By Richard Black
Environment correspondent, BBC News website
Some fear that a shortfall in the Asian monsoon is linked to climate change
Protecting societies against the impacts of climate change will be much more expensive than previously believed, according to a new analysis.
In 2007, the UN climate convention came up with a sum of $49-171bn per year.
The new report says the UN sums omitted important factors and the true cost will be two to three times higher.
Developing nations want rich countries to provide major sums for adaptation as part of the new UN climate deal due to be agreed in Copenhagen in December.
"The amount of money on the table at Copenhagen is one of the key factors that will determine whether we achieve a climate change agreement," said lead author Martin Parry, a visiting research fellow with the Grantham Institute for Climate Change at Imperial College London.
"But previous estimates of adaptation costs have substantially misjudged the scale of funds needed."
Professor Parry co-chaired the Intergovernmental Panel on Climate Change (IPCC) working group on climate impacts for its 2007 assessment.
The new report - issued under the aegis of the International Institute for Environment and Development (IIED) and the Grantham Institute - says that some aspects of the UN estimates were wrong by a factor of more than 100.
Call to caution
The UN climate convention (UNFCCC) made its assessment in 2007 after the IPCC concluded the task was too difficult; and Professor Parry suggested it had been done in a hurry, with some vital caveats ignored in subsequent deliberations.
A spokesman for the UNFCCC defended the process but said the organisation had a responsibility to be conservative.
The authors recall that Africa already has a "development gap"
"Looking at what was out there in 2007, these were best estimates, which we simply collected; and we had to err on the side of caution," he said.
Recently the UNFCCC's executive secretary Yvo de Boer has tended to use the figure of $100bn (£60bn) per year.
Professor Parry told BBC News that a key point of the new report, Assessing the Costs of Adaptation to Climate Change, was that it included "bottom-up" as well as "top-down" analyses.
"One study shows the cost of adapting a single watershed in China - that's one of the few case studies that's been done - comes in at a billion dollars a year," he said.
"So when you start adding up the various figures you soon start to exceed the global number (in the UN's analysis)."
He said UNFCCC calculations had taken into account only half of the extra disease burden due to emerge from climate change, assumed low levels of future development in Africa (so giving less infrastructure to protect), used low estimates for sea level rise and had not included the economic costs of nature loss.
Mr de Boer told BBC News he was happy that thinking and understanding on the issue was evolving.
"For me, the priority is not to try and determine with scientific precision exactly how much is going to be needed to cover the cost of adaptation in 2030," he said.
"The point is to put in place a robust architecture for raising adaptation funds and allocating them over time."
Securing funds for climate protection, or adaptation, is a key priority for developing countries in the run-up to the Copenhagen conference, which is supposed to secure a new global agreement supplanting the Kyoto Protocol.
Earlier this week the African Union suggested it would be asking for $67bn (£40bn) per year for Africa alone.
"Finance is the key that will unlock the negotiations in Copenhagen," said IIED director Camilla Toulmin.
"But if governments are working with the wrong numbers, we could end up with a false deal that fails to cover the costs of adaptation to climate change."
Most is expected to be raised through levies on carbon trading, but Mr de Boer believes developed countries will need to pledge an up-front sums in the region of $10bn to enable poorer nations to begin assessing their needs and priorities.
Further analyses from the World Bank and management consultants McKinsey and Company are due out before the Copenhagen talks.