The U.S.’s exit from the historic 2015 Paris climate agreement takes effect today, capping four years of President Donald Trump aggressively rolling back the Obama administration’s climate-change-mitigation policies. The acceleration of the country’s greenhouse gas emissions on Trump’s watch has been blunted by state- and city-level efforts, a burgeoning renewable energy market and the COVID-19 pandemic’s economic downturn. But the Trump-era rollbacks could still ultimately lead to more heat-trapping carbon entering the atmosphere over the next decade or more.

The U.S.’s position contrasts starkly with that of much of the rest of the world. Other industrialized nations “are crafting detailed policies that are suited to their individual national context and making commitments that are high-ambition and delivered in many different ways with the policies that they will enact,” says Rachel Cleetus, policy director of the climate and energy program at the Union of Concerned Scientists. Just last week, for example, Japanese prime minister Yoshihide Suga pledged to reduce his country’s carbon emissions to zero by 2050. And China and the European Union have made similar commitments. “At this moment, the U.S. is globally isolated,” Cleetus adds. “No other country has left the agreement.”

When the Paris Agreement was concluded in 2015, it was hailed as a historic commitment, among nearly 200 nations, to reduce greenhouse gas emissions to stave off the worst effects of climate change. The pact’s stated aim was to keep global average temperatures from increasing by more than two degrees Celsius above preindustrial levels by the end of this century and to pursue efforts to keep that temperature rise below 1.5 degrees C. Contrary to Trump’s assertions, the agreement (which took effect in November 2016), is not prescriptive; signatory countries are asked to implement their own emissions-reduction strategies.

Signing the agreement was something of a capstone to the ambitious climate policies championed by President Barack Obama. U.S. emissions had steadily increased for decades before he was elected, and his administration reversed this trend. It took actions such as setting stricter fuel economy standards for vehicles and setting out a Climate Action Plan that “outlined systematic emission reductions for almost every sector of the economy,” says Kate Larsen, a director at the Rhodium Group, an independent research organization. Larsen recently co-authored an analysis that found Trump’s climate policy rollbacks could add the equivalent of one third of the country’s 2019 greenhouse gas emissions to the atmosphere by 2035. “We estimate that had all of the Obama policies been put in place, the U.S. would have come close to meeting the [2009] Copenhagen [Accord] commitment of a 17 percent reduction [in carbon emissions below 2005 levels] in 2020,” she says. Obama’s policies would not have brought about the Paris Agreement’s more rigorous goal of 26 to 28 percent reductions below 2005 levels by 2025, but they “would have been an important first start.”

Trump began attempting to unravel his predecessor’s legacy almost immediately after taking office. In March 2017 he ordered the Environmental Protection Agency to scrap the Clean Power Plan—an Obama-era policy that aimed to reduce U.S. emissions from power generation by 32 percent of 2005 levels by 2030—and under this directive, the agency replaced it with the Affordable Clean Energy rule. According to the Rhodium Group analysis, that policy change alone could result in as much as an extra 624 million metric tons of greenhouse gas emissions by 2035. The Trump administration has also weakened fuel efficiency requirements and rolled back numerous regulations related to methane, a potent greenhouse gas, among other actions. Collectively, those rule changes will result in an excess 1.8 billion metric tons of CO2 equivalent by 2035, according to the Rhodium report. “The last four years of the Trump administration has not only been a reversal of many of the Obama policies that would have put us on track with meeting our initial obligations,” Larsen says, “[it has also been] four years of lost opportunities to continue the progress of the Obama administration.”

Despite that lost time, a confluence of factors has prevented Trump’s rollbacks of emissions standards from manifesting their worst possible outcomes. Efforts by states and municipalities to fill the gap—along with the growing economic competitiveness of renewable energy sources—have helped curb emissions in the absence of federal regulations. Joseph Aldy, an economist at Harvard University’s John F. Kennedy School of Government, who served as Obama’s special assistant for energy and the environment from 2009 to 2010, says the result is that emissions output has plateaued, rather than spiked, during Trump’s tenure. “The Trump administration has slowed progress considerably, but they haven’t reversed progress,” Aldy says. “They’re certainly not consistent with their [campaign] rhetoric to bring back coal” as an energy source. (Coal-fired power plants produce more than twice as much CO2 emissions as natural gas plants.) The combined market forces of cheap natural gas, wind and solar power mean “there’s just no economic reason” to be investing in coal, he says.

COVID-19 shutdowns have also dampened the U.S.’s 2020 emissions output. According to a study published in Nature Climate Change in May, global CO2 emissions decreased by about 17 percent in the first four months of 2020, compared with the previous year. The largest reductions were caused by sharp decreases in aviation and surface transport. Another paper, published in October in Nature Communications, found CO2 emissions began to rise again in May and June 2020, resulting in a global emissions decline of about 9 percent, compared with 2019. Eri Saikawa, an environmental scientist at Emory University, who was not involved in either study, says the fact that the COVID-19 shutdowns were the single largest factor in reducing emissions is rather bleak news. “We really need to do everything we can to reduce [emissions],” she says. “COVID cannot do that.” If the federal government continues to ignore climate change after the pandemic subsides, grassroots movements could pressure local governments to move forward on their own, she adds.

Federal climate change action now depends largely on who takes office in January 2021. The Trump administration is continuing to enact rollbacks to emissions regulations and appointing officials who have made statements casting doubt on widely accepted climate science to key government positions. But former vice president Joe Biden has pledged to enact climate-change polices that are even more ambitious than Obama’s. He has also said he will bring the U.S. back into the Paris pact. Doing so would be a relatively simple process and would take place 30 days after Biden sends notification of his intent to rejoin. But even if a Biden administration aggressively tackles carbon emissions, Trump’s impact on climate policy could be lasting.

“There is a longer-term effect of failing to act over the past four years,” Aldy says. “We need a serious, whole-of-the-economy, whole-of-the-government approach to public policy [on climate change]. It’s the delay in getting that going which makes it all the more challenging. But as [Trump has] demonstrated with COVID, it’s hard for him to actually do hard work to deal with challenges.”