Politico reports today that the Sierra Club is set to launch a $5 million campaign to fight construction of the 221 natural gas-fired power plants planned to be constructed across the country. Sierra Club argues that the world cannot afford more carbon dioxide emissions from combustion of fossil fuels.

So where will the power come from?

Since 2010, 531 coal-fired power plants in the United States have shuttered, converted to natural gas or announced that they will be closing. This is 38 percent of all coal-fired power plants in the country, and 28 percent of the coal-fired generating capacity (many of the closing units were older, smaller units). These announcements occurred largely before the Obama administration’s Clean Power Plan, which, if upheld in court, would result in many more coal-fired plants’ closing down.

Numerous nuclear power plants are at risk of closure across the country. The Nuclear Energy Institute says more than a dozen nuclear plants are at risk of closure or have announced they will close. This is roughly a quarter of the nuclear plants operating in competitive bid-based wholesale electricity markets. Some market observers believe the number of nuclear units at risk could be much higher. Market prices, which are in part affected by bids from subsidized renewable sources, are not high enough for nuclear and other plants to continue to turn a profit. Nuclear plants, it should be noted, emit zero CO2.

In just the past few years, natural gas has risen to become the primary electricity-producing fuel in the United States. It now produces 34 percent of US power, up from under 20 percent just a few years ago. This is due to a combination of factors: gas is very inexpensive by historical standards; gas-fired power plants are relatively low cost and low regulation to build and operate compared with coal and nuclear; and environmental and safety regulations have increased costs for coal-fired and nuclear generation, respectively.

Many environmental groups have long opposed coal and nuclear power. Now the Sierra Club is gearing up to oppose natural gas-fired power as well. As older power plants continue to retire and electricity demand continues to grow as our country grows, where will the new power come from?

The power grid needs “always on” power sources to maintain voltage and frequency. Intermittent sources like wind and solar have grown substantially in recent years to provide more than 5 percent of electricity in the United States in 2015. Maintaining “always on” power sources is critical both to making it feasible to integrate electricity from intermittent renewables and to maintaining electric reliability.