The threat of climate change caused by human activity could turn out to be a minor problem by comparison with a scarcely acknowledged natural hazard.
Geologists say there is a real risk that sooner or later a supervolcano will erupt with devastating force, sending temperatures plunging on a hemispheric or even global scale.
A report by the BBC Two programme Horizon on one supervolcano, at Yellowstone national park in the US, says it is overdue for an eruption.
Yellowstone has gone off roughly once every 600,000 years. Its last eruption was 640,000 years ago.
Professor Bill McGuire, of the Benfield Greig Hazard Research Centre at University College, London, told BBC News Online: "We're getting ready for another eruption, unless the system has blown itself out.
"But the ground surface deformation and other signs measured by satellite suggest it's still active, and on the move."
Typically, supervolcanoes are not mountains but depressions, huge collapsed craters called calderas, which are hard to detect.
The Yellowstone caldera is 70 kilometres long and 30 km wide. Eight km beneath the Earth's surface lies a huge magma chamber, containing vast amounts of molten rock.
As pressure rises in the chamber, the surface is also rising and there is a measurable increase in heat. But vulcanologists do not know when Yellowstone will blow.
Supervolcanoes are relate to giant calderas
Professor McGuire, whose book, Apocalypse! A natural history of global disasters, portrays a possible Yellowstone explosion in 2074, says there have been two such events every 100,000 years for the last two million years.
The areas where supervolcanoes are most likely to be found, he says, are subduction zones, where the Earth's plates are dipping below one another. The Pacific Rim and southeast Asia are especially vulnerable.
But there is a caldera in the Phlegraean Fields near Naples in southern Italy. Dr Ted Nield, of the Geological Society of London, told BBC News Online: "It could do the same as Yellowstone, though on a smaller scale".
"When a supervolcano goes off, it is an order of magnitude greater than a normal eruption. It produces energy equivalent to an impact with a comet or an asteroid.
"You can try diverting an asteroid. But there is nothing at all you can do about a supervolcano.
"The eruption throws cubic kilometres of rock, ash, dust, sulphur dioxide and so on into the upper atmosphere, where they reflect incoming solar radiation, forcing down temperatures on the Earth's surface. It's just like a nuclear winter.
"The effects could last four or five years, with crops failing and the whole ecosystem breaking down. And it is going to happen again some day."
Ice-core records show that the eruption of Toba in Sumatra about 74,000 years ago may have caused global cooling of from three to five degrees Celsius, and perhaps as much as 10 degC during growing seasons in middle to high latitudes.
Even ordinary volcanoes can affect the climate.