The relationship between national carbon dioxide emissions intensity (CO 2 emitted per unit of Gross Domestic Product) and level of economic development has changed from essentially linear in 1962 to strongly curvilinear in 1991. The inverted-U curve reached statistical significance briefly in the early 1970s and increasingly since 1982. This is the result not of groups of countries passing through stages of development, but of efficiency improvements in a small number of wealthy countries combined with worse performance in poor and middle-income countries. The curvilinear relation is deepening and is likely to persist due to constraints on poorer countries in the world economy.