James E. Hansen:

Attack both health and climate threats by focusing on non-CO₂ air pollution first.

James E. Hansen, prominent climate scientist at the NASA Goddard Institute for Space Studies, proposed an alternative scenario for handling the impact of greenhouse gases and human pollutants on climate in a controversial paper in the Proceedings of the National Academy of Sciences¹ His goal was to put forth a companion to the Business-as-Usual scenario, which assumes no special efforts at controlling the greenhouse effect of human gas emissions. He intended to propose a plausible set of actions that would have a modest rather than a large effect on climate.

Hansen proposed that it is possible to limit the extra climate forcing to 1 Watt per square meter (1 W/m²) beyond what now exists, due to additional CO₂. He also asserts that it is possible to have no additional climate forcing from all other gases and pollutants, including greenhouse gases, soot, and aerosols. The effect is shown as Alternative Scenario in the exhibit below. It is like illuminating a one-watt Christmas tree light bulb above every square meter of the Earth's surface.

In contrast, the Business-as-Usual scenario, below would allow 3 W/m² of climate forcing beyond what now exists, of which two units are due to the effects of additional CO₂. Hansen’s alternative would have only one-third the direct effect on climate as the business-as-usual scenario, which would be similar to hanging three light bulbs over every square meter of Earth. Hansen prefers quick action on non-CO₂ gases and slow, deliberate action to reduce CO₂. Referring to non--CO₂ greenhouse gases, he says, These gases are probably the main cause of observed global warming, with methane causing the largest net climate forcing [after CO₂]. The exhibit below, adapted from Hansen's papers, compares the total climate forcing of all greenhouse gases, dust, aerosols, human effects on land surfaces and cloud cover, and natural changes in the strength of the sun and amount of volcanic ash. While carbon dioxide exerts the largest forcing, notice that the other four types of greenhouse gases together cause a forcing that is equal to or greater than CO₂.

While black soot and methane are larger problems than the Intergovernmental Panel on Climate Change (IPCC) has indicated, he says their emissions are easily controlled. Hansen asserts that the warming effect of black carbon soot is 1 W/m², ten times greater than IPCC calculated. On the exhibit below, notice that black carbon has a climatic warming effect roughly equal to that of methane, and these two together have an effect as large as CO₂. Quick action to reduce soot pollution from black carbon should lead to quick results. Of the other air pollutants, ozone has a warming effect on climate, while aerosol particles have a cooling effect - but the Kyoto protocol addresses neither. Methane, a large part of the problem, forces a climate change one-half as large as that caused by CO₂ but has received little attention.

Hansen would like to see immediate action to reduce air pollutants, both to reduce human impacts on climate and to improve human health. Developing nations, particularly, could enlist support for dealing with climate change and air pollution together.

Hansen’s strategy is published on the Web by Natural Science (footnote ²), while his rebuttal to Wuebbles’E arguments appears in Climatic Change (footnote ⁴).

Donald J. Wuebbles:

Hansens’s scenario is too simple to be a strategy

Donald J. Wuebbles, a professor of atmospheric sciences at the University of Illinois, criticized the paper by James Hansen’s scenario (at left) because it overlooks other careful studies of emissions and strategies in the future. Thus, it is inadequate to serve as a strategy. He also underlines that Hansen’s paper has been widely misunderstood by the press, as Hansen himself took pains to document. Wuebbles critique appears in this year’s Climatic Change³

Noting that Hansen seems to suggest that one can balance off the warming effects of CO₂ and the cooling effects of aerosols, Wuebbles added that the net effect of aerosols can either be a warming or a cooling effect in different regions, because of the extremely inhomogeneous way that the particles are distributed. Also, there has been little change in the radiative effects of aerosols over the last 30 years, while the effects of CO₂ have been increasing; clearly there is no balancing of the radiative effect of CO₂ and aerosols.

According to Wuebbles, Hansen did not compare his optimistic scenario with the many published scenarios for managing greenhouse gas effects. Further, he says that Hansen’s proposed increase of1Wm2 in climate forcing is smaller than any proposed in IPCC scenarios and that a zero increase in methane emissions is achievable but difficult.

Glossary

Climate Forcing is a phrase often used in place of radiative forcing. Both phrases refer to the change of net energy entering the Earth and atmosphere, due to some imposed change, such as an increased amount of a greenhouse gas or reduced amounts of dust. However, the temperatures of the Earth and atmosphere are considered to be fixed during this measurement. We consider only the change in radiation energy, including visible light, infrared, and microwave radiation. Net energy means the incoming energy minus the outgoing energy. We measure the change in net energy at jet-stream level and above the lowest, well-mixed layer of the atmosphere, which is called the troposphere.