David Biello, writing for Scientific American:
According to a new analysis published online in Environmental Research Letters on January 9, the world now likely needs 12 wedges [a reduction of 1 billion metric tons of CO2 emissions maintained for 50 years] just to get back to the business-as-usual scenario outlined by Socolow and Pacala in 2004. Add another nine billion ton wedges if you want to stabilize emissions at last year’s levels, and add another 10 if you’d like to keep greenhouse gas concentrations from rising above 500 parts-per-million. Already, concentrations are at 394 ppm, up from 280 ppm before the dawn of widespread fossil fuel burning. That’s if you also want the global economy to continue to grow at the same time.