Paying landowners to let forests grow is promoted by the UN as a viable way to fight global warming, but experts first have to puzzle out how to insure trees against going up in smoke. Under UN plans, owners will get carbon credits to slow the destruction of tropical forests. But fires caused by lightning - along with other hazards such as storms, insects and illegal logging - are a big risk for insurers and investors.

A new UN climate treaty to include granting forest owners tradeable carbon credits will be discussed by about 190 nations in Poznan, Poland, from Monday to December 12. The credits could be worth billions of dollars for those agreeing not to cut down trees.

Burning forests to clear land for farming releases about a fifth of all the greenhouse gases blamed for causing climate change. If trees die, the carbon stored as they grew would be released, rendering carbon credits worthless.

"From a formal point of view insurance shouldn't be a problem," said Wojciech Galinsky, who works on UN projects to promote green investment in developing countries. "If Tina Turner's legs can be insured, why not forests?"

But there is wide disagreement on how to assess the risks under the new UN treaty, due to be agreed by end of next year.

Forest owners want full access to credits as fast as possible. But insurers suggest that half be retained in buffer funds in case forests vanish in a few decades. If a forest disappeared, the credits in the funds would go to them.

"How much land-managers will see of the price is what the excitement is about," said Frances Seymour, head of the Centre for International Forestry Research in Indonesia. Placing a value on forests could give developing nations in Africa, Latin America and Asia a big incentive to do more to slow rising greenhouse gas emissions. But the economic slowdown may make rich nations reluctant to take part.