Transport impacts on atmosphere and climate: Metrics

Abstract

The transport sector emits a wide variety of gases and aerosols, with distinctly different characteristics which influence climate directly and indirectly via chemical and physical processes. Tools that allow these emissions to be placed on some kind of common scale in terms of their impact on climate have a number of possible uses such as: in agreements and emission trading schemes; when considering potential trade-offs between changes in emissions resulting from technological or operational developments; and/or for comparing the impact of different environmental impacts of transport activities. Many of the non-CO 2 emissions from the transport sector are short-lived substances, not currently covered by the Kyoto Protocol. There are formidable difficulties in developing metrics and these are particularly acute for such short-lived species. One difficulty concerns the choice of an appropriate structure for the metric (which may depend on, for example, the design of any climate policy it is intended to serve) and the associated value judgements on the appropriate time periods to consider; these choices affect the perception of the relative importance of short-and long-lived species. A second difficulty is the quantification of input parameters (due to underlying uncertainty in atmospheric processes). In addition, for some transport-related emissions, the values of metrics (unlike the gases included in the Kyoto Protocol) depend on where and when the emissions are introduced into the atmosphere-both the regional distribution and, for aircraft, the distribution as a function of altitude, are important. In this assessment of such metrics, we present Global Warming Potentials (GWPs) as these have traditionally been used in the implementation of climate policy. We also present Global Temperature Change Potentials (GTPs) as an alternative metric, as this, or a similar metric may be more appropriate for use in some circumstances. We use radiative forcings and lifetimes from the literature to derive GWPs and GTPs for the main transport-related emissions, and discuss the uncertainties in these estimates. We find large variations in metric (GWP and GTP) values for NO x , mainly due to the dependence on location of emissions but also because of inter-model differences and differences in experimental design. For aerosols we give only global-mean values due to an inconsistent picture amongst available studies regarding regional dependence. The uncertainty in the presented metric values reflects the current state of understanding; the ranking of the various components with respect to our confidence in the given metric values is also given. While the focus is mostly on metrics for comparing the climate impact of emissions, many of the issues are equally relevant for stratospheric ozone depletion metrics, which are also discussed.

Transport impacts on atmosphere and climate: Land transport

“…The contribution of each climate gas depends on the chosen time horizon and becomes relatively more important on longer time horizons in the case of a long-lived gas such as CO 2 . For a detailed discussion compare also Fuglestvedt et al (2010). Fuglestvedt et al (2008) use emissions from the EDGAR database for 2000 and calculate that, on a 100-year time horizon, the contribution from CO 2 is nearly 400 mW m À2 yr for road transport, compared to about 100 mW m À2 yr for shipping and aviation combined.…”

“…Further assessments give an overview of Metrics for estimating emission impacts at different time scales (Fuglestvedt et al, 2010), aviation impacts on atmosphere and climate and shipping impacts on atmosphere and climate (Eyring et al, 2010).…”

Comparison of physically- and economically-based CO<sub>2</sub>-equivalences for methane

“…Finally, we assume that the RF by CO 2 follows a Gaussian distribution with a standard deviation set to 5 % of the RF value (Forster et al, 2007). In a second step we convert the time profile of RF into a time profile of GMST change, through the integration of a GMST impulse response function, as done in Boucher and Reddy (2008) and Fuglestvedt et al (2010):…”

“…Boucher et al, 2009;Reisinger et al, 2010;Gillett and Matthews, 2010;Reisinger et al, 2011). Only the concept of GTP has recently been gaining some momentum as an alternative (IPCC, 2009;Fuglestvedt et al, 2010).…”

Short/medium-term prediction for the aviation emissions in the en route airspace considering the fluctuation in air traffic demand

“…The chemical and aerosol effects caused by aviation emissions contribute to the RF change, which ultimately results in the climate change [8][9] [10] . Therefore, this paper proposes to address the predicted effect of aviation emissions on climate change, which has not been studied before.…”