1 

## 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 Re search Court, Suite 3500 College Park, Maryland 207402 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 chang es are considered: CO 2 , CH 4 , N 2 O, VOC 2 , NO x , and SO 2 . 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 Thir d 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 ant hropogenic 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 emissi ons 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, an d Prather (2002).



### Method and Data Sources The primary data needed for this calculation are: 1) a refer ence scenario for all emissions and 3 ) a set of emissions reductions. The reference scenario is the MiniCAM reference (no clima te 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 wi ll 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

- CO 2 and pol lutant 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/3 The emissions reductio ns 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 exogeno usly 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 PM 2.5 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 as sumed 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 21 st century greenhouse gas concentrations, radiative forcing, temperature change, and

sea

- level r ise. 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 s pecified LDV emissions changes on global CO 2 concentrations.



### Results The result of the calculation is shown in the Figure s for climate sensitivites ranging from 1.5 to 6.0 °C per CO 2 doubling . The specified emissions reductions reduce CO2 concentrations by 1 ppmv in 2050 and an average of 3 ppmv by 2100. Radiative forcing d ecreases by 0.01 W/m 2 by 2050 and 0.02 W/m 2 by 2100. The curvature in the forcing result is due to the non

- linear relationships between CO 2 , CH 4 , and N 2 O radiative forcing and concentrations. Due to this non

- linearity, the same concentration increment in a n 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 emi ssions 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 t he 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.

2 Inventories_from_ref_CY_20100204.xls456 

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