The U.K.’s Astronomer Royal Martin Rees made part of the dire prediction during a debate at the Edinburgh International Science Festival.

Beyond the tangible threat of a terrorist nuclear attack on a major city, the eminent astrophysicist points to climate change and overpopulation as the world’s leading manmade perils.

But Astronomer Royal for Scotland, John Brown, cautions that the planet is somewhat overdue for an asteroid impact.

He also fears electromagnetic solar storms that could “fry” electronics in orbit and on the ground. Such an event could cripple the modern devices that have become crucial to human survival and commerce.

“Suppose they knocked out a quarter of the world’s power supplies — that would create a potential threat to millions of people,” Brown told the gathering.

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