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Global warming and the press

September 29, 2000: Mean global surface temperature has increased by approximately half a degree Celsius since 1975 and is expected to keep rising. It is widely believed that this warming trend is mainly a consequence of human activities that raise atmospheric concentrations of greenhouse gases such as carbon dioxide, methane and chlorofluorocarbons. It has generally been assumed that of all human activities, the combustion of fossil fuels, which has raised atmospheric carbon dioxide concentration from the pre-industrial value of 280 parts per million to around 350 parts per million today, has been the chief factor driving climate warming. However, according to an article by James Hansen and others in the August 29 issue of the Proceedings of the National Academy of Sciences, the effect on global temperature of past fossil fuel combustion may have been negligible relative to the effects of anthropogenic increases in greenhouse gases other than carbon dioxide (1).

From this assessment, several major news sources concluded that carbon dioxide has not been a factor in climate warming. Thus, according to a New York Times report, Hansen et al. "conclude that warming seen in recent decades has been caused mainly by other heat trapping gases," i.e., other than carbon dioxide (2). A Washington Post editorial reached the same conclusion, saying "most of the global warming so far observed actually has come from other greenhouse gases" (3), while Canada's National Post wrote of "The recanting this week by James Hansen, the father of climate change theory" and continued "Mr. Hansen, who declared in 1988 that man-made global warming had indubitably arrived, has now released a study indicating that carbon dioxide was not the main culprit."

Climate forcings

In fact, the Hansen et al. study indicates no such thing, as is apparent from the bar graph above, which is based on Figure 1 of the Hansen et al. paper. Not only does the increase in atmospheric carbon dioxide concentration from 1850 to the present represent by far the largest single contributor to climate warming (positive forcing), but its effect is estimated to be equal to that of all other positive greenhouse gas forcings combined. Thus, it is simply false to say that anthropogenic carbon dioxide emissions have not comtributed to climate warming.

That media reports have assumed otherwise apparently stems from a failure to distinguish between the effects on global temperature of rising atmospheric carbon dioxide concentration and of fossil fuel combustion, the latter having multiple effects, both warming and cooling. Hansen et al. do not deny that rising atmospheric carbon dioxide concentration has had a climate warming effect. What they argue is that the warming influence of recent increases in atmospheric carbon dioxide concentration has been largely offset by the cooling effects of fossil fuel burning, in particular the emission of sulfates and other particulates.

But what Hansen et al. propose is strictly a hypothesis or, as they call it, a scenario, concerning which they emphasize two vital reservations. First, that the magnitude of the cooling effect of particulate emissions from fossil fuel combustion is highly uncertain; and second, that particulate emissions are most unlikely to offset the warming effect of future increases in carbon dioxide concentration, the magnitude of which are themselves uncertain, although they will certainly be substantial.

Particulate emissions affect temperature in many ways. Sulfate particles, for example, which are produced as a result of the combustion of sulfur-containing oil, and coal, reflect sunlight to space and promote the formation of clouds, which also reflect incoming solar radiation, thereby cooling the atmosphere. However, black soot particles from the combustion of coal and diesel absorb sunlight, thereby warming the atmosphere, reducing cloud formation and increasing the amount of solar energy reaching the earth's surface. The uncertainty about the effect of particulate emissions on global temperature is indicated by Hansen et al.'s error bar relating to "Forced cloud changes" (see Figure above), which is as large as the estimated value itself.

As to the future, Hansen et al. note that while carbon dioxide emissions will continue to escalate, the emission of atmospheric particulates is already being curtailed and this will continue because particulate emissions now constitute a severe health hazard in areas near major industrial sources in Asia and elsewhere.

Thus, contrary to the National Post's claim, Hansen et al. confirm that measured climate warming in recent decades is consistent with climate change theory and that, of all factors contributing during the last 150 years to a positive planetary energy balance, the increase in atmospheric carbon dioxide has been by far the largest. Further, they show that during the next 50 years, rising atmospheric carbon dioxide concentration will continue to be the largest single climate warming factor.

The contribution that the Hansen et al. paper makes to the policy debate on global warming is the argument that, during the next 25 to 50 years, action to limit anthropogenic emissions of heat-absorbing carbon particles and greenhouse gases other than carbon dioxide is likely to provide a more cost-effective approach to the control of climate warming than massive reductions in fossil fuel use. Thus they state "A key feature of this strategy is its focus on air pollution, especially aerosols and tropospheric (ground level) ozone, which have human health and ecological impacts."

Concerning carbon dioxide emissions they say "Business-as-usual scenarios understate the potential for carbon dioxide emission reductions" and "Based on this potential [i.e., for reducing carbon dioxide emissions] and current carbon dioxide growth trends, we argue that limiting the carbon dioxide forcing increase to 1 Watt per square meter [indicative, all other things being equal, of a global temperature rise of approximately 0.75 degrees Celsius] in the next 50 years is plausible."

Nevertheless, Hansen et al. emphasize the importance of ensuring lower growth in carbon dioxide emissions. Thus they say "achieving the level of [carbon dioxide] emissions needed to slow climate change significantly is likely to require policies that encourage technological developments to accelerate energy efficiency and decarbonation trends."

If this story has a moral, it is that the near-paranoia of many in the scientific community about media presentation of science is largely justified. Fortunately, the Internet now provides an antidote to media misinformation, by providing the public with direct access to the source of many scientific news stories. A link to the full text of the Hansen et al. paper is provided below.

References

(1) James Hansen, James, Makiko Sato, Reto Ruedy, Andrew Lacis, and Valdar Oinas. 2000. Global warming in the twenty-first century: An alternative scenario. Proc. Natl. Acad. Sci. USA 97:9875-9880.

(2) Revkin, Andrew C. 2000. Study proposes new strategy to stem global warming. The New York Times, Saturday, August 19.

(3) Editorial. 2000. Hot news on warming. Washington Post, Monday, August 28.

(4) Peter Foster. 2000. Global warming: the cloud thickens. The National Post, Saturday, August 19.

Related articles

Links to other naturalSCIENCE articles relating to global warming.

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