New federal figures show the amount of heat-trapping carbon dioxide in the air jumped dramatically in 2012, making it very unlikely that global warming can be limited to another 2 degrees. Many governments set a 2-degree increase as the upper limit.
Scientists say the rise in CO2 reflects the global economy revving up and burning more fossil fuels, especially in China.
U.S. government scientists report that carbon dioxide levels jumped by 2.67 parts per million for a total of just under 395 parts per million compared to 2011.
That's the second highest rise in carbon emissions since records started being kept in 1959. Only 1998 had a bigger increase.
Scientists say hopes of limiting warming to 2 degrees are fading away to almost nothing.