LOS ANGELES (Reuters) - This is the way the world might end: A genetically engineered pathogen is released, debris from an erupting “supervolcano” blocks the sun or scientists in the biggest "bioerror" of them all accidentally trigger a matter-squeezing "big bang."

The demise of civilization has been predicted since it began, but the odds of keeping Planet Earth alive and well are getting worse amid a breakneck pace of scientific advances, according to Martin Rees, Britain's honorary astronomer royal.

Rees calculates that the odds of an apocalyptic disaster striking Earth have risen to about 50 percent from 20 percent a hundred years ago.

The 60-year-old scientist, author of the recently published “Our Final Hour,” says science is advancing in a far more unpredictable and potentially dangerous pattern than ever before.

He lists as mankind's biggest threats: nuclear terrorism, deadly engineered viruses, rogue machines and genetic engineering that could alter human character. All of those could result from innocent error or the action of a single malevolent individual.

By 2020, an instance of bioterror or bioerror will have killed a million people, Rees contends.

“There is a growing gap between doors that are open and doors that should be open,” Rees, a professor at Britain's Cambridge University, said in a recent interview.

The cosmologist concedes that natural disasters have always loomed—so-called supervolanoes could explode at any time, and asteroids could slam into the planet, causing massive climate changes—but says the most frightening risks are probably man-made.

“A hundred years ago, the nuclear threat wasn't even predicted ... but that threat still hasn't gone away,” he said.

The arms race, after all, was fueled by science, and the field has a responsibility to inform a wide public of the risks in deciding how to apply scientific breakthroughs, he added.

“For the first time ever, human nature itself isn't fixed. Biotech drugs and genetic engineering are empowering individuals more than ever before,” Rees said.

With rapidly advancing DNA technology, “even a single person could cause a disaster,” Rees warned, noting that the United States, after the Sept. 11, 2001, attacks and anthrax scare, is well aware of this threat.