Radiative-forcing analysis: more mitigation effort in road transport
Carbon dioxide is not the only greenhouse gas. Coemitted air pollutants also significantly affect global climate. By various interactions, their absolute effect can be complicated to evaluate. In principle, however, most air pollutants have a relatively short time span in the atmosphere, and hence, can be classified as short lived species. Many air pollutants, including organic aerosols have a global cooling effect, whereas black carbon and ozone contribute to global warming. The effect of the short lived species is not marginal. In fact, their combined radiative forcing may outweight that of carbon dioxide (Forster et al., 2007). In view of this observation, there is increased attention on the mitigation potential of some air pollutations, such as ozone and black carbon, short lived species with high radiative forcing. On the other hand, there are other aerosols, excluding black carbon, which exert a cooling effect that may have masked about 50% of the global warming by GHG. The fight against air pollution and for public health can, hence, have an unwanted impact by inducing accelerated global warming. Starting with this background, Unger et al. from NASA ask in PNAS in their article Attribution of climate forcing to economic sectors the following question: How is the total radiative forcing effect organized according to economic sector, the drivers of emissions?
The authors point out that emissions of black carbon (positive RF) and organic aerosols (negative RF) are often coupled. Hence, the ratio between both emittants is important to evaluate the total radiative forcing in a specific sector. An analysis of sectors according to this ratio reveals significant differences across sectors:
- There are sector such as the power industry that have high emissions of species with both positive and negative radiative forcing.
- Other sectors, such as road transport, are dominated by species with positive radiative forcing. More specifically, the ratio between black carbon and organic carbon aerosols is relatively high in road transport.
The accumulated climate impact over all sectors is visualized in the figure below (Source: Unger et al., 2010). In fact, in the short term (2020), road transport dominates the accumulated climate impact. In the long run (2100), the power sector dominates as greenhouse gases persists significantly longer in the atmosphere than short lived species.
From this technical analysis, effective climate change mitigation can most easily be obtained in on-road transportation - with significant co-benefits as air pollutants from transport are more harmful than from other sectors. This is for example due to a higher intake fraction (fraction of pollutants that is inhaled, e.g. Marshall et al., 2005), but can have more general benefits for public health and overall mobility (Creutzig and He, 2009). However, from a climate perspective road transportation is underregulated. For example, in Europe road transport is not part of the emission trading scheme, and, by this, is positively discriminated against electrified rail transport. A clever mix of instruments that prices the harmful parts of mobility while increasing its beneficial aspects (e.g. accessibility, thence, could have a near and long term positive impact).
Thanks to Jan Minx for pointing out the PNAS article.
Forster P, et al. (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, eds Solomon S, et al. (Cambridge Univ Press, New York)
Unger, N., Bond, T. C., Wang, J. S., Koch, D. M., Menon, S., Shindell, D.T., Bauer, S. (2010) Attribution of climate forcing to economic sectors PNAS 2010 : 0906548107v1-6
Marshall, J.D., Teoh, S.K., and Nazaroff, W.W. (2005) Intake fraction of nonreactive vehicle emissions in US urban areas. Atmospheric Environment 39 (7), 1363
Creutzig, F. and He, D. (2009) Climate change mitigation and co-benefits of feasible transport demand policies in Beijing Transportation Research D 14: 120-131
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