Here's an important energy milestone: For the first time since 1979, America’s cars, trucks, and airplanes emit more carbon dioxide than its power plants do. The chart below comes from Sam Ori, executive director of the Energy Policy Institute at the University of Chicago:

You can see the full data here. The story here is that the United States has made remarkable progress in greening its electricity sector since 2005. Whenever you see exciting headlines about renewable energy growth or the plunge in US emissions, those articles are usually talking about electricity.

But power plants are only one-third of America’s CO2 emissions. Transportation, another third (and now the biggest source), remains tougher to address. In fact, since 2013, transport emissions have been creeping upward again.

Why power plants have been easier to clean up than transportation

Over the last decade, utilities have been slashing CO2 emissions from power plants mainly by switching over to cleaner fuels.

Back in 2005, about half of US electricity came from burning coal, which emits a staggering amount of CO2. Since then, spurred by new pollution rules and pricing shifts, utilities have switched to cheaper natural gas (which produces just half the CO2 as coal when burned, though with some offsetting methane emissions), as well as wind and solar. Coal's share of electricity has plummeted down to one-third.

It's fair to expect this trend to continue, especially if Obama’s Clean Power Plan takes effect and utilities have to keep pushing down emissions. Substituting in natural gas, wind, and solar for coal is a (relatively) manageable task, and the transition is aided by the fact that 1) renewables keep getting cheaper, 2) new technologies and policies are facilitating more flexible grids, and 3) electricity demand in the United States has stagnated for years, thanks to improvements in energy efficiency. (One big caveat here is that many of America’s carbon-free nuclear plants are in danger of closing, which could slow or halt the drop in power-sector emissions.)

Transportation is trickier. Oil remains by far the dominant source of fuel for cars, trucks, and planes, and there’s no readily available substitute here. On Twitter, Costa Samaras, an assistant professor in the Department of Civil and Environmental Engineering at Carnegie Mellon University, posted this chart breaking down transportation emissions:

(This chart only goes to 2013, so it doesn't include the uptick in vehicle emissions over the past two years seen in Ori’s chart.)

We’ve certainly been trying to get transport emissions down: Over the past decade, the United States has been using more corn-based ethanol instead of gasoline in its cars. But ethanol is currently stuck at around 10 percent of the gasoline supply — a contentious issue known as the "blend wall" — and there’s a fierce debate over whether ethanol is actually an improvement, climate-wise, over gasoline.

The Obama administration has also enacted fuel-economy standards that require cars to emit less CO2 per mile. Those rules have certainly kept a lid on emissions. But, right now, they're being offset by the impacts of low oil prices. Americans are shifting back to gas-guzzling SUVs and they’ve been driving more miles over the past two years, which helps explain why transportation emissions have been nosing upward since 2013, despite efficiency rules.

Over the long term, the real hope is that electric cars will catch on and help drive down overall emissions by relying more heavily on the quickly-greening power sector. Right now, electric vehicles are only 0.7 percent of the US car fleet, and turnover is fairly slow, but many analysts expect that falling battery prices should help speed up the shift by making EVs more cost-competitive with traditional vehicles.

(The advent of self-driving cars could also hasten this transition, though it's not yet clear how many autonomous vehicles will be electric and whether they’ll lead to more driving or less. Plus, self-driving cars still have serious hurdles to overcome before they become widespread.)

But even if we do get an electric-car revolution, that would still leave air travel and long-haul trucking, which are about one-third of transport emissions. For the foreseeable future, it won’t be practical for either mode to be powered by limited-range batteries, which means that engineers will have to figure out how to whittle down fuel use bit by bit. See here for an exploration of what that might look like for aviation.

There are plenty of ideas out there for cutting transportation emissions. Cities could reduce vehicle travel through changes in land-use planning that promote density and expanded transit, for instance. (Differences in density are one reason why per capita transportation emissions are dramatically lower in Europe.) We could try to shift more freight from diesel-burning trucks to electric rail. We could try to develop hydrogen-powered trucks.

There's no shortage of papers and technical reports on this subject. It’s just more complicated than cleaning up power plants, and will remain one of the biggest challenges as the US tries to push down emissions to fend off severe global warming.

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P.S. Notice I said power plants are (roughly) one-third of US greenhouse-gas emissions and transportation is another third. Those are the two biggest sources. But, of course, that leaves a lot of other stuff, such as industry (cement and steel plants both emit a lot of CO2 directly), fossil-fuel consumption for heating in homes and buildings, livestock and agriculture (a big source of methane), as well as forestry. If we really want to decarbonize the US economy, we'd need to tackle all of these areas.

Further reading: The rapid rise of electric cars worldwide, in 4 charts