Planning for future energy resources.

WE AGREE WITH M. I. HOFFERT ET AL. (“Advanced technology paths to global climate stability: energy for a greenhouse planet,” Review, 1 Nov., p. 981) that stabilizing atmospheric CO2 concentrations at 550 parts per million (ppm) or below will require investment in energy research and development well in excess of current levels. However, their conclusion—that known technological options are not up to the task— suffers from two shortcomings related to how much decarbonization is required and how soon we need it. First, they do not consider uncertainty in future energy demand, basing their analysis on a single reference scenario (1). In contrast, the most recent Intergovernmental Panel on Climate Change (IPCC) report on emissions scenarios (2) foresees a wide range of plausible development paths leading to global primary power demand of anywhere from 20 to 50 TW by 2050. Relative to these scenarios, as quantified by six different integrated assessment modeling teams, stabilizing at 550 ppm may not require any additional energy from carbon-free technologies over the next 50 years beyond that produced by known technologies for reasons unrelated to climate change. Or it could require that additional zero-carbon generating capacity deliver nearly 600 TWyears of energy over that same period. Policy responses to climate change should be robust across this wide range of uncertainty. Second, we doubt whether the development and implementation of the radically new technologies such as fusion or solar power satellites advocated in the article are feasible within the time horizon necessary for CO2 stabilization. The process from invention, to demonstration projects, to significant market shares typically takes between five and seven decades (3). Fundamentally new technologies that have not been demonstrated to be feasible even on a laboratory scale today would therefore likely come much too late to contribute to the emissions reductions necessary by 2050, particularly for stabilization at 450 ppmv or below (4). We believe that the appropriate mix of investments must include an initial focus on technologies with proven feasibility if we are to embark on a path to stabilization. At the same time, we should begin to explore new energy sources that might then be available in the long term to finish the job. BRIAN O’NEILL, ARNULF GRÜBLER, NEBOJSA NAKICENOVIC, MICHAEL OBERSTEINER, KEYWAN RIAHI, LEO SCHRATTENHOLZER, FERENC TOTH International Institute for Applied Systems Analysis, Schlossplatz 1,A-2361 Laxenburg,Austria.