Carbon-dioxide emissions from the United States spiked sharply in 2018, bucking a three-year trend and making it more likely that the country will fail to meet its promises under the Paris Agreement on climate change, according to a preliminary analysis published Tuesday.

The new study—written by the Rhodium Group, an energy-research firm—found that emissions of the heat-trapping gas leaped by 3.4 percent, the second-largest increase in more than two decades. The emissions surge was driven by a number of factors: Americans flew more, shipped more goods by truck, and burned more heating oil during a frigid winter.

Even emissions from power plants went up, after years of declines. A record number of U.S. coal plants closed in 2018, but a booming natural-gas sector gobbled up those declines and generated most of the year’s growth in electricity, too.

Where Are U.S. Carbon Emissions Coming From?

U.S. carbon emissions are still down from their historic peak in 2005. But they have not fallen nearly far enough. Under the Paris Agreement, the Obama administration voluntarily promised to cut U.S. emissions by at least 26 percent by 2025, as compared with 2005 levels. Though President Donald Trump pulled out of the Paris Agreement in 2017, the United States is still technically subject to the treaty until next year, and dozens of liberal states and cities have pledged to uphold its terms.