Acclaimed British astrophysicist warns humanity has only a 50 percent chance of surviving the 21st Century

One of the world's leading scientists says technology gives a handful of bad actors the power to do serious harm on a global scale -- and he gives humanity a 50-50 chance of surviving the 21st Century.

Martin Rees, Britain's astronomer royal and a Cambridge University cosmologist, has written a new book called "Future: Prospects for Humanity" that gives reason for optimism about humankind's chances for survival, but he told Vox that his hopes rely heavily on a series of good decisions getting made.

"I’m an optimist in that I believe that the ability of technology to provide a good life for everyone, not just in our countries, but throughout the world, is going to grow," Rees said. "But I’m also an ethical pessimist in that I recognize that this is not happening in the way that it should."

His previous book, 2003's "Out Final Hour," explored all the ways humans could destroy themselves through technology, and he said the rapid explosion of technology with little regard for ethics has made him even more concerned about societal disruption triggered by small groups or individuals.

"This is a relatively new thing, and I’m not sure we fully appreciate the dangers," Rees told Vox. "Technology has not only increased the ways we could destroy ourselves, it’s also made it much easier for us to do it. So that means we’re always close, potentially, to a global disaster."

He said the demand for energy and other resources was unsustainable, but he said individual bad actors pose a potentially greater threat in the short run.

"I worry about the disruptive effects of cyber attacks or some form of biological terror, like the intentional release of a deadly virus," he said. "These kind of events can happen right now, and they can be carried out by small groups or even an individual. It’s extremely hard to guard against this kind of threat."

Those threats are potentially catastrophic, but Rees said humans are not socially prepared to handle them.

"Our societies are more brittle now and less tolerant of disruption," Rees explained. "In the Middle Ages, for example, when the Black Plague killed off half the populations of towns, the others sort of went on fatalistically."

"But I think if we had some sort of pandemic today," he added, "and once it got beyond the capacity of hospitals to cope with all the cases, then I think there would be catastrophic social disruption long before the number of cases reached 1 percent. The panic, in other words, would spread instantly and be impossible to contain."

Technological changes are rapidly changing human society, and Rees said economic pressure to pursue new advances exerts stronger pressure than ethical considerations for using them responsibly.

"Is it too fast for society?" Rees said. "I don’t know. I do know that we’re struggling to cope with all these technologies. Just look at the impact of social media on geopolitics right now, and the risks of artificial intelligence and biotechnology far exceed social media. But these things also have potentially huge benefits to society, if we can manage them responsibly."