Coal-fired electricity generation increased for the first time in six years due to “significantly higher” natural gas prices in 2021, but the trend is not expected to continue, according to the U.S. Energy Information Administration.

The EIA said coal-fired electricity generation increased 22% due to natural gas prices more than doubling between 2020 and 2021.

The agency said the increase is not expected to continue. The EIA estimates coal-fired generation will decrease 5% due to coal-fired power plant retirements and low production from coal mines.

Coal-fired generation emits toxic mercury and other heavy metals as well as pollutants like sulfur dioxide, nitrogen oxides and particulate matter, all of which cause negative human health effects.

Burning coal for energy also emits large amounts of carbon dioxide, a greenhouse gas that traps heat in the atmosphere, spurring changes in the climate that alter the way people live and work and are expensive to mitigate.

Coal-fired generation had been on the decline since at least 2010 due to the relatively low price and stability of natural gas prices, but higher natural gas prices due to tight supplies kicked off by extreme weather and increased exports have at least temporarily stopped the trend.

The North American Electric Reliability Corp., the entity that oversees the six regional electric grids in the continental U.S. and parts of Canada and Mexico, said there was a “high probability of insufficient resources and energy to serve electricity demand” as early as summer due to extreme weather-related events and issues incorporating renewable energy systems onto the existing grid.

“When we look at events over the last several years, it is clear that the bulk power system is impacted by extreme weather, and that it is happening more often,” said John Moura, NERC director of Reliability Assessment and Performance Analysis in a press release. “Prioritizing reliability as climate change policies are developed will support a transition that assures electric reliability in an efficient, effective and environmentally sensitive manner.”

The EIA found the U.S. installed more wind turbine capacity in 2020 than in any other year, but energy grids are currently not prepared to move the energy produced to where it is needed.

A 2021 report by the Carmel, Indiana-based Midcontinent Independent System Operator, which oversees the electrical grid for Indiana, 14 other states and the Canadian province of Manitoba, found that “transformational change” would be required to incorporate renewable energy into the grid, but that up to 50% of electricity from renewables was ultimately possible with coordinated action from energy providers.