Via Andrew Revkin, the Energy Information Administration has published a fascinating chart that offers a look at how the U.S. electric sector has changed over time. Basically, the EIA looked at all power plants in operation (as of 2010) and charted them out by fuel type and the date that they came online:
As you can see, the vast majority of U.S. coal plants in operation were built between 1950 and 1990. The same goes for our nuclear fleet. Those two sources still provide about 40 percent of the nation's electricity, but very few new facilities are coming online. (Right now, only two new nuclear reactors have been approved.) Utilities have also exploited all the major hydropower spots, though there's still hope for small-scale hydro.
Since the early 1990s, utilities have mostly stopped building coal and nuclear plants, thanks to a combination of costs, regulation and pressure from outside groups. The Sierra Club, in particular, has done a lot of work to prevent utilities from building new coal-fired plants, and only one was built this year—an 800-megawatt unit at the Prairie State Energy Campus in Illinois.
Instead, power companies switched their focus to natural gas plants, which are often smaller, less polluting and easier to build. (By the way, that dip in new natural gas generation in the 1980s came after Congress made natural gas plants illegal for a few years. True story.) Moreover, since the mid-2000s, after a series of tax incentives from Congress, solar and especially wind power have made up a bigger and bigger chunk of new capacity. These days, the plants being built are pretty much natural gas, wind and a bit of solar.
As the EIA chart below shows, natural gas and, to a lesser extent, renewables are quickly taking a larger share of the power sector, mainly at the expense of coal. That's predicted to continue for the near future, especially since power companies are expected to retire up to 17 percent of their coal fleet between now and 2020:
These two graphs go a long way toward explaining why greenhouse gas emissions in the United States have been falling of late.