1. Inadvertent Climate Modification (Report of the Study of Man's Impact on Climate) (MIT Press Cambridge MA 1971).
2. Ramanathan V., et al., Rev. Geophys.25, 1441 (1987).
3. The IPCC Working Group I Fourth Assessment Report Summary for Policymakers was released on 2 February 2007 (www.ipcc.ch/SPM2feb07.pdf); the full Working Group I report will be released in May 2007.
4. A cocktail of pollutants not all of them greenhouse gases have contributed to higher levels of ozone—which is a greenhouse gas—in the lower part of the atmosphere indirectly adding a further radiative forcing of 0.35 W m −2 (3). Through similar effects ozone in the stratosphere has been reduced contributing −0.05 W m −2 (3). Human activity has also changed the concentrations of particles in the atmosphere leading to a more uncertain cooling effect estimated to be about −1.2 W m −2 (3). We focus here only on greenhouse gases that are emitted directly into the atmosphere.
5. Fuglestvedt J. S., et al., Clim. Change58, 267 (2003).