Radiative forcing in the S1 and S2 scenarios with a 20-year timeframe. Unger et al. (2009) Click to enlarge.

The study applied the NASA Goddard Institute for Space Studies (GISS) model for Physical Understanding of Composition-Climate INteractions and Impacts (G-PUCCINI). This model has been used previously to understand the role of short-lived air pollutants in climate change for past, present and future atmospheres.

For an emissions inventory, they used anthropogenic CO, NO_x, NMVOCs and SO₂ emissions from EDGAR3.2 for the year 1995 (Olivier and Berdowski, 2001), and BC and OC emissions (which are not available in the EDGAR3.2 database) from another global inventory for the year 1996.

In the first scenario (S1), ORT emissions were reduced by 50% in the US and globally, with the assumption that the replacement energy to supply the fleet is provided by a clean zero emission source. In the second scenario (S2), ORT emissions were reduced by 50% in the US and globally, and they assumed that the replacement energy to supply the fleet is provided by the PG sector in its current state. The scenarios were run with 20-year and 100-year time frames.

In running the models, they found that the net non-CO₂ and CO₂ RF for each scenario and each time frame was negative, indicating a cooling effect on global climate in the short and long terms. However, the relative roles of non-CO₂ and CO₂ forcing agents depend on how the replacement energy is supplied.

A global reduction of 50% in ORT emissions with no change to PG emissions yields a total RF (including CO₂ and non-CO₂ effects) of -82 mWm^-2 for the 20-year time horizon (40% due to non-CO₂ effects) and -176 mWm^-2 for the 100-year time horizon (25% due to non-CO₂ effects) and could contribute to global warming mitigation as part of a multi-pronged strategy (Pacala and Socolow, 2004). For the global S1 scenario, the non-CO₂ cooling effects are dominated by reductions in O₃ and BC. Similarly, for emissions changes in the US (S1_US), the total RF is -33 mWm^-2 for the 20-year time horizon (50% due to non-CO₂ effects) and -68 mWm^-2 for the 100-year time horizon (40% due to non-CO₂ effects). Reduction of ORT emissions in the US also imposes a negative indirect CH₄ RF that is of the same magnitude as the O₃ RF response. In the short-term, for a 50% reduction in US ORT emissions, the non-CO₂ and CO₂ agents play an equal role in the climate forcing impacts.

—Unger et al. (2009)

The net non-CO₂ RF is always important relative to the CO₂ RF and outweighs the CO₂ RF response in the S2 scenario for both time horizons. S1 offers additional improvements to ozone and black carbon air quality whereas S2 implies substantial degradation of sulfate air quality.

Whether or not the emissions profiles modeled are actually achievable is unclear, the authors note in their discussion. For example, a 2007 EPRI study they cite concluded that the US PHEV average fraction of vehicle miles traveled using battery electricity would be about 20%. “Thus, a 50% reduction in ORT emissions may not be possible to achieve even with complete conversion of the current US fleet to PHEV,” the authors write. On the other hand, improved battery technology and the advent of electric vehicles could alter that. There are also complex uncertainties in quantifying the consequences of adding load from PHEVs onto the electric grid in the US.

Until higher confidence estimates of the impacts of various technology change options (including a large PHEV fleet) on the ORT and PG sectors are obtained in the future, our results may be interpreted to serve as a guide to their climate impacts. In the specific case of a PHEV fleet, the development of an adequate battery that would allow the widespread introduction of PHEVs or fully electric vehicles is yet to come. However, the manufacturers remain committed to the development of advanced battery technology. The positive impacts on climate via non-CO₂ and CO₂ effects revealed in this study from ORT emission reduction demonstrate that is a worthwhile commitment.

...this study does demonstrate the critical importance of considering climate effects of O₃ and fine aerosol particles in mitigation strategies and environmental policy. Our model results indicate that full assessment of the environmental impacts of technology and policy changes designed to counter global climate change must consider the climate effects of O₃ and aerosol air pollution that may outweigh CO₂ effects depending on the replacement energy source.

—Unger et al. (2009)

Resources

• Nadine Unger, Drew T. Shindell and James S. Wang (2009) Climate forcing by the on-road transportation and power generation sectors. Atmos. Environ., in press, doi: 10.1016/j.atmosenv.2009.03.021