 Capping carbon levels alone won't work. Research out of the MIT Global Change Institute assumed a $20 tax per ton of carbon pollution, increasing 4% every year, and found that it would only modestly curb emissions. This carbon tax scenario falls well short of even the White House's goal of cutting carbon emissions by 80% by 2050. And it would come nowhere close to staving off the projected temperature increases that could make our planet over the extinction tipping point.