LONDON, Aug 27 (AlertNet) - Adapting to the effects of climate change such as floods and droughts will probably cost many times more than the United Nations estimates, a report said on Thursday ahead of a major U.N. summit in December.

The U.N. climate change secretariat, UNFCCC, puts the global costs of adaptation, through measures such as growing drought-resistant crops and limiting the spread of diseases, at $40 billion to $170 billion a year until 2030.

The range is so broad because of a large degree of uncertainty over some of the costs.

The estimate has been used at U.N. climate meetings this year in the run-up to the December summit in Copenhagen, whose goal is a new international agreement on how to tackle global warming, the study's authors said.

"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," said Camilla Toulmin, director of the International Institute for Environment and Development.

It co-published the review of the U.N. numbers with the Grantham Institute for Climate Change at Imperial College London.

The report said UNFCCC had produced its numbers too quickly -- "in a matter of weeks" according to the lead author Martin Parry -- and covered the sectors it included only partially.

The authors took six months to update the U.N. estimate, and had it reviewed by seven leading adaptation scientists, including the lead authors of the original U.N. study.

"Just looking in depth at the sectors the UNFCCC did study, we estimate adaptation costs to be two to three times higher, and when you include the sectors the UNFCCC left out the true cost is probably much greater," Parry said in a statement.

"(The costs) are going to be in several hundreds of billions," he later told a news conference, adding that it was difficult to be specific as more studies needed to be done.

The United Nations omitted the costs of adaptation and protection for sectors such as energy, tourism, ecosystems, manufacturing, retailing and mining, the new report said.

The bulk of the adaptation measures and the associated costs will befall developing countries, who are the hardest-hit by global warming, Parry said in an interview.

"No one has estimated that but at a guess it will be at least two thirds (of all costs)," he said.

Poorer nations should be able to draw some of the money from a U.N. Adaptation Fund set up to help them.

The fund is expected to grow from around $80 million in 2009 to about $300 million per year by 2012 -- a paltry sum compared to what developing countries, the United Nations, aid groups and the new report's authors say is necessary.

(Editing by Elizabeth Fullerton)

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