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appendix
Scientific Concern over Greenhouse
Gas Buildup
Scientists have documented increasing concentrations of greenhouse
gases (GHGs) in the atmosphere resulting from human activity. Con-
centrations in terms of carbon dioxide equivalent values (CO2-eq) have
increased from about 280 parts per million by volume (ppmv) in 1750
to be about 390 ppmv today, with most of the growth coming from the
burning of fossil fuels and deforestation.1 Scientists have also connected
the GHG buildup to rising global temperatures, the melting of terrestrial
snow and ice, and rising sea level (Parry et al. 2007).
International policy goals for limiting climate change were established
in 1992 under the United Nations Framework Convention on Climate
Change, in which the United States and more than 190 other nations set
the goal of “stabilization of GHG concentrations in the atmosphere at
a level that would prevent dangerous anthropogenic interference with
the climate system.” Subsequent scientific research has sought a better
understanding and quantification of the links among GHG emissions,
atmospheric GHG concentrations, changes in global climate, and the
impacts of those changes on human and environmental systems.
The United Nations Intergovernmental Panel on Climate Change
(IPCC) has examined a range of reduction pathways leading to GHG
concentrations stabilizing at different levels and over different time
frames, each associated with different types and magnitudes of climate
1
http://www.esrl.noaa.gov/gmd/ccgg/trends/.
193
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194 Policy Options for Reducing Energy Use and Greenhouse Gas Emissions from U.S. Transportation
impacts. The uncertainties in scientific understanding of the world’s
climate system render exact relationships between GHG atmospheric
concentrations and temperature changes impossible to define. Linking
global temperature change with a target GHG concentration involves
a number of physical processes that are not fully understood, and thus
there is uncertainty surrounding this linkage. The most recent Synthe-
sis Report of IPCC (IPCC 2007) indicates that global GHG concentrations
may need to be limited to around 450 ppmv CO2-eq to keep global tem-
peratures, within a reasonable likelihood, from rising more than 2°C.2
A higher emissions target of 550 ppmv CO2-eq is associated with a 3°C
increase in temperature. The IPCC report recognizes that such concen-
trations could be associated with temperature changes that are well above
or below these figures.
The stabilization range of 450 to 550 ppmv CO2-eq has been exten-
sively analyzed by the scientific and economic communities and is a
focus of international climate policy forums. To stabilize atmospheric
concentrations of CO2-eq at or below 450 ppmv by 2050 implies that
global cumulative worldwide emissions must not exceed about 650 giga-
tons (Gt) over the next 40 years (Meinshausen et al. 2009). Determining
what U.S. emissions allocations are consistent with achieving this global
mitigation goal is complicated by a range of uncertainties, including the
degree of international action and the many forces that will influence
global emissions over a period of decades such as changes in popula-
tion, economic development, and technology. The recent Stanford Uni-
versity Energy Modeling Forum Study 22 (EMF-22)3 used many of the
nation’s leading integrated assessment models to explore the relationship
between global and regional GHG emissions reduction and long-term
climate goals. The EMF-22 model runs suggest that the equivalent U.S.
share of required global emissions reduction needed to stabilize concen-
trations at 450 ppmv CO2-eq corresponds roughly to reducing 2050 U.S.
emissions to 80 percent below current levels (if emissions are reduced
at a linear rate over 40 years). In comparison, the higher emissions tar-
2
GHGs are not the only anthropogenic influence on global temperatures. Others are surface albedo
changes and aerosols.
3
See http://emf.stanford.edu/research/emf22/ and Clarke et al. 2009.
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195
Scientific Concern over Greenhouse Gas Buildup
Cumulative emissions
10,000 2012 to 2050
9,000
2012
8,000
Annual U.S. emissions (MtCO2e)
287 GtCO2e
7,000 If current emissions stabilize,
Recent trend leading to concentrations > 600 ppmv
6,000
5,000
4,000
203 GtCO2e
3,000
Needed to achieve atmospheric
2,000 concentration of 550 ppmv
167 GtCO2e
1,000
Needed to achieve atmospheric
concentration of 450 ppmv
0
1990 2010 2030 2050
figure a-1
U.S. emissions budgets to achieve stability in atmospheric concentrations of CO2-eq.
1 Gt = 1,000 million metric tons.
SOURCE: Fawcett et al. 2009; Clarke et al. 2009.
get of 550 ppmv CO2-eq, associated with a 3°C increase in temperature,
would require roughly a 50 percent reduction in annual emissions by
2050 (Fawcett et al. 2009; Paltsev et al. 2007).
The EMF-22 linear paths to stabilize concentrations at 450 or
550 ppmv correspond to a cumulative U.S. CO2-eq emissions “budget”
for the next 40 years of about 167 Gt and 203 Gt, respectively, as shown
in Figure A-1.4 To meet the 167-Gt budget would require that emissions
be 80 percent lower in 2050 than they are today. They would drop from
7.1 Gt CO2-eq per year to about 1.1 Gt CO2-eq per year. The pursuit of
a less stringent 203 Gt budget (with the accompanying greater risk of a
higher global temperature) would require that annual U.S. emissions be
4
These estimates are based on a “least cost” model calculation for distributing emissions reduction
burdens among countries. The question of what constitutes a fair share of emissions reductions
for the United States involves numerous economic, scientific, political, and ethical considerations.
The estimates assume full international participation in emissions reductions. Without significant
international participation, the United States will have to do more to reach any long-term emissions
reduction goal.
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196 Policy Options for Reducing Energy Use and Greenhouse Gas Emissions from U.S. Transportation
reduced by about 50 percent by 2050. Because of the long-lived nature of
most GHGs, even zero growth in CO2-eq emissions would lead to atmo-
spheric concentrations in excess of 600 ppmv by 2050, which would risk
temperature increases exceeding 3°C.5
Even stabilizing emissions at current levels for the next four decades
presents difficult challenges because of expected increases in population
and economic development. The rapid growth in energy use in large
industrializing countries such as China and India will be critical to the
prospects for stabilization. Even today, as one of the largest individual
emitters of GHGs, the United States cannot substantially reduce global
emissions. The EMF-22 results indicate that atmospheric GHG concen-
trations can only be kept below 450 ppmv CO2-eq if the United States and
other high-income countries, along with China, India, and many other
low- and middle-income countries, take aggressive mitigation measures.
References
abbreviation
IPCC Intergovernmental Panel on Climate Change
Clarke, L., J. Edmonds, V. Krey, R. Richels, S. Rose, and M. Tavoni. 2009. International
Climate Policy Architectures: Overview of the EMF 22 International Scenarios.
Energy Economics, Vol. 31 (Supplement 2), pp. S64–S81.
Fawcett, A. A., K. V. Calvin, F. C. de la Chesnaye, J. M. Reilly, and J. P. Weyant. 2009.
Overview of EMF 22 U.S. Transition Scenarios. Energy Economics, Vol. 31,
pp. S198–S211. http://emf.stanford.edu/files/res/2369/fawcettOverview22.pdf.
IPCC. 2007. Climate Change 2007: Synthesis Report. Contribution of Working Groups
I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on
Climate Change. Geneva.
Meinshausen, M., N. Meinshausen, W. Hare, S. C. B. Raper, K. Frieler, R. Knutti,
D. J. Frame, and M. R. Allen. 2009. Greenhouse-Gas Emission Targets for Limit-
ing Global Warming to 2°C. Nature, Vol. 458, pp. 1158–1162. http://www.nature.
com/nature/journal/v458/n7242/full/nature08017.html.
5
As mentioned above, these emission budgets are for gross emissions in the United States and do
not include sources and sinks from land use, land use change, and forestry. If these factors contribute
to net emissions, attaining the budgets will be more difficult. If they act as net sinks, which is the
current trend, attaining the budgets will be somewhat easier.
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197
Scientific Concern over Greenhouse Gas Buildup
Paltsev, S., J. M. Reilly, H. D. Jacoby, A. C. Gurgel, G. E. Metcalf, A. P. Sokolov, and
J. F. Holak. 2007. Assessment of U.S. Cap-and-Trade Proposals. National Bureau
of Economic Research, Cambridge, Mass.
Parry, M. L., O. F. Canziani, J. P. Palutikof, P. J. van der Linden, and C. E. Hanson
(eds.). 2007. Contribution of Working Group II to the Fourth Assessment Report
of the Intergovernmental Panel on Climate Change. Cambridge University Press,
Cambridge, United Kingdom. http://www.cambridge.org/features/earth_
environmental/climatechange/wg2.htm.
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