Human-induced climate change is now the central health issue facing humanity. The World Medical Association recently adopted the Declaration of Delhi, committing the medical profession to mitigate and adapt to the effects of climate change. This is new professional territory for many doctors. Even so, the profession has often engaged with issues outside 'the health sector' when the stakes are high, for example leaded petrol, road safety, tobacco, and nuclear weapons. The scientific basis to the declaration merits scrutiny in light of commonly used contrary arguments. Decisions in medicine, as elsewhere, must be taken on the evidence to hand, weighing up the risks, given that complete knowledge is seldom available and time is precious. There are strong analogies between clinical experience and our approach to planetary climate. The relevant context for scientific observations on climate is the world's multi-gigatonne annual CO2-equivalent greenhouse gas emissions. Emissions drive changes in concentrations of greenhouse gases, which matters when they are rapid or prolonged. The current variation in global temperature is alarming, even when within 'normal range'. Climate models inform and guide present-day decision-making, and perform well in explaining observed warming. They corroborate other evidence that tells us that CO2 and other greenhouse gases are harmful at current atmospheric concentrations. As a profession and as global citizens, we need to move beyond dissent and denial about anthropogenic climate change. The WMA correctly says that circumstances now require us all to take action.