In 2001, the Intergovernmental Panel on Climate Change (IPCC) issued its Third Assessment Report (TAR)¹ on the prospects for and likely impact of increases in global average temperature over the next century. The summary report of Working Group (WG) I,² on science, widened the range of likely temperature increase, compared with the IPCC's Second Assessment Report (SAR)³ of five years earlier, to 1.4-5.8 degrees Celsius ([degree] C), with the increase in the upper end of the range receiving wide public attention. The summary report of WG II,⁴ on impacts, sketched a somber picture of how both human settlements and nonhuman ecologies might be adversely affected by the rise in temperature and an accompanying rise in sea level.

A close reading of the WG I report, however, reveals that the wider range, and in particular the increase in the upper end of the range, was not at all due to a reassessment of the scientific evidence accumulated and closely studied since the mid-1990s. Rather, it was due to a change in the way that emissions over the next century of greenhouse gases (GHGs), mainly carbon dioxide (CO₂) from fossil fuel consumption and deforestation, and methane (CH[4]) from agriculture and waste disposal, were characterized. Instead of the single "business as usual" emissions trajectory used in the SAR, the TAR produces six different scenarios, depending on the evolution both of the world economy and of its energy system over the next 100 years. This refinement is in some respects an improvement over the single assumed trajectory, but to present the outlook as likely more serious than the previous assessment is, to say the least, misleading.