Climate scientists, used to dealing with sceptics, are under siege like never before, targeted by hate emails brimming with abuse and accusations of fabricating global warming data.

Some emails contain thinly veiled death threats.

Across the Internet, climate blogs are no less venomous, underscoring the surge in abuse over the past six months triggered by purported evidence that global warming is either a hoax or the threat from a warmer world is grossly overstated.

A major source of the anger is from companies with a vested interest in fighting green legislation that might curtail their activities or make their operations more costly.

"The attacks against climate science represent the most highly coordinated, heavily financed, attack against science that we have ever witnessed," said climate scientist Michael Mann, from Pennsylvania State University in the US.

"The evidence for the reality of human-caused climate change gets stronger with each additional year," Mann told Reuters in emailed responses to questions.

Greenpeace and other groups say that some energy companies are giving millions to groups that oppose climate change science because of concerns about the multi-billion dollar costs associated with carbon trading schemes and clean energy policies.

For example, rich nations including the US, Japan and Australia, are looking to introduce emissions caps and a regulated market for trading those emissions.

More broadly, the United Nations is trying to seal a tougher climate accord to curb emissions from burning fossil fuels and deforestation blamed for heating up the planet.

Other opponents are drawn into the debate by deep concerns that governments will trample on freedoms or expand their powers as they try to tackle greenhouse gas emissions and minimise the impacts of higher temperatures.

"There are two kinds of opponents -- one is the fossil fuel lobby. So you have a trillion-dollar industry that's protecting market share," said Stephen Schneider of Stanford University in California, referring to the oil industry's long history of funding climate sceptic groups and think tanks.

"And then you have the ideologues who have a deep hatred of government involvement," said Schneider, a veteran climate scientist and author of the book "Science as a contact sport".

The result is a potent mix that has given the debate a quasi-religious tone with some climate critics coming from the right-wing fringe and making arguments as emotive as those raised in the abortion and creationism debates in the US.

The debate has largely become drawn along political lines, at least in the US, where opponents in the Republican Party question climate science and raise doubts over the need to implement greener policies such as those espoused by climate change campaigner and former Vice President, Al Gore.

In a party conference in April, Republican firebrand Sarah Palin, a potential 2012 presidential nominee, mocked what she called the "snake-oil-based, global warming, Gore-gate" crowd.

The green lobby is also to blame. Exaggerations by some green interest groups, which have at times over-played the immediacy of the problem to bring about a groundswell of support for a new UN climate treaty and green policies, have given sceptics plenty of ammunition.

Sceptics also point to admissions in a 2007 report by the UN's Intergovernmental Panel of Climate Change that there is a 10 percent chance global warming is part of a natural cycle.

The same report says there's a 90 percent probability that climate change is due to human activities led by burning fossil fuels. Nevertheless, the sceptics demand 100 percent certainty, something that researchers say is impossible.

"This time it's different"

Scientists and conservationists say some anti-climate change lobbyists are funded by energy giants such as ExxonMobil, which has a long history of donating money to interest groups that challenge climate science.

According to a Greenpeace report released last month, ExxonMobil gave nearly $9 million to entities linked to the climate denialist camp between 2005 and 2008.

The report, using mandatory SEC reporting on charitable contributions, also shows that foundations linked to Kansas-based Koch Industries, a privately owned petrochemical and chemicals giant, gave nearly $25 million.

Koch said the Greenpeace report mischaracterised the company's efforts. "We've strived to encourage an intellectually honest debate on the scientific basis for claims of harm from greenhouse gases," the company said in a note on its website.

ExxonMobil makes no secret of funding a range of groups, but says it has also discontinued contributions to several public policy research groups.

"We contribute to an array of public policy organisations that research and promote discussion on climate change and other domestic and international issues," the company says on its website.

Stanford's Schneider has dealt with sceptics for years. But this time, he says, it's different.

"I don't see it stopping," said Schneider by telephone. "I see it intensifying. The ugliness is what's new."

One of the thinly veiled death threats that Schneider has received says: "You communistic dupe of the UN who wants to impose world government on us and take away American freedom of religion and economy -- you are a traitor to the US, belong in jail and should be executed."    

Hacked emails

Scientists say there is a wealth of data showing the planet is warming, that it's being triggered by rising levels of greenhouse gases such as carbon dioxide and that man is to blame.

Sceptics counter this by saying that rising CO2 levels is natural and harmless and that it's impossible for mankind to influence the way the planet functions. Others play up doubts or errors in some scientific studies to undermine it all.

Many also say warming has stalled, pointing to the recent burst of cold weather in the Northern Hemisphere as evidence of global cooling, even though satellite data show that, overall, Nov. 2009 to Jan. 2010 was the warmest Jan-Nov the world has seen since satellite temperature data began in 1979.

Then came the release of emails hacked late last year from a British climate research unit.

The "climategate" emails, totalling more than 1,000, were stolen from the University of East Anglia's Climate Research Unit (CRU), and involve correspondence between director Phil Jones and other leading climate scientists, including Schneider and Mann.

The emails led to allegations the scientists fudged data to bolster the case for mankind causing global warming, setting off a surge of criticism across the Internet accusing climate scientists of a massive hoax.

"This whole thing has gone viral on the Internet," said Cindy Baxter of Greenpeace, author of a recent report "Dealing in Doubt: The Climate Denial Industry and Climate Science."

"You've got all those voices out there on the blogosphere who are then picked up and echoed," she told Reuters.

The University of East Anglia has been a particular target.

"There have been an awful lot of abusive emails since 'climategate' broke," said university spokesman Simon Dunford.

Sceptics were accused of very selectively choosing only a small number of the hacked emails and taking comments out of context to misrepresent the scientists' meaning.

A British government inquiry cleared Jones of any wrongdoing, but said CRU was wrong to withhold information from sceptics.

Mann, who was accused of falsifying data, was cleared of any wrongdoing by an internal investigation by Penn State University. – Reuters