Scientists reacted to last weekend’s Paris Agreement on climate change with restrained optimism. The goal set out in the final document was strikingly clear—“holding the increase in the global average temperature to well below 2°C above preindustrial levels and pursuing efforts to limit the temperature increase to 1·5°C”. The Lancet’s recent Commission on Health and Climate Change concluded that “Tackling climate change could be the greatest global health opportunity of the 21st century.” But the final agreement left much room for criticism amid the understandable relief that a final document was agreed at all. There are no deadlines for specific emissions targets. Indeed, there are no specific emissions targets. There is no specified date when emissions should peak. There is no plan for how the world will achieve a 1·5–2°C ceiling in temperature rise. Serious action by countries is not required before 2020 and current national pledges fall far short of what is needed to meet their stated goal. All in all, the Paris Agreement left climate scientists anxious. It was too vague on detail and too much the product of lowest-common-denominator diplomacy.