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Last week, environmentalists got a rare bit of good news: U.S. CO2 emissions dropped 3.8 percent in 2012, according to the U.S. Energy Information Administration. That brings emissions to their lowest level since 1994, and represents the fifth drop in seven years. And unlike some previous years, the drop was not the silver lining to a shrinking economy: The economy grew by 2.8 percent in 2012. That means carbon intensity, which is CO2 emissions per dollar of gross domestic product, dropped by an impressive 6.5 percent.

But before you get too comfortable, it’s worth looking at those numbers in the context of what we need to achieve to avert catastrophic climate change. There are three reasons to stay worried, and vigilant:

• 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. 
• CO2 is not the only greenhouse gas.

To keep average global temperatures from rising more than 2 degrees Celsius above pre-industrial levels, and thereby prevent the worst of climate change, we need to keep atmospheric CO2 concentrations around 450 parts per million (and that’s a generous estimate). As the Presidential Climate Action Project (PCAP) noted in 2011, “In order to stabilize CO2 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.”