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Impact of Vehicle Emissions Reductions on Global Climate 

Steven J. Smith

February 2010

Prepared for the U.S. Environmental Protection Agency

under Contract DE-AC06-76RL01830

Joint Global Change Research Institute

5825 University Research Court, Suite 3500

College Park, Maryland 20740
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The impact of a specified set of emissions reductions from light duty vehicles on climate 

change is calculated using the MAGICC 5.3 climate model. The integrated impact of the 

following emissions changes are considered: CO2, CH4, N2O, VOC2, NOx, and SO2. 

This brief summarizes the assumptions and methods used for this calculation

Overview of MAGICC 

The MAGICC simple climate model (Wigley and Raper 1992, 2002; Raper et al., 1996) 

as used in the IPCC Third Assessment Report (Cubasch et al., 2001) was used for these 

calculations. The version of MAGICC used here uses parameterizations updated for use 

in the IPCC AR4. The carbon-cycle component of the MAGICC model operates with a 

balanced global carbon cycle for both historical and future time periods. Input 

assumptions are specified for net anthropogenic deforestation and ocean fluxes for the 

decade of the 1980s as well as the strength of temperature-feedbacks. The MAGICC 

model then adjusts the strength of the carbon-dioxide feedback in order to balance the 

carbon-cycle over this decade (Wigley 1993). Terrestrial carbon-cycle feedbacks are 

included as temperature dependent reductions in carbon-pool timescales. A temperature 

feedback on respiration and gross primary productivity is also included. While the 

detailed behavior of climate feedbacks on the carbon cycle is undoubtedly complex, the 

representations used in MAGICC are capable of reproducing the range of results from 

more complex carbon-cycle models (e.g. Smith and Edmonds 2006). Central values for 

carbon-cycle parameters were used for this calculation. The impact of pollutant emissions 

is felt both through changes in atmospheric chemistry that impact the lifetime of methane 

and some HCFC species, and through changes in tropospheric ozone, a greenhouse gas. 

These impacts are included using the methodology descried in Wigley, Smith, and 

Prather (2002).

Method and Data Sources

The primary data needed for this calculation are: 1) a reference scenario for all emissions 

and 3) a set of emissions reductions.

The reference scenario is the MiniCAM reference (no climate policy) scenario used as 

the basis for the Representative Concentration Pathway RCP4.5. This scenario is used 

because it contains a comprehensive suite of greenhouse and pollutant gas emissions 

including carbonaceous aerosols. 

The four RCP scenarios will be used as common inputs into a variety of Earth System 

Models for inter-model comparisons leading to the IPCC AR5 (Moss et al. 2008; Moss et 

al. 2010). The MiniCAM RCP4.5 is based on the scenarios presented in Clarke et al. 

(2007) with non- CO2 and pollutant gas emissions implemented as described in Smith 

and Wigley (2006) and land-use change emissions as described in Wise et al. (2009). 

Base-year information has been updated to the latest available data for the RCP process. 

The final RCP4.5 scenario is available at the IAMC scenario web site.1

 1 www.iiasa.ac.at/web-apps/tnt/RcpDb/
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The emissions reductions were supplied by EPA in spreadsheet form.2 Emissions 

reductions were supplied for 15 pollutant and GHG species. The impact of HFC 

reductions were not considered in this calculation since the impact of these emissions is 

calculated separately and exogenously included in MAGICC. Emissions of all VOC 

species were summed to one aggregate NMVOC emissions change (1,3-Butadiene, 

Acetaldehyde, Acrolein, Benzene, Formaldehyde, VOC). 

Emissions changes in PM2.5 were also provided, but these were not considered at the 

request of EPA. These emissions changes could be converted into BC and OC emissions 

changes, but the overall impacts of these changes for this sector would be a very small 

fraction of the total radiative forcing change.

All emissions reductions were assumed to begin in 2012, with zero emissions change in 

2011 and linearly increasing to equal the value supplied for 2020. The emissions 

reductions past 2050 were scaled with total US road transportation fuel consumption 

from the MiniCAM reference scenario. This was chosen as a simple scale factor given 

that both direct and upstream emissions changes are included in the emissions reduction 

scenario provided. Road transport fuel consumption past 2050 does not change very 

much, dropping to 93% of the 2050 value in 2095, which results in a nearly constant level 

of reductions past 2050.

The calculation consisted of using the MAGICC model to determine two pathways for 

21st century greenhouse gas concentrations, radiative forcing, temperature change, and 

sea-level rise. The first pathway uses the reference scenario emissions and the second 

uses a scenario where the specified emissions changes were subtracted from global 

emissions for the years 2000-2100. The difference between these two results is the impact 

of the specified LDV emissions changes on global CO2 concentrations. 

Results

The result of the calculation is shown in the Figures for climate sensitivites ranging from 

1.5 to 6.0 °C per CO2 doubling. The specified emissions reductions reduce CO2 

concentrations by 1 ppmv in 2050 and an average of 3 ppmv by 2100. Radiative forcing 

decreases by 0.01 W/m2 by 2050 and 0.02 W/m2 by 2100. The curvature in the forcing 

result is due to the non-linear relationships between CO2, CH4, and N2O radiative forcing 

and concentrations. Due to this non-linearity, the same concentration increment in an 

earlier year will result in a larger forcing change than the same increment in a latter year 

where concentrations are higher.

The forcing impact of pollutant emissions reductions is relatively minor due to the much 

larger reductions in greenhouse gas emissions in this scenario, although there is an 

indirect effect of these reductions on methane concentrations. Global-mean temperature 

decreases range from 0.003 to 0.005 degrees centigrade in 2050 and 0.007 to 0.016 

degrees centigrade by 2100 depending on the climate sensitivity. Sea-level rise decreases 

by 0.02-0.03 cm by mid-century, and 0.06–0.14 cm by 2100. The response of sea-level 

rise is different than that for temperature due to the long time scale for thermal expansion 

of the oceans.

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References

Bond, T. C., D. G. Streets, K. F. Yarber, S. M. Nelson, J.-H. Woo, and Z. Klimont (2004) “A technology-based global 

inventory of black and organic carbon emissions from combustion” J. Geophys. Res. 109, D14203, 

doi:10.1029/2003JD003697.

Clarke, L., J. Edmonds, H. Jacoby, H. Pitcher, J. Reilly, R. Richels, (2007) Scenarios of Greenhouse Gas Emissions 

and Atmospheric Concentrations. Sub-report 2.1A of Synthesis and Assessment Product 2.1 by the U.S. Climate 

Change Science Program and the Subcommittee on Global Change Research (Department of Energy, Office of 

Biological & Environmental Research, Washington, DC., USA, 154 pp.).

Durbin, T.D., J.M. Norbeck, M.R. Smith, and T.J. Truex (1999) “Particulate emission rates from light-duty vehicles in 

the South Coast Air Quality Management District” Environmental Science and Technology 33 4401-4406.

Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, G. Myhre, 

J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland (2007) Changes in Atmospheric Constituents and in 

Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the 

Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, 

Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, 

United Kingdom and New York, NY, USA.

Kim, S.H., J. Edmonds, J. Lurz, S. J. Smith, and M. Wise (2006) “The ObjECTS Framework for Integrated 

Assessment: Hybrid Modeling of Transportation” Energy Journal, (Special Issue #2).

Moss, Richard, Mustafa Babiker, Sander Brinkman, Eduardo Calvo, Tim Carter, Jae Edmonds, Ismail Elgizouli, Seita 

Emori, Lin Erda, Kathy Hibbard, Roger Jones, Mikiko Kainuma, Jessica Kelleher, Jean Francois Lamarque, 

Martin Manning, Ben Matthews, Jerry Meehl, Leo Meyer, John Mitchell, Nebojsa Nakicenovic, Brian O’Neill, 

Ramon Pichs, Keywan Riahi, Steven Rose, Paul Runci, Ron Stouffer, Detlef van Vuuren, John Weyant, Tom 

Wilbanks, Jean Pascal van Ypersele, and Monika Zurek (2008) Towards New Scenarios for Analysis of Emissions, 

Climate Change, Impacts, and Response Strategies (Intergovernmental Panel on Climate Change, Geneva) 132 

pp.

Moss RH, JA Edmonds, K Hibbard, T Carter, S Emori, M Kainuma, T Kram, M Manning, J Meehl, J Mitchell, N 

Nakicenovic, K Riahi, S Rose, SJ Smith, R Stouffer, AM Thomson, D VanVuuren, J Weyant, and T Wilbanks 

(2010) Representative Concentration Pathways: A New Approach to Scenario Development for the IPCC Fifth 

Assessment Report Nature 463 pp. 747–756.

Raper, S.C.B., Wigley T.M.L. and Warrick R.A. 1996. in Sea-Level Rise and Coastal Subsidence: Causes, 

Consequences and Strategies J.D. Milliman, B.U. Haq, Eds., Kluwer Academic Publishers, Dordrecht, The 

Netherlands, pp. 11–45.

Smith, Steven J. and J.A. Edmonds (2006) The Economic Implications of Carbon Cycle Uncertainty Tellus B 58 (5), 

pp. 586–590.

Smith, Steven J. and T.M.L. Wigley (2006) “Multi-Gas Forcing Stabilization with the MiniCAM” Energy Journal

(Special Issue #3).

Wigley, T. M. L. 1993. Balancing the Carbon Budget - Implications for Projections of Future Carbon-Dioxide 

Concentration Changes Tellus 45B, 409-425.

Wigley, T. M. L. and Raper, S. C. B. 2001. Interpretation of high projections for global-mean warming. Science

293(5529), 451-454.

Wigley, T.M.L. and Raper, S.C.B. 1992. Implications for Climate And Sea-Level of Revised IPCC Emissions 

Scenarios Nature 357, 293–300.

Wigley, T.M.L. and Raper, S.C.B. 2002. Reasons for larger warming projections in the IPCC Third Assessment Report 

J. Climate 15, 2945–2952.

Wigley, T.M.L., Steven J. Smith, and M.J. Prather (2002) “Radiative Forcing due to Reactive Gas Emissions” Journal 

of Climate 15(18), pp. 2690–2696.

Wise, M. A, Calvin, K. V, Thomson, A. M, Clarke, L. E, Bond-Lamberty, B., Sands, R. D, Smith, S. J, Janetos, T. C, 

Edmonds, J. A (2009) "Implications of Limiting CO2 Concentrations on Land Use and Energy" Science 324 pp.

1183–1186