champagne bottle
Keep the champagne corked for now.

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.”

That means, using 2010 as a baseline, the U.S. would have to cut emissions 2 percent per year every year until 2050. All other industrialized countries would have to do the same. The U.S. managed to make cuts in 2011 and 2012, but it’s looking like the country’s CO2 emissions will be up for 2013 and might continue to rise over the next few years.

Developing nations would need to start cutting their emissions too, most notably China and India, but also smaller countries. And as PCAP points out, “In addition, we would have to reduce emissions of the non-CO2 greenhouse gases (methane, nitrous oxide, soot, halocarbons, etc.).”

There are several reasons U.S. CO2 emissions dropped last year, and they highlight why it will be hard to continue cutting them. We reduced transportation emissions, due to a shift away from driving (vehicles miles traveled remained flat even as the population and economy grew) and more efficient cars. We also reduced emissions because of the shift from coal to natural gas for electricity generation. Finally, global warming itself played a role: Shorter, milder winters mean we burned less oil and gas for home heating.

Except for the last of those, unfortunately, we can’t expect present trends to continue. The U.S. is currently experiencing the benefits of higher fuel-efficiency standards for cars and trucks. But cutting transportation sector emissions to where they need to be will require driving less, and that will require a level of investment in mass transit and denser building patterns that the government has not yet committed to. The natural-gas fracking boom hasn’t peaked yet, but it will, and to keep energy generation emissions going downward we’ll need much more capacity from renewables than we currently have. (Of course, the overarching solution to both of these problems would be a cap-and-trade system for emissions, but good luck finding the votes for it in this Congress.)

Also, while fracking can help cut greenhouse gases on the one hand, it can increase them on the other. Burning natural gas is about half as carbon intensive as burning coal. However, when drilling for natural gas, methane can escape. Methane that is released into the atmosphere is an especially potent greenhouse gas. According to the EPA, “Pound for pound, the comparative impact of CH4 [methane] on climate change is over 20 times greater than CO2 over a 100-year period.” And in states with a lot of fracking going on, there has been an uptick in methane emissions.

As Climate Central reports, “Some states, particularly those producing shale oil and gas, are seeing some increases in methane emissions, the most potent greenhouse gas. Leaks in the oil and gas drilling, production and distribution process are suspected to be a major source of methane emissions. Texas, home to several major shale oil and gas plays and by far the nation’s biggest industrial air polluter, saw its methane emissions increase from 12.96 million tons to 13.41 million tons between 2011 and 2012.”

So, by all means, be happy that we’re making some progress, but don’t think we’re on track yet to solve the problem.