Climate change is responsible for the deaths of 300,000 people every year and costs 125 billion dollars (90 billion euros) annually, a new report said on Friday.
The study, from the Global Humanitarian Forum, claims to be the first to measure the impact of climate change on people globally -- and says it is 325 million of the poorest who suffer most.
It highlights the plight of people in Bangladesh, where millions face regular flooding and cyclones, Uganda, where farmers are plagued by drought and some Caribbean and Pacific islands facing obliteration due to rising seas.
This is despite the world's 50 least developed countries contributing less than one percent of global carbon emissions.
Speaking at the report's launch in London, ex UN secretary-general Kofi Annan said it showed the need for a "bold, post-Kyoto agreement to protect the world" at crunch international climate change talks in Copenhagen in December.
"The alternative is mass starvation, mass migration, mass sickness and mass death," Annan, the forum's president, added.
"If political leaders cannot assume responsibility for Copenhagen, they choose instead responsibility for failing humanity."
Annan described climate change as "the greatest emerging humanitarian challenge of our time" and said the world was "at a crossroads" on how to tackle the issue.
The report projects that by 2030, deaths worldwide due to climate change will rise to nearly half a million a year and the cost will hit 300 billion dollars.
It urges developing countries, which account for 99 percent of climate change casualties, to scale up their efforts to adapt for climate change "by a factor of 100."
The vast majority of deaths are caused by gradual environmental degradation which causes problems like malnutrition rather than natural disasters, it said.
The Global Humanitarian Forum was founded in 2007 to work on issues including climate change. The report is also backed by British charity Oxfam.