With its presidential elections in full swing, the United States has become the first nation in the world to formally withdraw from the Paris Agreement on climate change. President Trump had announced the move in 2017, but United Nations rules meant that it comes into effect today.

The Paris Agreement was drafted in 2015 to strengthen the global response to the threat of climate change. It aims to keep global temperature rise well below 2ºC above pre-industrial levels, and to pursue efforts to limit the temperature increase even further to 1.51C.

The delay of the US exiting the agreement is due to the baked-in complex of its rules, so written specifically due to the possibility of a country deciding to exit. In the past, internal US politics had influenced other climate pacts as well, such as the Kyoto Protocol, signed in 1997. The Clinton administration couldn’t secure Senate backing for it.

No country was allowed to leave the climate agreement before three years had passed from the date of ratification (after at least 55 countries representing 55% of global emissions had ratified it). This happened on 4 November 2016. Still, member states had to serve a 12-month notice period on the United Nations prior to exiting.

“Being out formally obviously hurts the US reputation,” Andrew Light, a former climate change official in the Obama administration, told BBC. “This will be the second time that the US has been the primary force behind negotiating a new climate deal – with the Kyoto Protocol we never ratified it, in the case of the Paris Agreement, we left it.”

Although this has been long in the making, there’s still a sense of disappointment among climate diplomats and environmental activists, who believe that climate change is the biggest global challenge we’re facing and that the US should be leading the fight against it. The US now represents around 15% of global greenhouse gas emissions.

“The decision to leave the Paris agreement was wrong when it was announced and it is still wrong today,” Helen Mountford from the World Resources Institute (WRI), an environmental organization, told BBC. “Simply put the US should stay with the other 189 parties to the agreement, not go out alone.”

President Trump had made leaving the Paris Agreement a key part of his election platform in 2016. He included it into a vision of a revitalized US with booming energy production, especially coal and oil. His understanding was that the climate deal was unfair to the US, allowing developing countries like India to continue using fossil fuels. As Trump announced the decision to leave the Paris Agreement in 2017, a number of states and businesses have pledged to continue cutting carbon and to try and make up for Trump’s decision. They presented America’s Pledge program, through which states and cities would help cut US emissions by 19% compared to 2005 levels by 2025.

Now, climate activists and delegates are worried that the US withdrawal will see other countries adopt a go-slow attitude, at a time when scientists are saying that climate efforts should be speeded up. Countries such as Saudi Arabia and Russia have shown a willingness to side with US efforts to push back on climate science.

“They are biding their time, they are saying that if the US is not in then we don’t need to rush to do anything at this time’,” Carlos Fuller, lead negotiator from the Alliance of Small Island States, told BBC. “I think they are hedging their bets to see what kind of a better deal they can get out of it, and not actually withdraw.”

But the US involvement in the Paris Agreement isn’t necessarily over. The country could choose to return, and Democratic candidate Joe Biden has promised to do just that “on day one” if he wins the election. If he were to do so, the US could officially resume its leadership role under the Paris Agreement in mid-February.