Noted physicist Stephen Hawking has said that climate change was as great a threat to the world as terrorism and nuclear war, and ought to be tackled urgently.

He was speaking to the group of scientists who run the Doomsday Clock, a countdown to Armageddon that was begun in 1947. The clock's hands were moved two minutes closer to stand at five minutes to midnight to reflect climate change and the nuclear programmes of North Korea and Iran.

"Since Hiroshima and Nagasaki, no nuclear weapons have been used in war, though the world has come uncomfortably close to disaster on more than one occasion. But for good luck, we would all be dead," The Times quoted Prof Hawking as saying.

"As we stand at the brink of a second nuclear age and a period of unprecedented climate change, scientists have a special responsibility once again to inform the public and advise leaders about the perils that humanity faces. We foresee great peril if governments and society do not take action now to render nuclear weapons obsolete and prevent further climate change.

"As scientists, we understand the dangers of nuclear weapons and their devastating effects, and we are learning how human activities and technologies are affecting climate systems in ways that may for ever change life on earth. As citizens of the world, we have a duty to alert the public to the unnecessary risks that we live with every day, and to the perils we foresee if governments and societies do not take action now to render nuclear weapons obsolete and to prevent further climate change," he said.

The Doomsday Clock is operated by the Bulletin of the Atomic Scientists and has been adjusted only 17 times in its 60-year history, most recently in 2002 when it was advanced to seven minutes to midnight after the events of September 11, 2001, and the US withdrawal from the Anti- Ballistic Missile Treaty.

He said the dangers posed by climate change were nearly as dire as those posed by nuclear weapons.

"The effects may be less dramatic in the short term than the destruction that could be wrought by nuclear explosions, but over the next three to four decades climate change could cause irremediable harm to the habitats upon which human societies depend for survival," he said.