While there would be some positive effects, "the balance of health effects is on the negative side," Alistair Woodward, a professor at the University of Auckland, told a regional meeting of the World Health Organisation.
Woodward was a lead writer for the fourth assessment report of the Inter-Governmental Panel on Climate Change.
Giving examples in a speech, he said that in China's Jiangsu province the winter freezing zone has moved northwards. The water snail that transmits schistosomiasis had also shifted northwards, putting perhaps 20 million people at risk of the parasitic disease also known as bilharziasis.
In France extreme heat in August 2003 led to about 25,000 deaths. In the WHO's Western Pacific region, a heat wave in summer 1998 increased mortality in Shanghai threefold.
Globally, said Woodward, the largest effect would be under-nutrition. "There will be some winners and losers, but overall, climate change is expected to have a negative effect on food production."
In the Western Pacific, changes in temperature and rainfall would make it far harder to control dengue fever, he said.
"Empirical modelling suggests the climate that is likely to apply in 2085 will put an extra billion people at risk of dengue worldwide, and perhaps half that number will be in this region."
Water supplies would be an increasingly serious concern, with the percentage of the world's land area suffering drought increasing perhaps tenfold by the end of the century.
Small Pacific island states would be especially vulnerable to rising sea levels and changes in rainfall patterns.
Woodward said the health sector must be at the forefront on climate change.
He called for studies on water management in low-lying Pacific islands, community-based disaster preparedness, and on efforts to reduce the impact of rural drought.
"The most difficult change of all is a change of will. We should not be daunted by the size of the task," Woodward said.
WHO director general Margaret Chan, in a speech Monday afternoon, said that even if greenhouse gas emissions were to stop immediately the changes already being seen would go on throughout this century.
"Climate changes will affect, in profoundly adverse ways, some of the most fundamental determinants of health: food, air, water," she said.
"Developing countries will be the first and hardest hit. Subsistence agriculture will suffer the most. Areas with weak health infrastructures will be the least able to cope."