He said major emitters of greenhouse gases could no longer argue they were unaware of the risks after top climate scientists said on Friday they were 90 percent sure humans were to blame for most global warming since 1950.

"Carrying on with business as usual could be viewed as negligent in future," he said. Until now, countries or firms could say there was doubt because the UN climate panel had been just 66 percent sure of a link to human activities.

Lawyers noted it took years for courts to crack down on health dangers such as asbestos and tobacco, even when there was clear scientific evidence of harm.

Greenhouse gases, led by carbon dioxide from burning fossil fuels, mix into the atmosphere. That makes it hard to quantify which percentage of any claimed damage could be reasonably pinned on emissions by any country or company.

A "problem is that it's damage from a diffuse set of sources ... so it's less easy to establish causation," said Vanessa Havard-Williams, a partner at international law firm Linklaters.

SUPREME COURT

Among prominent lawsuits, the US Supreme Court heard a case in November 2006 brought by a dozen states and 13 environmental groups saying greenhouse gases from burning fossil fuels should be regulated by the government.

It is expected to rule by mid-2007. The state of California last year accused six automakers of causing billions of dollars in damages in the state because of greenhouse gases from the tailpipes of their vehicles.

"The California damages case has made people think that (climate lawsuits) are a serious possibility," said Peter Roderick, director of the Climate Justice group, which is linked to environmental group Friends of the Earth.

He said governments were increasingly worried by potential claims for damages. "There is more talk than 2-3 years ago about a 'Compensation Protocol' to the UN Climate Convention," he said.

Sheppard said global warming was unlikely to trigger a slew of lawsuits such as against many tobacco producers, who covered up evidence that smoking kills.

Lung cancer was statistically easier to link to smoking than tying global warming to more storms, droughts or floods since extreme weather occurs naturally.

Most climate cases are civil actions which require merely 51 percent certainty to win -- US law seeks a "preponderance of evidence" and British law a "balance of probabilities".

Roderick said the law was slow to match scientific warnings. In 1898, British factory inspector Lucy Deane wrote a report documenting "evil effects" of asbestos dust because of the "sharp, glass-like jagged nature of the particles", he said.

"Asbestos is an interesting lesson," he said. "It was socially useful because it can save lives by preventing fires. Carbon dioxide is the same -- we need it but too much is harmful."