This summer, the nation sweltered in a deadly heat wave.

High temperatures hovered near 100 degrees, the heat buckled highways in several states, and firefighters in Indianapolis evacuated 300 people from a senior living community when the air conditioning failed. Cities from New York to Seattle set up cooling centers as demand for electricity hit all-time highs.

This fall, the nation froze in near-record cold.

Temperatures in the upper Midwest recently plummeted to -31 degrees. Freezing temperatures and ice storms snarled air traffic across the nation. Overnight lows in the normally temperate Puget Sound region plunged into the teens and 20s — and stayed there for a week. Cities across the state opened public buildings during the night for the homeless, and electricity demand skyrocketed as homeowners pushed up the thermostat to fend off the bone-chilling cold.

The one constant in these two extremes has been a high demand for electricity. In suffocating heat or freezing cold, ample power supplies are literally a matter of life or death.

Yet, if some environmental activists get their way, the U.S. will eliminate almost 90 percent of its electricity.

The Sierra Club and others have mounted an aggressive national campaign to eliminate the use of all fossil fuels. They started with coal and quickly expanded their purity crusade to all forms of fossil fuels, including clean-burning natural gas. Their “War On (insert-any-fossil-fuel-here)” targets anything related to the use of fossil fuels — from coal mining to gas exploration to power plant siting to pipelines to transmission lines to supply trains to shipping terminals.

But renewables — wind, solar, geothermal and hydropower combined — produce only 12 percent of our energy needs. Yet even that is under attack in some quarters.

Hydropower is the single largest form of renewable energy, producing 6.7 percent of the nation’s electricity. Here in Washington, hydropower produces more than 75 percent of our electricity. Nevertheless, many activists want to severely restrict hydropower production because of the dams. They even push to tear down key dams.

They have been successful to some degree. For example, our state law does not allow large utilities to include hydropower in their mandated renewable energy portfolio. Even environmentally conscious California embraces hydropower.

If these anti-hydropower activists and their fellow “no-fossil-fuel” warriors succeed, they will eliminate the source of 95 percent of our energy.

What then?

How do they intend to replace it? What is their plan? How will they ensure that we can heat and cool 135 million homes, businesses, grocery stores, hospitals and schools?

They need to answer these questions, or we will freeze in the winter and swelter in the summer.

Yes, we must embrace all practical forms of energy, and the use of renewable energy has increased significantly over the years. But that increase comes with a cost — a cost that has been subsidized by taxpayers.

According to the Congressional Budget Office, the federal government awarded $16 billion in energy subsidies in 2013 — 74 percent of it for renewable energy and energy efficiency. Fossil fuels received 20 percent of the total, and nuclear power received 7 percent. We also need to realize that, as costs for alternative energy rise, so do our electric bills, hitting the poorest and most vulnerable families the hardest.

Finally, how will the opponents of fossil fuels solve the massive job losses and economic collapse that would follow the destruction of the U.S. energy industry?

The American Petroleum Institute reports that the oil and gas industry supports 9.2 million jobs in the U.S. and pays $86 million a day in taxes. How do the “no-fossil-fuel” purists plan to replace that?

We need to have the answers to these questions before we lurch blindly forward on a campaign to eliminate the use of fossil fuels. Otherwise, we’ll all be left out in the cold.

Don Brunell is the president of the Association of Washington Business (www.awb.org).