By Richard Black
Environment correspondent, BBC News website
The authors believe climate change will increase rates of malnutrition
Failure to agree a new UN climate deal in December will bring a "global health catastrophe", say 18 of the world's professional medical organisations.
Writing in The Lancet and the British Medical Journal, they urge doctors to "take a lead" on the climate issue.
In a separate editorial, the journals say that people in poor tropical nations will suffer the worst impacts.
They argue that curbing climate change would have other benefits such as more healthy diets and cleaner air.
December's UN summit, to be held in Copenhagen, is due to agree a new global climate treaty to supplant the Kyoto Protocol.
But preparatory talks have been plagued by lack of agreement on how much to cut greenhouse gas emissions and how to finance climate protection for the poorest countries.
"There is a real danger that politicians will be indecisive, especially in such turbulent economic times as these," according to the letter signed by leaders of 18 colleges of medicine and other medical disciplines across the world.
"Should their response be weak, the results for international health could be catastrophic."
Earlier in the year, The Lancet, together with University College London (UCL), published a major review on the health impacts of climate change.
Some of the headline findings were that rising temperatures are likely to increase transmission of many infectious diseases, reduce supplies of food and clean water in developing countries, and raise the number of people dying from heat-related conditions in temperate regions.
Changing fuel can improve women's lives as well as curbing emissions
But it also acknowledged some huge gaps in research - for example, that "almost no reliable data for heatwave-induced mortality exist in Africa or south Asia".
Nevertheless, the main conclusion was that in a world likely to have three billion new inhabitants by the second half of this century: "Effects of climate change on health will affect most populations in the next decades and put the lives and wellbeing of billions of people at increased risk".
The current Lancet and BMJ editorial that accompanies the letter from doctors' organisations argues that climate change strengthens the cases that health and development charities are already championing.
"Even without climate change, the case for clean power, electric cars, saving forests, energy efficiency, and new agriculture technology is strong.
"Climate change makes it unanswerable."
Written by Lord Michael Jay, who chairs the health charity Merlin, and Professor Michael Marmot of UCL, the editorial argues that there are plenty of "win-win solutions" available.
"A low-carbon economy will mean less pollution. A low carbon-diet (especially eating less meat) and more exercise will mean less cancer, obesity, diabetes, and heart disease.
"Opportunity, surely, not cost."