Aim for low world warming despite hardship: scientist

LONDON (Reuters) - The world must aim to limit the temperature rise due to global warming to just two degrees Celsius (4 F) despite the near impossibility of achieving it, World Bank Chief Scientist Robert Watson said on Monday.

Scientists say that at atmospheric concentrations of 450 parts per million of carbon dioxide -- the main greenhouse gas from burning fossil fuels -- temperatures will rise by two degrees Celsius. At 550 ppm it will be three degrees or more.

Current levels are already over 400 ppm and rising at around two ppm per year.

"We should aim at the 450 ppm target. Whether we can get to it is another question," Watson told a climate change investment conference in London's financial district. "In practice I don't think we can stabilize at two degrees."

"It is going to take a major change in the way we generate and use electricity to even stabilize between two and three degrees," he added. "The time for action is now."

Research from the charity WWF last year showed that a rise of two degrees would put between 90 and 200 million more people at risk from malaria, while over three degrees the figure shoots above 300 million.

Likewise, a two degree rise puts up to 50 million people at risk from rising sea levels due to melting ice caps, while at three degrees the figure surges to 180 million people.

Last October former World Bank chief economist Nicholas Stern said acting now to cut carbon gas emissions would cost roughly one percent of global economic output, with the cost from delaying rising sharply.

Dimitri Zenghelis, an economic advisor to the British government and part of Stern's team, told the conference organized by the Tyndall Center for Climate Change Research it was already too late to stop at two degrees.

"Three degrees we can do. For two degrees it is too late," he said. "But 550 ppm is a nasty place to be."

Watson, former head of the Intergovernmental Panel on Climate Change which is in the throes of updating its analysis of the global warming threat, said major investments were needed now to save the planet from devastating and irrevocable change.

"We first need to use all the technologies that we already have to get to a low carbon economy," Watson said. "Then we need to invest in the new technologies. We have to mitigate as quickly as possible while also having to adapt."

But governments couldn't do it on their own and private finance was reluctant in the absence of a long-term global regulatory framework, he added.

To help achieve that governments had to come up with a long-term plan to cut emissions and create a stable price for carbon.

At the moment the only global plan is the Kyoto Protocol. But that expires in 2012, was rejected in 2001 by the United States -- the world's biggest polluter -- and is not binding on boom economies China and India.

"We need something long-term, not Kyoto plus five, not Kyoto plus 10, but something much longer," Watson said.

(Additional reporting by Gerard Wynn)