What do Boston, London, New York, Frankfurt, San Francisco, and Paris have in common? They have all been destroyed by asteroid impacts—in the movies. The death toll was enormous in every case. Understandably, people are worried that such a catastrophe might actually occur within their lifetime.

Actually, says climate scientist Mark B. Boslough, “if you’re going to stay up late at night worrying about something, worry about climate.” Boslough, a researcher at the Sandia National Laboratories in Albuquerque, New Mexico, US, says, “you are much more likely to die from climate change than from an asteroid impact by a factor of something like a thousand.” He presented his findings at the Fall Meeting of the American Geophysical Union in San Francisco, California.

Boslough cannot put a precise number on that ratio, because climate science is still replete with gaps in the relevant data. He is much more confident about the asteroid threat, though. He says that there is no evidence that anyone has died from an asteroid or meteorite impact—ever. He does not make a distinction between asteroids and meteors, saying the latter are basically fragments of the former and there is no size boundary to distinguish one from the other.

Looking at worst case scenarios for asteroid impacts and climate change over the next 100 years, Boslough estimates that the largest asteroid to impact Earth would be around 50 metres in diameter. It would explode in the atmosphere and not make an impact crater, he says, and it would kill no one. Asteroids larger than 50 metres across and likely to cross Earth’s orbit are rare, and the larger they are, the rarer they are. They are also well tracked. Smaller ones are more numerous, but even less likely to cause death and destruction on Earth.

Scientists can make the same kind of probability study with regard to climate change, Boslough says, “but the nature of the problem is very different.” Climate modelers do not have sufficient data to construct high resolution models, he says, including such key questions as: what are the relevant feedbacks? And how do aerosols (fine particles) affect the formation of clouds?

The best approach at present, he says, is to use an ensemble of models to produce an estimate of the consequences, including fatalities, for a given degree of climate change. Still, even with the present uncertainties, Boslough says, the World Health Organisation estimates that 150,000 people per year will die from warming at the current rate, compared with none from asteroid impacts.

Boslough acknowledges a perception gap among the general public, thanks to the media, which he says commonly focuses on “exceptionally unlikely impact scenarios.” When climate scientists talk about worst case scenarios, he says, “often times they are portrayed as alarmist. When we in the asteroid impact business talk about big impacts, like the ones depicted in the movies, no one ever accuses us of being alarmist.” He hopes his analysis of the relative threats of both impact and climate will help put both threats into perspective.

Harvey Leifert, freelance science writer