This paper explores a finding that emerged in the early phases of a multidisciplinary project applying population health and psychology to issues of social progress and sustainability. Across 180 countries and half a century of data, the levels of carbon emissions per capita that maximise life expectancy fall within a tight band averaging only 6.6 tonnes—considerably less than wealthier countries emit per person. Various tests fail to break down the curves and, although the authors remain cautious, the stability since 1960 offers implications for the carbon budget leading up to 2050. This is the first time these curves have been contextualised against established climate science, with timely implications for international negotiations on sustainability and development.