A Trump administration will have the ability to weaken or do away with many of the policies.

But even if all these strategies are put into effect, the United States will not reach its goal. Other actions , like energy markets and new state laws, may close the gap.

A large part of the American pledge relies on an Obama administration policy, called the Clean Power Plan , that will steer power plants toward renewable energy.

Federal regulations governing energy efficiency, hydrofluorocarbons and pollution from methane and other gases could contribute another 31 percent.

Many strategies are in place or proposed to reach this goal. California state policy on greenhouse gases gets national emissions 5 percent of the way to the pledge.

But the country is poised for big reductions. Last year, when 195 nations agreed to reduce emissions together, the United States pledged to lower its share to 26 to 28 percent below 2005 levels by 2025.

If the United States continues to create greenhouse gases at the so-called business as usual level, net emissions could reach 6.9 gigatons by 2025.

During the presidential campaign, Donald J. Trump said he would take the United States out of the Paris agreement and get rid of the Clean Power Plan, which the Supreme Court suspended earlier this year.

A Trump administration may decide to move against efforts undertaken by the Obama administration — by limiting the budget of the E.P.A. or slowing progress on federal actions, for example.

In choosing Scott Pruitt, a close ally of the fossil fuel industry, to run the Environmental Protection Agency, Mr. Trump has signaled a desire to dismantle President Obama’s actions to counter climate change, despite discussing the issue with Al Gore earlier this week at Trump Tower.

Mr. Trump’s decisions will be important but not the sole factor in how close the United States gets to reaching the emissions goal.

Energy markets are shifting; coal power has been declining while wind and solar power use have made gains recently.

This summer, California strengthened policies to limit greenhouse gas emissions. Nine northeastern states began working together in 2009 under a cap-and-trade program.

Last month, more than 300 companies called for the United States to continue with its emission reduction plans.

Andrew Jones, co-director and co-founder of Climate Interactive, says that reducing emissions requires a number of strategies. “The contributors to global warming are diverse,” Mr. Jones said. “Therefore the response to it by the United States is going to be diverse as well.”

Estimated global emissions in 2025