murphy, 1995; Murphy and Mitchell,
1995; Senior, 1995 |
| UKMO |
HADCM2 |
X7 (z) |
2.5°x3.75° |
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) |
4°x5° |
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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