Good news: The United States is back on the front lines fighting against climate change. That is the message Barack Obama tried to send when announcing a “new national climate action plan” at his Georgetown University speech in Washington on Tuesday, June 25.

Reducing greenhouse gas emissions from the most polluting power plants, implementing renewable energies and a plan to adapt to extreme climate events in a country that is still scarred by the ravages of Hurricane Sandy: This speech has been long awaited by Americans who are disappointed by the mediocre results of Obama’s first term.

It has also been awaited by those who think that the United States, through its idleness, has been holding international climate negotiation hostage for several years. The Americans are not the only ones responsible, but the fact remains that without them — the second highest polluters on the planet after China — an ambitious agreement is impossible.

Time, however, is running out. Since the 2009 U.N. climate change conference in Copenhagen — during which the infamous 2 degrees Celsius (35.6 degrees Fahrenheit) maximum global warming threshold was recognized — statistics have repeatedly shown that commitments are not being upheld. What will occur between now and the end of the century is an average temperature increase of nearly 4 degrees Celsius (39 degrees Fahrenheit).

Can Mr. Obama’s promises make a difference? Since 2009, right after his election, the Democratic president established the fight against global warming as one of his top priorities. He declared that deferring it was not an option. Two years later, faced with intransigence from Congress, he abandoned his carbon-trading project. The climate emergency was put on the back burner.

This time around, it is no longer a question of attacking the Republican opposition head on. To avoid the obstacle that is Congress, Mr. Obama opted for the regulation route. The Environmental Protection Agency will be in charge of establishing, by 2015, pollution standards for coal-fired plants, which — despite the rise of shale gas — still provide 40 percent of electricity in the United States.

This is good, but nothing guarantees that these future standards will be ambitious enough. It is just as difficult to evaluate the extent of the set of measures presented at Georgetown. In the best case scenario, these measures should allow the United States to reach the goals that were set in 2009, meaning a 17 percent reduction in greenhouse gas emissions by 2020 compared to the 2005 levels. Beyond that, Mr. Obama has said nothing. What about after 2020? This is the challenge in an international negotiation.

The president, who confirms the United States’ ambition to lead the fight against climate change on the global scene, will not be able to avoid debates concerning the future if he wants to be deemed credible by his partners. After years of American inaction, things are moving. We should rejoice. Especially since China just announced that it would be moving in that direction. This is important: Without the number one and number two global economies, nothing is possible.