This paper uses data for 134 countries for the period 1960–2010 to document an energy ladder that nations ascend as their economies develop. On average, economic development results in an overall substitution from the use of biomass to energy sourced from fossil fuels, and then increasingly towards nuclear power and certain low-carbon modern renewables such as wind power. The process results in the carbon intensity of energy evolving in an inverse-U manner as per capita incomes increase. Fossil fuel-poor countries climb more quickly to the low-carbon upper rungs of the national-level energy ladder and so typically experience larger reductions in the carbon intensity of energy as they develop. Leapfrogging to low-carbon energy sources on the upper rungs of the national-level energy ladder is one route via which developing countries can reduce the magnitudes of their expected upswings in carbon dioxide emissions.

The June 2012 issue of Environment and Development Economics published a symposium with considerable focus on our paper, ‘Sustainability and the measurement of wealth’. The Symposium also contained five articles in which other researchers offered valuable comments on our paper. The present note replies to those comments. It clarifies important issues and reveals how important questions relating to sustainability analysis can be fruitfully addressed within our framework. These include questions about the treatment of time, the use of shadow prices and the treatment of transnational externalities. This note also offers new theoretical results that help substantiate our earlier empirical finding that the value of human health is something very different from the value of the consumption permitted by health and survival.