Global Warming Causes

Emissions Concentrations and Climate Change

EMISSIONS

We next examine the impact of different policies on the climatic variables. Table 5-6 and Figure 5-6 show the aggregate industrial CO2 emissions per decade. Projections of baseline or uncontrolled industrial CO2 emissions in DICE-2007 continue to rise rapidly in coming decades, reaching 19 billion tons of carbon (gigatons of carbon, or GtC) annually in 2100. In the optimal case, emissions are limited to 12.5 GtC annually in 2100.

Annual emissions follow a hump-shaped pattern for the scenarios with emissions reductions, with the hump being around 2100 for the optimal case and around 2050 for the climate restrictions. None of the efficient paths—even the one restricting the temperature increase to 2°C—calls for declining emissions paths from the start. By comparison, the ambitious programs of Gore and Stern call for immediate emissions reductions or limitations. The front-loaded emissions reductions in the ambitious proposals lead to much more costly profiles than the ones that are efficiently constructed and hump shaped.

CONCENTRATIONS

Atmospheric concentrations of CO2 are shown in Table 5-7 and Figure 5-7. Beginning at an atmospheric concentration of

Table 5-6. Global Emissions of Industrial C02 per Decade by Policy

2005 2015 2025 2035 2045 2055 2065 2075 2085 2095 2105

Policy (Billions of Metric Tons of Carbon per Decade, Industrial Sources)

No controls

250-year delay	74-3	87.4	99-7	111.5	123.1	134-7	146.5	158.6	171.1	184.1	197-5
50-year delay	74-3	87.5	99-7	111.5	123.1	99.0	103-3	106.8	109.7	111.7	112.8
Optimal	74-3	73-7	81.6	88.3	94.2	99-3	103.6	107.2	110.1	112.1	113.1
Concentration limits
Limit to 1.5 X CO,	74-3	50.1	41.6	30.7	28.7	27.0	25.6	24.5	23-5	22.7	22.0
Limit to 2 X CO,	74-3	73-1	80.6	86.6	91.4	94-5	95.6	93-7	87.0	72.8	60.9
Limit to 2.5 X CO,	74-3	73-7	81.6	88.3	94.2	99-3	103.6	107.2	110.1	112.1	113.1
Temperature limits
Limit to i.5°C	74-3	56.7	54-5	46.8	32.7	12.8	0.0	0.0	16.1	2.8	9.0
Limit to 2°C	74-3	68.8	73-4	75.2	73-5	67.2	55-5	37-7	16.4	4.2	16.4

Limit to 3°C 74.3 73.5 81.3 87.8 Kyoto Protocol

Kyoto with 74.3 79.8 90.7 101.7

United States

United States

Strengthened 74.3 78.5 81.6 80.0

Stern Review 43.1 43.2 42.7 40.9 discounting

Gore proposal 74.3 65.9 55.2 39.3

Note: Policies are assumed to be introduced in 2008 unless otherwise stated.

78.1

32.7

Optimal Baseline <2°c

< 2 xC02 Stern

Kyoto w U.S. Gore

2005 2015 2025 2035 2045 2055 2065 2075

2085 2095

2105

Figure 5-6. Global emissions of industrial CO, per decade under different policies. The global emissions of industrial CO, under different policies over the next century. The figure for 2005 is the actual value.

2005 2015 2025 2035 2045 2055 2065 2075

2085 2095

2105

Optimal Baseline <2°c

< 2 xC02 Stern

Kyoto w U.S. Gore

Figure 5-6. Global emissions of industrial CO, per decade under different policies. The global emissions of industrial CO, under different policies over the next century. The figure for 2005 is the actual value.

Results of the DICE-2007 Model Runs

Table 5-7. Atmospheric CO2 Concentrations by Policy

Policy	2005	2015	2025	2050	2100	2200
		(Atmospheric Concentrations, Parts per Million of Carbon)
No controls
250-year delay	379.8	405.2	432.7	507.9	685.9	1,182.6
50-year delay	379.8	405.2	432.7	507.9	602.9	667.6
Optimal	379.8	405.2	426.2	480.9	586.4	658.5
Concentration limits
Limit to 1.5 XCO2	379.8	405.2	415.1	420.2	420.2	420.2
Limit to 2 XCO2	379.8	405.2	425.9	479.0	557.8	558.0
Limit to 2.5 XCO2	379.8	405.2	426.2	480.9	586.4	658.5
Temperature limits
Limit to 1.5°C	379.8	405.2	418.2	434.4	400.4	388.2
Limit to 2°C	379.8	405.2	423.9	466.2	464.9	442.2
Limit to 2.5°C	379.8	405.2	425.7	477.3	544.4	504.6
Limit to 3°C	379.8	405.2	426.1	480.4	579.3	575.7
Kyoto Protocol
Kyoto with United States	379.8	405.2	429.1	496.0	660.3	1,166.2
Kyoto w/o United States	379.8	405.2	431.7	505.6	684.0	1,181.5
Strengthened	379.8	405.2	428.5	474.9	543.8	629.2
Stern Review discounting	379.8	390.5	400.0	417.0	404.4	361.2
Gore proposal	379.8	405.2	422.5	430.9	399.2	399.4
Low-cost backstop	379.8	370.3	363.3	352.2	340.3	325.2

380 ppm in 2005, baseline concentrations rise to 686 ppm in 2100 and 1,183 ppm in 2200. In the optimal control case, concentrations are limited to 586 ppm in 2100 and 659 ppm in 2200. Most of the differences between the CO2 concentrations in the economic optimum and in the climatic-limits cases come after 2050.

1000 4

Optimal Baseline <2°C < 2 xC02 Stern

Figure 5-7. Atmospheric CO, concentrations under different policies. The atmospheric concentrations of CO, under different policies over the next century. The figure for 2005 is the actual value.

1000 4

Figure 5-7. Atmospheric CO, concentrations under different policies. The atmospheric concentrations of CO, under different policies over the next century. The figure for 2005 is the actual value.

Optimal Baseline <2°C < 2 xC02 Stern

Results of the DICE-2007 Model Runs 105

TEMPERATURE INCREASE

The increases in global mean temperature are shown in Table 5-8 and Figure 5-8. The baseline temperature increase of 0.73°C in 2005 (relative to the 1890-1910 average). The projected increase for the baseline scenario is 3.06°C by 2100 and 5.30°C by 2200. Clearly, according to the DICE-model projections, major warming is in store because of past emissions and climatic inertia. By comparison, the economic optimum has a projected increase of 2.61°C by 2100 and 3.45°C by 2200.

Except for the ambitious policy proposals of Gore and Stern, all runs have very similar concentration and temperature trajectories through the middle of the twenty-first century. After 2050, the scenarios with economic or climatic limits begin to trend downward relative to the other paths. The ambitious programs show a much sharper downward tilt, with warming for both cases peaking at around 1.6°C. The most successful emissions limitation is, of course, the low-cost backstop technology, which has zero effective emissions. Even with zero future emissions, however, the global temperature increase is close to 1°C.

One of the sobering results of integrated assessment analyses shown in these figures is how difficult it is to have a major impact on the temperature trajectory over the next century because of inertia in the economic and climate systems. The optimal path reduces global mean temperature by about 0.5°C relative to the baseline in 2100. Even if emissions were reduced 50 percent relative to the baseline by the mid-twenty-first century, global temperature change would still be at least 2°C. Only the ambitious paths, with excess abatement costs of $25 trillion to $34 trillion in present value (1.2 to 1.7 percent of global output), make a very large dent in global warming by

106 Results of the DICE-2007 Model Runs

Table 5-8. Projected Global Mean Temperature Change by Policy

	2005	2015	2025	2050	2100	2200
Policy	(Temperature		increase	from 1900, degrees C)
No controls
250-year delay	0.73	0.96	1.20	1.82	3.06	5.30
50-year delay	0.73	0.96	1.20	1.81	2.72	3.52
Optimal	0.73	0.95	1.17	1.68	2.61	3.45
Concentration limits
Limit to 1.5 X CO2	0.73	0.94	1.10	1.36	1.61	1.78
Limit to 2 X CO2	0.73	0.95	1.16	1.67	2.48	2.84
Limit to 2.5 X CO2	0.73	0.95	1.17	1.68	2.61	3.45
Temperature limits
Limit to 1.5°C	0.73	0.94	1.12	1.43	1.50	1.50
Limit to 2°C	0.73	0.95	1.15	1.61	2.00	2.00
Limit to 2.5°C	0.73	0.95	1.16	1.66	2.41	2.50
Limit to 3°C	0.73	0.95	1.17	1.68	2.57	2.99
Kyoto Protocol
Kyoto with
United States	0.73	0.96	1.18	1.76	2.94	5.23
Kyoto w/o
United States	0.73	0.96	1.20	1.81	3.05	5.29
Strengthened	0.73	0.95	1.17	1.66	2.39	3.26
Stern Review
discounting	0.73	0.89	1.03	1.31	1.52	1.27
Gore proposal	0.73	0.95	1.14	1.42	1.49	1.58
Low-cost backstop	0.73	0.80	0.84	0.86	0.90	0.83

Note: Increases are relative to the 1900 average.

2005 2025 2045 2065 2085 2105 2125 2145 2165 2185 2205

Figure 5-8. Projected global mean temperature change under different policies. Increases are relative to the 1900 average.

2005 2025 2045 2065 2085 2105 2125 2145 2165 2185 2205

Figure 5-8. Projected global mean temperature change under different policies. Increases are relative to the 1900 average.

108 Results of the DICE-2007 Model Runs

2100. However, the efficient policies have a more substantial impact over the longer run. Relative to the baseline, the temperature reductions in 2200 of the optimal path, the CO2-concentration-doubling target, and the 2.5°C temperature target are 1.85, 2.46, and 2.80°C, respectively.

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