Climate forcing from the transport sectors

doi:10.1073/pnas.0702958104

Climate forcing from the transport sectors

*Center for International Climate and Environmental Research–Oslo (CICERO), P.O. Box 1129 Blindern, N-0318 Oslo, Norway; and
^‡Department of Geosciences, University of Oslo, P.O. Box 1022 Blindern, N-0315 Oslo, Norway

Edited by Christopher B. Field, Carnegie Institution of Washington, Stanford, CA, and approved October 5, 2007 (received for review March 30, 2007)

Abstract

Although the transport sector is responsible for a large and growing share of global emissions affecting climate, its overall contribution has not been quantified. We provide a comprehensive analysis of radiative forcing from the road transport, shipping, aviation, and rail subsectors, using both past- and forward-looking perspectives. We find that, since preindustrial times, transport has contributed ≈15% and 31% of the total man-made CO₂ and O₃ forcing, respectively. A forward-looking perspective shows that the current emissions from transport are responsible for ≈16% of the integrated net forcing over 100 years from all current man-made emissions. The dominating contributor to positive forcing (warming) is CO₂, followed by tropospheric O₃. By subsector, road transport is the largest contributor to warming. The transport sector also exerts cooling through reduced methane lifetime and atmospheric aerosol effects. Shipping causes net cooling, except on future time scales of several centuries. Much of the forcing from transport comes from emissions not covered by the Kyoto Protocol.

Footnotes

^†To whom correspondence should be addressed. E-mail: j.s.fuglestvedt{at}cicero.uio.no
Author contributions: J.F., T.B., G.M., and K.R. designed research; J.F., T.B., G.M., K.R., and R.B.S. performed research; J.F., T.B., G.M., K.R., and R.B.S. analyzed data; and J.F., T.B., G.M., K.R., and R.B.S. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0702958104/DC1.
Freely available online through the PNAS open access option.