LONDON — The world must halt greenhouse gas emissions and reverse them within two decades or watch the planet spiralling towards destruction, scientists said on Monday.

Saying that evidence of catastrophic global warming from burning fossil fuels was now incontrovertible, the experts from oceanographers to economists, climatologists and politicians stressed that inaction was unacceptable.

"Climate change is worse than was previously thought and we need to act now," Henry Derwent, special climate change adviser to British Prime Minister Tony Blair, said at the launch of a book of scientific papers on the global climate crisis.

Researcher Rachel Warren from the Tyndall Centre for Climate Change Research, who contributed to the book "Avoiding Dangerous Climate Change", said carbon dioxide emissions had to peak no later than 2025, and painted a picture of rapidly approaching catastrophe.

Global average temperatures were already 0.6 Celsius above pre-industrial levels, and a rise of just 0.4C more would see coral reefs wiped out, flooding in the Himalayas and millions more people facing hunger, she said.

A rise of 3C -- just half of what scientists have warned is possible this century -- would see 400 million people going hungry, entire species being wiped out and killer diseases such as dengue fever reachning pandemic proportions.

"To prevent all of this needs global emissions to peak in 2025 and then come down by 2.6 percent a year," Warren said.

"But even then we would probably face a rise of 2 degrees because of the delay built into the climate system. So we have to start to plan to adapt," she added.

Already the effects of the change are becoming visible, with more extreme weather events and people in coastal areas put at risk from rising sea levels due to melting ice caps.

The first phase of the global Kyoto protocol on cutting greenhouse gas emissions runs until 2012, and negotiations have only just started on finding a way of taking it beyond that.

The United States, the world's biggest polluter, has rejected both the protocol in its current form and any suggestion of expanding or extending it.

Instead it has set up with Australia, India, China, Japan and South Korea the Asia Pacific Partnership on Clean Development.

Source: Reuters