An expert panel said that climate change might wipe out the gains in global health over the last 50 years by ramping up heat waves, flood, drought, hunger and disease.
"Climate change is a medical emergency," said Hugh Montgomery, a University College London professor of health who co-chaired a commission set up by The Lancet journal.
Tackling the carbon emissions, which cause the problem, would yield many benefits for health. It would reduce air pollution that causes respiratory disease and cardiac stress, the panel said.
Peripheral measures such as encouraging cycling and walking would also cut deaths from road accidents and rates of obesity and diabetes.
"The implications of climate change for a global population of nine billion people threaten to undermine the last half-century of gains in development and global health," the experts said.
The direct effects of climate change include increased heat stress, floods, drought and increased frequency of intense storms. The indirect effects on health come from urban smog, food insecurity and spread of mosquito-borne disease as insects spread to warmer habitats.
Fixing the source of the problem "could be the greatest global health opportunity of the 21st century," the review suggested.
Many choices are "no-regret options, which lead to direct reduction in the burden of ill health, enhance community resilience, alleviate poverty and address global inequity."
It is the third such warning to be sounded by specialists in less than a last year, coming on the heel of landmark reports by the UN's Intergovernmental Panel on Climate Change (IPCC) and Britain's Royal Society.