In their Commentary ‘Dangerous assumptions’ (Nature 452, 531–532; 2008), Roger Pielke and colleagues argue that the Intergovernmental Panel on Climate Change (IPCC) “seriously” underestimates the scale of the technological changes required to stabilize greenhouse-gas concentrations, and hence conveyed an inappropriate message to policy-makers on policies required for mitigation. We believe that this argument is based on a flawed analysis.

The authors repeat a 'thought experiment' done by the IPCC, to find out what would happen over the next century if technology were frozen at present levels. In that case, the emission reductions required to reach any of the assessed greenhouse-gas stabilization levels would far exceed those computed by any of the mitigation scenarios reviewed in the IPCC fourth assessment report (AR4).

It is most unlikely, however, that technology will be frozen. Over the past 30 years, the decrease in energy intensity has been 1.1% a year — well above the 0.6% a year assumed in 75% of the energy scenarios assessed by the IPCC.

Developments in China since 2000 do raise concerns that the rate of decrease in energy and carbon intensity could slow down, or even be reversed. However, similar short-term slow-downs in technical progress have occurred in the past, only for periods of more rapid development to compensate for them. India, for example, does not show the decreasing trend in energy efficiency seen in China.

The increase in fossil-resource prices triggered by high economic growth will lead to an increase in energy efficiency. Admittedly, a possible increase in carbon intensity caused by a renaissance of coal is a worst-case scenario for any climate policy. But the impact of increasing fossil-fuel prices on technological change and on mitigation costs, or policies, cannot be analysed in any meaningful way for policy-makers by assuming a 'frozen technology' scenario.

The IPCC's main policy conclusions stand: present technologies can stop the rise in global emissions. But they will depend on governments' policies to ensure that the technologies reach the market in time. A carbon price on emissions will promote investment in lower-carbon technologies, and climate policies will stimulate technological development to bring emissions and mitigation costs down further.

See also:

Long-range energy forecasts are no more than fairy tales

Energy assumptions were reasonable at the time, but not now

Future scenarios for emissions need continual adjustment

IPCC’s climate-policy assumptions were justified