Pielke et al. correctly point out in their Commentary ‘Dangerous assumptions’ (Nature 452, 531–532; 2008) that almost all of the Intergovernmental Panel on Climate Change (IPCC) emissions scenarios assume continuing improvements in the energy intensity of the global economy and in the carbon intensity of the global energy system, even in the absence of explicit climate policy. This assumption is not hidden, as Pielke et al. imply, and it is not unreasonable.
Over several decades before 2000, that is exactly what happened. Improved manufacturing and buildings, increased efficiency in power production, and increasing economic activity in services and other low-energy activities, all contributed. Especially with historically high energy prices, there is every reason to expect future improvements in the efficiency of energy generation and use, improvements that don't depend on climate policies. Consistent with its mandate, the IPCC developed scenarios that build on historical trends to characterize future possibilities.
Since 2000, the historical patterns have not held. The carbon intensity of the energy system and the energy intensity of the economy have both increased, driven partly by a surge in new electricity from coal, and partly by rapidly accelerating construction and manufacturing, especially in China and India. This trend, if it continues, will greatly increase the challenge of meeting an atmospheric concentration goal of 500 parts per million, or any other level. Pielke et al. are right to emphasize the stark contrast between the pattern before 2000 and that in the past few years.
The trends towards increased carbon and energy intensity may or may not continue. In either case, we need new technologies and strategies for both endogenous and policy-driven intensity improvements. Given recent trends, it is hard to see how, without a massive increase in investment, the requisite number of relevant technologies will be mature and available when we need them.