· The U.S. has to keep up this performance year after year, and it will get harder as emissions go down.
· The whole world has to do the same.
· Carbon dioxide is not the only greenhouse gas.
To keep average global temperatures from rising more than 2° Celsius above pre-industrial levels, and thereby prevent the worst of climate change, we need to keep atmospheric carbon dioxide concentrations around 450 parts per million. As the Presidential Climate Action Project noted in 2011, “In order to stabilize carbon dioxide concentrations at about 450 ppm by 2050, global emissions would have to decline by about 60% by 2050. Industrialized countries’ greenhouse gas emissions would have to decline by about 80% by 2050.” That means, using 2010 as a baseline, the U.S. would have to cut emissions 2% per year every year until 2050.