Oversimplifying the Greenhouse

A recent paper by James Hansen and colleagues from the NASA Goddard Institute for Space Studies has received a lot of attention from the media and from industrial lobbyists. This paper, entitled ‘Global warming in the twenty-first century: An alternative scenario’ was published in the Proceedings of the National Academy of Sciences in August, 2000. As an example of how the press has interpreted this article, the Daily Telegraph in London cites this paper by stating that ‘the scientist who alerted the world to the consequences of the greenhouse effect admits today that carbon dioxide from burning fossil fuels was not the main cause of rapid warming of the Earth in recent decades’. They also cite the paper for its optimistic viewpoint that ‘global warming can be prevented “without any economically wrenching actions” ’. Similarly misleading is the press release put out by the Global Climate Coalition which has the headline ‘NASA’s Hansen: CO2 not main cause of rapid global warming in recent decades’. While the Hansen et al. paper has clearly been misunderstood by the press, there are several lessons along the way that warrant discussion because they have led to these misinterpretations. The paper by Hansen et al. is divided into two primary sections, one reviewing the past radiative forcing on climate from greenhouse gases, from aerosols, and from other forcings, and the other section proposing a new ‘scenario’ for future forcing that could be met with only a 1 Wm−2 increase in radiative forcing over the next 50 years. Although it could be argued perhaps that such papers appearing in the Proceedings do not warrant much consideration because these papers are not peer reviewed, I think special discussion of this paper is warranted here because of concerns relating directly to potential oversimplifying interpretations that can be made of the issues associated with past changes in climate and projections of future changes in climate. In a simple tabular form, the paper reviews the radiative forcing on climate from 1850 to 2000 in a globally averaged sense. While this sort of analysis has been useful by the international assessments led by the Intergovernmental Panel On Climate Change (IPCC) as well as by others (I’ve used it myself in various papers, e.g., Wuebbles et al., 1999) to give a sense of the relative importance of various forcings on climate over this time period, it is also very misleading, particularly as used by Hansen et al. to suggest that one can balance off the warming effects of carbon dioxide (CO2) and the cooling effects of aerosols because they both come primarily from combustion sources. However, the radiative forcing of CO2 and the other greenhouse gases vary with latitude and season (e.g., Jain et al., 2000) so