In a rational world, the ultimate level of climate and thus GHG concentration stabilization would emerge from a political process in which the global community would weigh mitigation costs and the averted damages associated with different levels of stabilization. Also weighed would be the risks of triggering systemic changes in large geophysical systems, like ocean circulation, or other irreversible impacts. In reality, the political process will inevitably be influenced by the distribution of positive and negative effects of climate change, as well as by the costs of mitigation among countries, largely determined by how risks, costs, environmental values, and development aspirations are weighed in different regions and cultures. This process will be strongly influenced by new scientific and technical knowledge and by experience gained in making and implementing policy. The climate change literature contains a diversity of arguments as to why either a low level or a relatively high level of stabilization is desirable (IPCC, 2001b).

Given the large uncertainties that characterize each component of the climate change problem, it is impossible to establish a globally acceptable level of stabilized GHG concentrations today. Studies discussed in this section and summarized in Table 10.11 support the obvious expectations that lower stabilization targets involve exponentially higher mitigation costs and relatively more ambitious near-term emissions reductions, but, as reported by WGII (IPCC, 2001b), lower targets induce significantly smaller biological and geophysical impacts and thus induce smaller damages and adaptation costs.

Table 10.11: Selected studies on global mitigation costs for different stabilization targets
Study	Scenarios & dimensions	450ppmv	550ppmv	650ppmv	750ppmv	850ppmv	Notes
Nordhaus and Boyer (1999) RICE-98	billion 1990 US$						discounted (d) back to 1950 IAM
	net impacts on global welfare		335.00
	difference from base (=0)
	mitigation		459.00
	reduction in cl.damage		794.00
Valverde and Webster (1999) MIT-EPPA 1.6	in billion 1985 US$						1985-2100, 5 year time steps d= 5%
	500a global emissions path		(500ppmv)
	all nations equal % abatement
	OECD: no trade		272.50
	OECD: trade		272.40
	Non-OECD: no trade		216.30
	Non-OECD: trade		216.20
	Global: no trade		488.90
	Global trade		488.60
Manne and Richels (1999b) MERGE 3.0	billion of 1990 US$
	Kyoto followed by arbitrary reduc.		2400.00				consumption loss through 2100 d=5% to 1990
	Kyoto followed by least-cost		900.00
	least cost		650.00
Tol (1999c), FUND	percentage of world income, median			2.1%			average annual income loss 9 regions, 5 sectors in 2100
Ha-Duong et al. (1997) DIAM	percentage of 1990 GWP						d=3% average annual costs period 2000–21000
	inertia of 50 years	1.1%
Lecocq et al. (1998) STARTS	percentage						average annual costs period 2000-2100 differential inertia in sectors
	abatement costs as consumption loss
	compared to BAU in
	A: flexible sector		1,5%
	B: rigid sector		0.4%
Yohe and Wallace (1996) Connecticut	percent of GWP as costs		16.40 to 16.69	5.94 to 7.09	2.84 to 4.24	1.59 to 3.05	d, expected present value 7 scenarios
	in 1990, no benefit side
	one percentage point is
	~210 billion US$
Dowlatabadi (1998) ICAM-3	percentage of GDP as costs		0.05 to 0.48				period: 2000–20025 sequential learning framework mitigation costs for the USA and Canada only
	mitigation costs within 9 scenarios
	with different technical change in
	energy sector
Richels a. Edmonds (1995) Global 2100 & ERB	percentage of GWP as costs	(400)	(500)				d=5% stabilization in 2100 Global 2100 Manne/Richels ERB: Edmonds-Reilly-Barns (1992)
	Sc:500a: follow BAU through 2010		Gl 2100: 0.6 %; ERB: 0.7%
	Sc: 500b: between 500a & stab.		Gl 2100: 0.9 %; ERB: 0.95%
	Sc: Emission stabilization at		Gl 2100: 1.15 %; ERB: 1.1%
	1990 level	Gl: 0.9%; ERB:1.1%	Gl:0.6%; ERB: 0.7%
Plambeck, Hope (1996) PAGE95	in trillion US$						d= 5 %, 1990-2200 further scenarios not listedhere, e.g., non-linear etc.
	BAU + 100 GtC	2.50
	BAU	2.20
Yohe and Jacobsen (1999) Connecticut	trillion 1990 US$						d= 3 % through 2100 (Ramsey) cost study in terms of deadweight loss, no opt A: alt. sink specifications B: alt. emissions targets for 2010
	annual control costs of 7 sc
	Minimum cost: Sc3 to Sc7	A: 10.13 to 44.40	A: 2.11 to 16.12	A: 0.36 to 7.24
	Cost with Kyoto: Sc3 to Sc7	A: 10.47 to 47.04	A: 2.12 to 16.19	A: 0.40 to 7.26
	Minimum cost minus 10 % emis.	B: 10.13 to 44.40	B: 2.11 to 16.12	B: 0.36 to 7.24
	Cost with Kyoto minus 10 %	B: 10.40 to 46.77	B: 2.13 to 16.16	B: 0.42 to 7.28
Manne (1995) MERGE	trillion 1990 US$	(415 ppmv)					d= 5 % to 1990
	global damage	1.90
	benefits of stab. as reduced dam.	2.50
	costs of stab.	18.50
Manne and Richels (1997) MERGE 3.0	trillion 1990 US$						d= 5% to 1990 1990- 2100 non- market and market damages
	WGI: w/ o where flex.	14.20	9.00	5.00	3.00
	WGI: with where flex	7.00	4.00	2.00	1.2
	WRE: w/ o where flex	5.50	2.00	1.00	1.00
	WRE: with where flex	3.50	1.00	0.6	0.50
	least cost: with where flex		0.60
	WGI: Annex- 1- trade		5.90
	10% cut in 2010: A- 1- trade		2.30
	WRE: A- 1- trade		0.90
Tol (1999d), FUND 1.6	trillion net present costs in US$						5% through 2050 damage per year in billion US$: 216
	WGI: no trade		17.5	10.50
	WGI: trade		8.0	4.00
	WRE: no trade		16.0	10.00
	WRE: trade		4.0	2.00
Tol (1999a) FUND 1.6	in trillion US$		below 550				d= 5% per year to 1990 consumption losses p.a. period 1990- 2200
	Minimum Cost		2.4
	Min. Cost meeting Kyoto, trade		3.1
	Min. Cost meeting Kyoto		3.7
	2 % reduction, intern. trade		4.0
	meeting Kyoto, trade		4.4
	meeting Kyoto, no trade		14.6

IPCC Homepage