The Economist compares the risk of emitting trace amounts of CO2 to the risk "of blowing your head off in a game of Russian roulette"

International: Warming up | The Economist

Take that key question of the warming that can be expected for a doubling of carbon dioxide. Last time round the IPCC said it was likely to be between 2 and 4.5 degrees—meaning there was a 66% chance of its sitting in that range. That left 17% chances that it would be less, or more. As Robert Socolow, an energy and climate-policy analyst at Princeton, has pointed out, these extremes (in particular the high one) need to be dealt with more explicitly, because their consequences would be great.

And their likelihoods, though low, are not that low: 17% is the chance of blowing your head off in a game of Russian roulette.