CORRESPONDENCE/REBUTTAL pubs.acs.org/est
Times Matter!—Response to Wallington et al. ’ TIMES MATTER! In our paper we analyzed the temporal behavior for each transport mode, using the analytical concept of an emission pulse.1 For modes with a high share of short-lived greenhouse gases, like aviation and shipping, the net temperature change of an emission pulse varies strongly in the first years after the emission.2 Wallington et al. do not dispute the science, but find this strong temporal change confusing and unsuitable for policy advice; they suggest time horizons shorter than 20 years be disregarded. We thank them for their comments but for reasons of transparency, functionality, and rate of climate change, we prefer presenting results for time horizons of shorter than 20 years. First, for transparency reasons, we demonstrated the time dependence of the various forcings using pulse emissions and differing metrics. The commentary refers to our use of the emission metric Global Temperature change Potential (GTP) as introduced by Shine et al.3 As it measures temperature change at a chosen point in time, GTP is less sensitive to earlier forcings. Conversely, the Global Warming Potential (GWP) integrates all the forcings of a pulse up to the chosen time; thus the impact from short-lived forcings is carried on far beyond their physical adjustment time.3 This fundamental difference leads to very different weights being assigned to short- or long-lived forcings for the same time horizon.4-6 Therefore time horizons and metrics need to be determined simultaneously. Both determine the relative weights assigned to short- and long-term forcings. Thus, suggesting a time horizon (for a specific metric) involves an implicit value judgment. We do not argue for a specific time horizon, but simply illustrate what a specific choice could mean in terms of weights for the different modes. Indeed, focusing on a short-time horizon only would be misleading, giving short-lived greenhouse gases a higher weight and possibly suggesting mitigation actions be focused on them. As Wallington et al. point out, this could unjustly distract attention from CO2 which determines the warming impact in the long term. However, to disregard climate impacts on time scales shorter than 20 years, as Wallington et al. suggest, could lead to an equally unjustified bias, giving little weight and focus to shortlived greenhouse gases and more to long-lived ones. If mitigation actions consequently focused on the long-term forcing agents without addressing the strong though shorter-lived forcings, important contributors to climate change would simply be overlooked. Second, the pulses can be used to calculate temperature change due to sustained emissions. This can be calculated as integral over successive pulse emissions multiplied by their absolute temperatureR change potential (AGTP) for each comH emi(te) 3 AGTPi(tH-te)dte, where te is ponent i: ΔT(tH)=∑i tte=0 4 time of emission. AGTP values need to be known for all times up to tH. Indeed, for very short-lived forcings like contrails and cirrus clouds, the total temperature change at the distant year tH will be determined by the forcings from the few years just before tH. However, components with lifetimes much longer than the chosen tH, will contribute over the full time span. Therefore, even r 2011 American Chemical Society
with the long-term focus of Wallington et al. the impact (AGTP) of various components at times shorter than 20 years needs to be known. Third, the temperature change of a pulse emission after five years can be considered as a proxy for the rate of climate change. Wallington et al. question this because of its relatively small magnitude compared to the much larger natural variability. This is true for any single-source pulse emission or even for an annual total global emissions pulse, also for longer time horizons. However, transport emissions are an important part of total anthropogenic emissions and likely to remain so for several decades. These seemingly small numbers thus add up and become significant. The Kyoto Protocol and subsequent national communications cited by Wallington et al., cover only six long-lived greenhouse gases using GWP. We went beyond this basket of gases to include components and forcings not covered in the Protocol. For short-lived forcings particularly, the shorter time horizons will have a significant effect. Ignoring shorter times would mean avoiding seeing their impact, even though that may be comparable size-wise to the CO2 impact, as we demonstrated. We concede that including short-lived effects increases uncertainties and agree that the temporal dynamics and trade-off between warming and cooling agents complicates the picture. However, we prefer to more completely and transparently represent a complex reality rather than implicitly conceal it behind some standard definition choices. Cutting out this perspective results in an implicit judgment about the weights assigned to different forcings, for example, in the case of transport emissions, as pointed out in this paper. Jens Borken-Kleefeld,†,§,* Terje Berntsen,‡ and Jan Fuglestvedt‡ †
‡
IIASA—International Institute for Applied Systems Analysis, Schlossplatz 1, 2361 Laxenburg, Austria CICERO—Center for International Climate and Environmental Research-Oslo, P.O. Box 1129 Blindern, 0318 Oslo, Norway
’ AUTHOR INFORMATION Corresponding Author
*Phone: þ43 (2236) 870-570; fax: 870-530; e-mail Borken@ iiasa.ac.at. Notes
Previous address: DLR—Deutsches Zentrum fu.r Luft-und Raumfahrt e.V., Transportation Studies, Rutherfordstrasse 2, 12489 Berlin, Germany.
§
’ REFERENCES (1) Borken-Kleefeld, J.; Fuglestvedt, J.; Berntsen, T. Specific climate impact of passenger and freight transport. Environ. Sci. Technol. 2010, 44 (15), 5700–5706. Published: March 02, 2011 3167
dx.doi.org/10.1021/es200343n | Environ. Sci. Technol. 2011, 45, 3167–3168
Environmental Science & Technology
CORRESPONDENCE/REBUTTAL
(2) Berntsen, T.; Fuglestvedt, J. Global temperature responses to current emissions from the transport sectors. Proc. Natl. Acad. Sci 2008, 105, 19154–19159. (3) Shine, K. P.; et al. Alternatives to the global warming potential for comparing climate impacts of emissions of greenhouse gases. Clim. Change 2005, 68 (3), 281–302. (4) Shine, K. P.; et al. Comparing the climate effect of emissions of short and long lived climate agents. Phil. Trans. R. Soc. A 2007, 365, 1903–1914. (5) Fuglestvedt, J.; et al. Climate forcing from the transport sectors. Proc. Natl. Acad. Sci. 2008, 105, 454–458. (6) Fuglestvedt, J. S.; et al. Transport impacts on atmosphere and climate: Metrics. Atmos. Environ. 2010, 2010 (44), 4648–4677.
3168
dx.doi.org/10.1021/es200343n |Environ. Sci. Technol. 2011, 45, 3167–3168
