It's been a good week for environmentalists. Al Gore is headed for Hollywood's red carpet thanks to an Oscar nomination for "An Inconvenient Truth", a big-screen adaptation of his slide-show lecture on climate change. And even U.S. President George W. Bush, long known as a friend to the oil industry, has been waxing green. In his State of the Union speech yesterday, he urged Americans to cut their gasoline use by 20 percent over the next decade and called for tighter vehicle fuel efficiency standards.

Now David Cameron, head of Britain's opposition Conservative party, is set to lecture leaders at the World Economic Forum in Davos on the security risks associated with global warming. As chief of a party that has never been much known for its environmental credentials, Cameron will warn that a hotter world promises to be an incubator for violence and danger.

Cameron rehearsed his theme in an op-ed piece in Britain's Financial Times today, where he cited an Oxfam prediction that global warming could put 30 million people at risk of famine and disease.

"The demand for essential resources could exacerbate tensions within countries," he writes. "We are already seeing this: a contributing factor to the conflict in Darfur has been a change in rainfall that pitted nomadic herders against settled farmers. Such conflicts over resources within countries could easily turn into conflicts between countries - either directly as clashes between governments over a resource such as a shared river or indirectly through the pressure of refugees crossing borders."

This was a topic touched on in London today at a conference entitled "Climate Change - The Global Security Impact" and hosted by the Royal United Services Institute for Defence and Security Studies.

Paul Rogers, a professor of peace studies at Bradford University, said recent climatology work suggested global warming could increase migratory pressures by an order of magnitude - "in other words, about 400 million people, not the current 40 million people, desperate to cross borders".

According to Rogers, global economic growth has already widened socio-economic divisions between the haves and the have-nots, with 85 percent of the world's wealth in the hands of just 20 percent of the population.

"Now put that together with climate change," he said. "If in fact you find that in broad terms the ecological carrying capacity of the tropical and sub-tropical regions (of the planet) tend to decline over the next 20 to 70 years, what you have is a divided world, with the countries that can least cope with those changes being the countries where most of the marginalised people already are."

If you accept this thesis, then climate change spells trouble for rich and poor countries alike - and not only because competition for dwindling resources is likely to fuel localised conflict.

"The implications of that for security are massive because we live in a fully integrated world," Rogers concluded. "In fact 9/11 showed that in many different ways...The international security dimension is the combination of two things: the socio-economic divide and the increasing environmental constraints as they affect the marginalised majority."