A Brighter Future

Contrary to Wuebbles’ thesis (2002), most of the media did not misunderstand the thrust of our recent paper (Hansen et al., 2000). We do indeed assert that a scenario is feasible in which the rate of global warming declines. We also posit that, with an understanding of the significant climate forcings, it is possible to achieve such a climatically brighter path with actions that are not ‘economically wrenching’, indeed, actions that make economic sense independent of global warming. Our paper does not denigrate the ‘business-as-usual’ (BAU) scenario that has been popular in global climate model simulations. The BAU scenario provides a valuable warning of potential climate change if the world follows a path with climate forcings growing more and more rapidly. Our aim was to present a companion scenario that stimulates discussion of actions that help avoid a gloom and doom scenario. I tried to clarify our objectives in an ‘Open Letter’, which is made available from Natural Science (a web publication) at http://naturalscience.com/ns_let25.html. I summarize here key points of discussion. Black Carbon (BC). One of our assertions is that BC (soot) plays a greater role in climate change than has been appreciated. We believe that the forcing due to BC is of the order of 1 W/m2, rather than of the order of 0.1 W/m2, as assumed by IPCC (1996). My present estimate for global climate forcings caused by BC is: (1) 0.4 ± 0.2 W/m2 direct effect, (2) 0.3 ± 0.3 W/m2 semi-direct effect (reduction of lowlevel clouds due to BC heating; Hansen et al., 1997), (3) 0.1 ± 0.05 W/m2 ‘dirty clouds’ due to BC droplet nuclei, (4) 0.2 ± 0.1 W/m2 snow and ice darkening due to BC deposition. These estimates will be discussed in a paper in preparation. The uncertainty estimates are subjective. The net BC forcing implied is 1 ± 0.5 W/m2. Air Pollution. Aerosols and tropospheric ozone (O3) are not addressed by the Kyoto protocol. They should be. A reason proffered for excluding ozone is that its chemistry is so complex that ‘most scientists’ eyes glaze over’ (Revkin, 2000). Perhaps the latter assertion is true. But it is not adequate reason to exclude air pollution from international climate negotiations. Our estimated anthropogenic global climate forcing due to BC (1 W/m2) and O3 (0.4 W/m2) is comparable to the CO2 forcing (1.4 W/m2). One thesis in our paper is that halting the growth of air pollution can make a significant contribution to slowing global warming.