Several new university studies on the impacts of fracking further show the tradeoff of one set of environmental consequences for another as the U.S. increasingly relies on natural gas—with perhaps negligible reductions in total greenhouse gas emissions.

The U.S. Energy Information Administration recently released data showing that in 2015 carbon dioxide emissions from electricity generation dropped to the lowest level since 1993, or 21 percent below 2005 levels. “A shift on the electricity generation mix, with generation from natural gas and renewables displacing coal-fired power, drove the reductions in emissions,” the EIA reported. Burning natural gas releases about half as much CO2 as coal.

Source: U.S. Energy Information Administration, Monthly Energy Review. Notably, the data doesn’t include how energy efficiency has contributed to carbon dioxide reductions.

This shift to natural gas, the rapid rise of renewables, and the prospect of federal limits on CO2 emissions have combined to wallop the coal industry. Coal output hit a 30-year low in 2015. The EIA forecasts that 2016 will be the first year natural gas-fired generation exceeds coal generation; it did so for seven months of 2015. Companies accounting for about 45 percent of U.S. coal production have filed for bankruptcy. Nearly 34,000 jobs have been lost in the industry since 2011. Coal production from Powder River Basin mines, in Wyoming and Montana, was down 35 percent in the first quarter.

But as the market spurns coal and the U.S. increasingly relies on natural gas, the environmental risks associated with hydraulic fracturing, or fracking—the game-changing drilling technique that’s unlocked an abundance of oil and gas from shale formations around the country—are coming into sharper focus. Researchers at Stanford and Duke recently released studies revealing the extent to which fracking pollutes water and soil. Michigan researchers uncovered how fracking in the Bakken oil patch has led to rising global ethane levels. Harvard scientists are finding that the amount of methane—a much more potent greenhouse gas than CO2—seeping from oil and gas wells is far greater than previously thought. And University of Texas researchers have linked much of the recent earthquake activity in the state to the injection of fracking wastewater underground.

Taken together, these studies, all published since March, further illustrate the tradeoff of one set of environmental consequences for another as we increasingly rely on fracking in moving away from coal. The “bridge fuel to a clean-energy future,” as natural gas has often been described, is a precarious one. It doesn’t appear to be a bridge we’ll cross quickly, especially as the U.S. looks to export liquified natural gas, and as oil prices slowly tick up, fracking activity will, too.

A quick primer on fracking, via SciShow and Hank Green:

The EIA recently showed that natural gas from fracked wells now accounts for about two-thirds of total U.S. production. Fracking accounts for about half of crude oil production.