The Regional Impacts of Climate Change

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C.5. Future Climate Scenarios

Estimation of the potential impacts of global warming should utilize several future climate scenarios, since the magnitude, timing and spatial details of global warming vary among climate models. Most published impacts studies were based on atmospheric General Circulation Model (GCM) doubled CO2 radiative forcing equilibrium experiments with simple mixed-layer oceans. Doubled CO2 radiative forcing (2 x CO2) includes only about 50% actual CO2 forcing with the balance arising from other greenhouse gases. More recent, transient experiments with coupled atmosphere-ocean GCMs have suggested a global average increase in temperature of about 1.0-3.5C by the time of CO2 doubling, estimated as 60-70 years from now (described in the IPCC Second Assessment Report, SAR; IPCC 1996, WG I, Section 6; Annex B). The most recent GCMs include sulfate aerosols in some experiments, which can cool the climate. The analysis presented here will rely both on the older 2 x CO2 equilibrium GCM scenarios (described in the IPCC First Assessment Report, FAR; IPCC 1990, WG I, Section 3; Annex B), since most published analyses have relied on them, and on three new simulations, two from the Hadley Center (HADCM2GHG and HADCM2SUL; Johns et al., submitted; Mitchell et al., 1995; IPCC 1996, WG I, Sections 5, 6), and one from the Max Planck Institute for Meteorology (MPI-T106; Bengtsson, et al. 1995; Bengtsson, et al., 1996; IPCC 1996, WG I, Section 6), which have been made using coupled atmosphere-ocean GCMs and considering sulfate aerosol forcing.

To allow direct comparison with the previously completed VEMAP simulations over the conterminous U.S. (VEMAP Members, 1995), the same three equilibrium GCM scenarios were utilized for the global simulations: UKMO (Mitchell and Warrilow, 1987); GFDL-R30 (IPCC 1990, WG I, Section 3; IPCC 1990, WG I, Section 5); and OSU (Schlesinger and Zhao, 1989). The coarse grid from each model was interpolated to a 0.5 x 0.5, lat.-long. grid. Scenarios were constructed by applying ratios ((2 x CO2)/(1 x CO2)) of all climate variables (except temperature) back to a baseline longterm average monthly climate dataset (Leemans and Cramer, 1991). Ratios were used to avoid negative numbers (e.g., negative precipitation), but were not allowed to exceed 5, to prevent unrealistic changes in regions with normally low rainfall. Temperature scenarios were calculated as a difference ((2 x CO2) - (1 x CO2)) and applied to the baseline dataset.

The newer GCM scenarios are extracted from transient GCM simulations wherein trace gases were allowed to increase gradually over a long period of years, allowing the climate to adjust while incorporating inherent lags in the ocean-atmosphere systems. In order to run the equilibrium vegetation models under the newer transient GCMs, a control climate is extracted as an average of either 30 years (Hadley Center) or 10 years (Max Planck Institute) of model output associated with present climate (e.g. 1961-1990). Likewise, a 30 or 10 year average is extracted from the time period approximating 2 x CO2 forcing (e.g. 2070- 2099). These average climates are then used to drive the vegetation models. Note that because the vegetation models are equilibrium models, the results must be interpreted as indicating the potential vegetation, i.e., the climatically suitable vegetation. Time lags and transient responses of the vegetation to climate change are not considered here.

C.6. Interpretation of Biogeographic Model Simulations

Each of the ten IPCC regions was supplied with a set of MAPSS and BIOME3 output. Included were figures of vegetation distribution under current and future climate, vegetation density change (indexed by leaf area change), and runoff change. Also included were summary tables of the areas of the different biomes within each region under current and future climate, a change matrix indicating the area shifts from current biome type to other types, the areas within each biome expected to undergo an increase or decrease in vegetation density (change in LAI) and the areas within each biome expected to undergo an increase or decrease in annual runoff. These results were supplied for each vegetation model and for each GCM scenario. MAPSS and BIOME3 were both run under the Hadley Center scenarios; BIOME3 alone was run under the Max Planck Institute scenario; and, MAPSS alone was run under the older OSU, GFDL-R30 and UKMO scenarios. The Hadley and MPI simulations were run both with and without a direct CO2 effect (applied in the ecological models); while, the OSU, GFDL-R30 and UKMO scenarios were only run with the direct CO2 effects incorporated, in keeping with the VEMAP analyses.

Since the regional maps are of a much smaller extent and include quantitative information, the detailed interpretation will be left to the regions and the following discussion will only address general features of the simulations, particularly the differences between the older and newer GCMs and the MAPSS and BIOME3 intercomparisons. Although each region received the full set of figures, only a subset will be presented here. The MAPSS and BIOME3 results are sufficiently similar that the ranges presented in Tables C-1, C-2, C-3, C-4 and C-5 encompass the output from both models to indicate the full range of uncertainties within the scope of these experiments and models.


Table C-1: Potential future biome area (percentage of current) simulated by the MAPSS and BIOME3 biogeography models under three older (IPCC 1990, WG I), equilibrium 2 x CO2 GCM scenarios and under three newer (IPCC 1996, WG I), transient simulations from which 2 x CO2 scenarios were extracted. The reported ranges include both ecological models under several GCM scenarios. The baseline areas estimates are outputs from each model. Since BIOME3 does not differentiate Taiga/Tundra from Boreal Forest, two different aggregations are presented. The Taiga/Tundra summaries are MAPSS data only; while the "Boreal + Taiga/Tundra" and "Total Forest + Taiga/Tundra" summaries are from both models. The ranges of percent change for Boreal Conifer are from both models (except FAR scenarios, which are MAPSS output). The Taiga/Tundra under the MAPSS simulations decreases in area in all scenarios; while, Boreal conifer increases in area. Were these two vegetation zones aggregated in MAPSS, they would exhibit either increases or decreases, as in the BIOME3 simulations. The decreases in Boreal Conifer, shown in the table, are BIOME3 simulations.

 
Baseline Area (Mha)
With CO2 Effect
Without CO2 Effect
Biome Type
MAPSS
BIOME3
FAR Scenarios
SAR Scenarios
SAR Scenarios

Tundra
792
950
33-59%
43-60%
43-60%
Taiga/Tundra
999
35-62%
56-64%
56-64%
Boreal Conifer Forest
1,024
1,992
109-133%
64-116%
68-111%
Boreal + Taiga/Tundra
2,023
1,992
72-95%
64-90%
68-87%
Temperate Evergreen Forest
1,142
816
104-121%
104-137%
84-109%
Temperate Mixed Forest
744
1,192
125-161%
139-199%
104-162%
Total Temperate Forest
1,886
2,008
116-125%
137-158%
107-131%
Tropical Broadleaf Forest
1,406
1,582
71-151%
120-138%
70-108%
Savanna/Woodland
2,698
2,942
90-130%
78-89%
100-115%
Shrub-Steppe
994
1,954
61-70%
70-136%
81-123%
Grassland
2,082
554
109-126%
45-123%
120-136%
Total Shrub/Grassland
3,076
2,508
96-108%
105-127%
111-126%
Arid Lands
1,470
1,351
71-72%
59-78%
83-120%
Total Vegetation
13,351
13,333
100-101%
100-101%
100-101%

Note: FAR = First Assessment Report (IPCC 1990, WG I); SAR = Second Assessment Report (IPCC 1996, WG I).

 

Table C-2: Percentage area of current biomes which could undergo a loss of leaf area (i.e., biomass decrease) due to global warming under various older (FAR) and newer (SAR) GCM scenarios, and with or without a direct CO2 effect (see Table C-1 for details), as simulated by the MAPSS and BIOME3 biogeography models (ranges include both models). The losses in leaf area generally indicate a less favorable water balance (drought).

 
With CO2 Effect
Without CO2 Effect
Biome Type
FAR Scenarios
SAR Scenarios
SAR Scenarios

Tundra
1-3%
0-1%
0-2%
Taiga/Tundra
1-5%
1%
2%
Boreal Conifer Forests
39-67%
0-20%
3-69%
Temperate Evergreen Forests
24-57%
1-18%
28-51%
Temperate Mixed Forests
54-86%
1-29%
15-75%
Tropical Broadleaf Forests
5-63%
1-42%
26-33%
Savanna/Woodlands
10-21%
7-17%
38-75%
Shrub-Steppe
26-45%
1-24%
20-59%
Grasslands
33-37%
5-46%
43-75%
Arid Lands
8-12%
0-13%
0-29%

 

Table C-3: Percentage area of current biomes which could undergo a gain of leaf area (i.e., biomass increase) due to global warming under various older (FAR) and newer (SAR) GCM scenarios, and with or without a direct CO2 effect (see Table C-1 for details), as simulated by the MAPSS and BIOME3 biogeography models (ranges include both models). The gains in leaf area generally indicate a more favorable water balance.

 
With CO2 Effect
Without CO2 Effect
Biome Type
FAR Scenarios
SAR Scenarios
SAR Scenarios

Tundra
20-74%
20-58%
49-82%
Taiga/Tundra
91-98%
92-95%
91-94%
Boreal Conifer Forests
13-21%
36-93%
3-58%
Temperate Evergreen Forests
20-41%
46-67%
7-18%
Temperate Mixed Forests
4-26%
50-91%
9-21%
Tropical Broadleaf Forests
7-40%
16-87%
0-7%
Savanna/Woodlands
74-88%
46-84%
4-31%
Shrub-Steppe
46-64%
64-80%
16-42%
Grasslands
56-60%
45-78%
3-28%
Arid Lands
51-57%
53-80%
23-66%

 

Table C-4: Percentage area of current biomes which could undergo a loss of annual runoff due to global warming under various older (FAR) and newer (SAR) GCM scenarios, and with or without a direct CO2 effect (see Table C-1 for details), as simulated by the MAPSS and BIOME3 biogeography models (ranges include both models).

 
With CO2 Effect
Without CO2 Effect
Biome Type
FAR Scenarios
SAR Scenarios
SAR Scenarios

Tundra
19-32%
16-45%
28-46%
Taiga/Tundra
79-90%
71-79%
76-82%
Boreal Conifer Forests
1-25%
3-53%
33-81%
Temperate Evergreen Forests
12-21%
25-37%
33-67%
Temperate Mixed Forests
59-77%
51-66%
62-68%
Tropical Broadleaf Forests
11-40%
15-54%
23-68%
Savanna/Woodlands
14-19%
37-60%
31-46%
Shrub-Steppe
43-61%
23-44%
18-42%
Grasslands
34-38%
41-60%
33-56%
Arid Lands
24-26%
1-20%
2-20%

 

Table C-5: Percentage area of current biomes which could undergo a gain of annual runoff due to global warming under various older (FAR) and newer (SAR) GCM scenarios, and with or without a direct CO2 effect (see Table C-1 for details), as simulated by the MAPSS and BIOME3 biogeography models (ranges include both models).

 
With CO2 Effect
Without CO2 Effect
Biome Type
FAR Scenarios
SAR Scenarios
SAR Scenarios

Tundra
67-80%
36-82%
32-70%
Taiga/Tundra
10-20%
20-28%
18-23%
Boreal Conifer Forests
74-98%
41-95%
14-63%
Temperate Evergreen Forests
78-87%
58-73%
29-66%
Temperate Mixed Forests
23-41%
33-47%
11-37%
Tropical Broadleaf Forests
60-89%
46-85%
32-76%
Savanna/Woodlands
80-84%
31-60%
51-59%
Shrub-Steppe
23-44%
15-45%
23-48%
Grasslands
38-41%
19-32%
17-40%
Arid Lands
7-24%
4-15%
3-15%

 



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