Today (31 May 2013), Asteroid 1998 QE2 will come within 3.6 million miles of the earth, making it a “0” (No Hazard) on the Torino Impact Hazard Scale. And what is a 10 on the Torino scale? That would be a situation where "a collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it, whether impacting land or ocean. Such events occur on average once per 100,000 years, or less often." It sounds like the hazard we're facing after pushing atmospheric carbon dioxide concentrations beyond 400 parts per million (ppm). But there are at least three important differences.