The United States is setting new milestones that suggest a fundamental, and possibly permanent, shift in its energy economy.

Consider what happened last year alone. The amount of electricity from coal-fired power plants hit a record low while that from natural gas generators hit a record high. Also, renewable energy added the most new power to the electric grid, and annual carbon emissions reached a 20-year low.

“2015 clearly marked a turning point for American energy….We’ve entered a new era here in the United States,” says Lisa Jacobson, president of the Business Council for Sustainable Energy, which released on Thursday the fourth annual Sustainable Energy in America Factbook in partnership with Bloomberg New Energy Finance.

Colleen Regan, a senior BNEF analyst, expects many of the key changes will likely be “permanent shifts, rather than temporary adjustments.”

Indeed, in recent years, the U.S. oil and natural gas boom has sparked much talk about an American energy renaissance. Yet as prices for oil, wind and solar have plunged, the most recent U.S. short-term forecast doesn’t see production growth for all three. Rather, it sees oil falling for the next two years but solar and wind rising.

What this means is an energy mix in the world’s largest economy that’s increasingly efficient and clean with fewer heat-trapping carbon emissions.

For example, energy productivity—the ratio of U.S. gross domestic product to energy consumed – continues to improve. It rose 2.3 percent from 2014 to 2015, up from a 1.1 percent increase the prior year, according to the BNEF Factbook. This shows that the economy can grow without increasing energy use.

Coal-fired power plants, which have seen a record number of closures, are producing much less electricity. Last year, through November—the most recent month for which government data are available—they generated 33.6% of U.S. electricity, down from 39 percent for all of 2014 and 50 percent in 2005.

In contrast, cheap natural gas is surging. Last year, through November, its generators produced 32.5 percent of U.S. electricity and could actually overtake coal when December figures are published later this month by the Energy Information Administration.

This shift has climate benefits, because natural gas emits half as much carbon as coal when burned. So also does the boom in wind and solar power, which emits zero carbon.

Solar panels added a record amount of capacity to the power grid last year, and wind farms saw their capacity jump 65 percent as developers sought to take advantage of a federal tax credit slated to expire, according to the BNEF data. In December, Congress renewed wind and solar tax credits for another five years.

As a result of the growth in both natural gas and renewables, the U.S. power sector recorded the lowest yearly carbon emissions—down 4.3 percent from 2014—since 1995. In fact, these emissions are now 17.8% below 2005 levels, the benchmark against which the Environmental Protection Agency’s Clean Power Plan set a goal of a 32% emissions cut by 2030.

While the U.S. record looks promising, the Clean Power Plan—the major plank of President Barack Obama’s efforts to address climate change—has plenty of critics. It’s being challenged on Capitol Hill and in federal courts, and its fate may turn on this year’s presidential election. The Democratic candidates support it, but the Republicans oppose it. (Find out where the candidates stand on key topics.)

Besides, some of the world’s major carbon emitters such as India are developing countries that have done little to curb fossil fuel use despite severe air pollution.

“This group of countries has greatly expanded its use of fossil fuels,” says a new study by three prominent economists. They say global consumption of fossil fuels rose 7.5 percent for oil, 24 percent for coal, and 20 percent for natural gas from 2005 to 2014.

“In the absence of substantial greenhouse gas policies, the U.S. and the global economy are unlikely to stop relying on fossil fuels as the primary source of energy, “ write Thomas Covert and Michael Greenstone of the University of Chicago and Christopher R. Knittel of the Massachusetts Institute of Technology. In the short- and middle-term, they say solar and wind are unlikely to “play a major role in base-load electricity capacity or in replacing petroleum-fueled internal combustion engines.”

Their calculations are ominous. They figure that burning the currently known fossil fuels would increase global average temperatures by at least 10° to 15° Fahrenheit. They say that whether the voluntary agreements in the UN climate accord, reached in December in Paris, can help developing countries adopt cleaner energy “will be determined in the coming years and decades.”

The story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.