In its most pessimistic scenario, the Rhodium Group projected that emissions from the world's largest economy would decline slightly for the rest of the decade, then flatten out between 2030 and 2040 at just under 4.9 billion metric tons of carbon dioxide equivalent.

The research organization modeled low, mid, and high emissions pathways by weighing factors including economic growth, fossil fuel prices, and clean energy costs and performance.

"The first seven months of the second Trump administration and 119th Congress have seen the most abrupt shift in energy and climate policy in recent memory," the authors wrote in their latest annual Taking Stock report.

"After the Biden administration adopted meaningful policies to drive decarbonization, Congress and the White House are now enacting a policy regime that is openly hostile to wind, solar, and electric vehicles and seeks to promote increased fossil fuel production and use."

US emissions have been steadily falling since peaking in the mid-2000s at more than six billion tons, thanks to several factors.

The biggest driver has been coal's decline, replaced first by natural gas and more recently renewables. Efficiency gains in vehicles, appliances, and industry, alongside supportive policies, have also played a role.

According to the Rhodium Group, greenhouse gases are now projected to decline 26-35 percent by 2035 relative to 2005 levels -- a "meaningful shift" from last year's report, which forecast a far steeper 38-56 percent drop.

Since returning to office, President Donald Trump has pursued an agenda centered on boosting fossil fuels and restricting renewables.

A recent tax-and-spending law passed by Republicans in Congress repealed green energy tax credits enacted under former president Joe Biden, while the administration has also been rescinding approvals for wind projects already under construction.

Renewables will continue to grow substantially through 2030 as companies claim expiring clean electricity tax credits, the report forecast, but deployment then diverged under the three different emissions scenarios.

Transportation emissions are projected to fall more modestly, by 8-20 percent in 2040 compared to 2024, with zero-emission vehicle sales shares rising only slightly.

Meanwhile, oil and gas production is increasingly geared toward exports. The Rhodium Group estimated liquefied natural gas -- heavily promoted by the Trump administration as part of foreign trade deals -- will grow by 94-150 percent in 2040 compared to 2024.