The amount of carbon dioxide that humans will have released into the atmosphere by 2100 may be enough to trigger a sixth mass extinction, a new study suggests. The huge spike in CO2 levels over the past century may put the world dangerously close to a "threshold of catastrophe," after which environmental instability and mass die-offs become inevitable, the new mathematical analysis finds. Even if a mass extinction is in the cards, however, it likely wouldn't be evident immediately. Rather, the process could take 10,000 years to play out, said study co-author Daniel Rothman, a geophysicist at the Massachusetts...