Three hundred thousand people are already dying every year as a result of global warming, according to the most comprehensive report ever on the human impact of climate change.
The report by the Global Humanitarian Forum, set up by Kofi Annan, the former head of the United Nations, looked at the latest information on changing weather patterns and how they are affecting humans in terms of spreading disease and causing starvation and even conflict.
It found 325 million people are already seriously affected by climate change through damage to crops, homes and livelihoods at a total economic cost of $125 billion (£78 billion) every year.
Heatwaves, floods and forest fires caused by rising temperatures will be responsible for half a million deaths annually by 2030 and could impose a $300 billion (£188 billion) drag on the world economy.
Most of the deaths will be in developing countries like Bangladesh or Sudan that are vulnerable to flooding or drought. However developed countries will also see an impact with an increase in deadly phenomena like the heatwave that struck Europe in 2003.
In addition to the direct effects of global warming, changes to the world's climate could also spark conflict over scarce resources, for example water.
Mr Annan said the report was a "clarion call" to world leaders to negotiate an international deal on climate change in Copenhagen at the end of this year that keeps temperature rises below 2 degrees C (3.6 degrees F).
"Climate change is silent human crisis. Yet it is the greatest emerging humanitarian challenge of our time. Already today, it causes suffering to hundreds of millions of people, most of whom are not even aware that they are victims of climate change. We need an international agreement to contain climate change and reduce its widespread suffering," he said.
Barbara Stocking, Chief Executive of Oxfam Great Britain, said the suffering is mostly in developing countries.
"The world's poorest are hardest hit by climate change but they have done the least to cause it," she said.