The scientist Stephen Hawking issued a warning this week that the world is on the brink of a second nuclear age and a period of unprecedented climate change.

The University of Cambridge mathematician's comments came as the time on the doomsday clock, which counts down to nuclear Armageddon, was moved two minutes closer to midnight, reflecting concerns among scientists over the rise of new nuclear powers.

"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," said Professor Hawking. "But for good luck, we would all be dead.