Gas for years was called the "bridge fuel," a cleaner-burning low-cost alternative to coal that would provide energy to U.S. homes and power the nation's economy until zero-emissions solar, wind, batteries and nuclear became cheap and widespread enough to generate the country's electricity.

The other side of the crossing, however, may now be in sight – and sooner than most expected. Gas, once embraced across the political spectrum, is increasingly being seen by scientists and environmental advocates as part of the climate problem.

Last year U.S. emissions of heat-trapping carbon dioxide increased for the first time in three years, soaring more than 3 percent according to an analysis by the Rhodium Group consulting firm. The spike marked the country's second-largest annual gain in 20 years, behind only 2010 as the country was recovering from the Great Recession.

The increase in emissions was driven by renewed U.S. demand for electricity, fueled by a more robust economy as well as an unusually cold winter and hotter-than-average summer that drove Americans to use more heat and air conditioning.

What met that increased demand, however, was natural gas – far less carbon-intensive than coal but the source of close to a third of the power sector's carbon dioxide emissions, a figure that also does not account for the far more potent climate impacts of methane that leaks from pipelines, well sites and other gas infrastructure.

"We're getting to the point where, if we want emissions to go down further, we're going to have to do some carbon capture or deploy different technologies," says Joshua Rhodes, a research associate with the Energy Institute at the University of Texas at Austin. "Anytime gas replaces coal, there's fewer carbon emissions, but that benefit only exists as long as that coal plant would have been there."

Gas's rise came at the expense of coal's ongoing demise: Since 2010, more than half the country's 530 coal power plants have gone offline or been slated for retirement. By the end of 2017, carbon dioxide emissions from the coal sector had plummeted by more than a third; emissions from the energy sector overall had dropped by close to 8 percent.

Coal sector executives and trade groups, as well as conservative politicians, have often blamed environmental regulations for the sector's decline, but it's gas that's played the most pivotal role: The fuel is cheap and abundant, and, because it burns more cleanly, power producers and industrial firms that switch to gas can style themselves as environmentally friendly.

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By 2014, even President Barack Obama, who made addressing climate change a central plank of his second term, was heralding natural gas as a crucial "bridge fuel" as the U.S. transitioned away from coal: renewables weren't yet competitive, and nuclear, though it generates no emissions, remained bedeviled by astronomical upfront costs and public opposition. By the start of Obama's last year in office, as roughly one coal plant went offline every 10 days, the number of gas plants increased by 10 percent to 1,820 across the utility, industrial and commercial sectors.

Even by that point, however, support for extracting yet more of the gas had peaked and a groundswell of opposition was well-established. Americans living near well sites and compression stations voiced concerns about water contamination, air pollution and noise. At the national level, environmental groups increasingly highlighted not only the carbon dioxide emissions generated by natural gas plants but also the far more potent methane emissions rising from leaky pipelines, wells and other infrastructure.

The Sierra Club was among the groups that went on the offensive. The group's Beyond Coal campaign, launched in 2010, has used aggressive litigation, lobbying and public marketing to become a potent weapon in bringing down coal plants. In more recent years, however, the group has steadily expanded the campaign's focus to also put natural gas in its crosshairs.

The organization's regional offices and even individual employees now have yearly goals – sometimes included as part of workers' annual reviews – for how many megawatts of gas and coal power they're expected to force into retirement.

"We have realized that we need to expand our efforts to go after natural gas and make sure that gas itself is replaced by clean renewable energy," Sierra Club Executive Director Michael Brune says. "The strategies are similar, the tactics are almost identical, the work is pretty much the same. There's just more of it."

The work has gotten a boost from the new economics of renewables: Wind and solar, once uneconomic, presently rank as the cheapest sources of electricity in the Midwest and Southwest, respectively. And while batteries have long been the missing link, allowing utilities to store and control the energy that solar and wind produce – rather than simply when the sun is shining or the wind is blowing – the cost of installing batteries along with solar and wind has substantially fallen, too.

Notably, the average cost of utility-scale solar with storage now rivals that of so-called gas "peakers," expensive plants that can be quickly fired up to meet high levels of demand. Meanwhile, utilities in sun-drenched Nevada and Arizona in the past two years have entered agreements for solar-plus-storage plants at prices equivalent to even "combined-cycle" natural gas plants, which are typically the cheapest type to operate.

"We have a climate case and a public health case, and the economic case is equally strong," Brune says.

Some economists and energy experts are not so bullish on renewables' competitiveness with gas: Ken Medlock, fellow in energy and resource economics at the Baker Institute at Rice University, maintains that "we're not quite there yet."

"When you have to balance the swings you get with renewables, the amount of storage you need is astronomical, and that raises the overall system cost," Medlock says.

Coal, meanwhile, is far from dead: The fuel generated nearly 30 percent of the country's electricity in 2017, slightly less than the 32 percent produced by gas. It also enjoys particular favor from the Trump administration, which counts the coal sector as one of the president's most fervent sources of support. Nonetheless, there is growing consensus that the low-hanging fruit – old, heavily polluting and expensive coal plants – is mostly picked.

"What we had was a whole bunch of old inefficient coal plants ready to be pushed over the edge, and low-cost natural gas – and renewables – were the key drivers in doing that," says Noah Kaufman, research scholar at the Columbia University Center on Global Energy Policy. "That's running out, and if we want sustained emissions reductions, we're going to need strong market signals where the objective is to produce those emission reductions."

Those will take time to be seen, if they're implemented at all – even outside the partisan gridlock in the nation's capital, state-based proposals to implement policies such as prices on carbon emissions have encountered pushback. Voters in left-leaning Washington notably rejected a referendum in November that would have implemented such a scheme. Investments in infrastructure like natural gas, meanwhile, proceed over years, if not decades.