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The U.N.'s environment agency says the cost to poor countries of adjusting to ever-hotter temperatures will be twice or even three times higher than previously thought — and that assumes a best-case scenario in which greenhouse gas emissions are dramatically reduced.

The dispute over who pays the bills for the impacts of global warming has long been the thorniest issue at the U.N. negotiations, now in their 20th round in Lima, Peru.

Rich countries have pledged to help the developing world convert to clean energy and adapt to shifts in global weather.

Earlier reports indicated it would cost poor countries $70 billion to $100 billion to adapt by 2050.

The new U.N. report says the costs likely will be "two to three times higher," possibly as high as $500 billion.