The United States on Wednesday formally left the Paris Agreement, a global pact forged five years ago to avert the threat of catastrophic climate change.

The move, long threatened by U.S. President Donald Trump and triggered by his administration a year ago, further isolates Washington in the world but has no immediate impact on international efforts to curb global warming.

There are 189 countries that remain committed to the 2015 Paris accord, which aims to keep the increase in average temperatures worldwide "well below" 2 degrees Celsius (3.6 degrees Fahrenheit), ideally no more than 1.5 C (2.7 F), compared to pre-industrial levels.

A further six countries have signed but not ratified the pact. Scientists say that any rise beyond 2 degrees C could have a devastating impact on large parts of the world, raising sea levels, stoking tropical storms and worsening droughts and floods.

The world has already warmed 1.2 degrees Celsius since pre-industrial times, so the efforts are really about preventing another 0.3 to 0.7 degrees Celsius warming from now.

"Having the U.S. pull out of Paris is likely to reduce efforts to mitigate, and therefore increase the number of people who are put into a life-or-death situation because of the impacts of climate change: this is clear from the science," said Cornell University climate scientist Natalie Mahowald, a co-author of United Nations science reports on global warming.

The Paris accord requires countries to set their own voluntary targets for reducing greenhouse gases such as carbon dioxide, and to steadily increase those goals every few years. The only binding requirement is that nations have to accurately report on their efforts.

"The beauty of this system is that nobody can claim they were bullied into some sort of plan," said Nigel Purvis, a former U.S. climate negotiator in the administrations of Presidents Bill Clinton and George W. Bush. "They’re not negotiated. They’re accepted."

The United States is the world’s second-biggest emitter after China of heat-trapping gases such as carbon dioxide, and its contribution to cutting emissions is seen as important, but it's not alone in the effort. In recent weeks, China, Japan and South Korea have joined the European Union and several other countries in setting national deadlines to stop pumping more greenhouse gases into the atmosphere than can be removed from the air with trees and other methods.

Democratic presidential candidate Joe Biden has said he favors signing the U.S. back up to the Paris accord. Because it was set up as an executive agreement, not a treaty, congressional approval is not required, Purvis said.

White House spokesman Judd Deere said the accord "shackles economies and has done nothing to reduce greenhouse gas emissions." Should the U.S. continue to remain outside the pact, it's likely that other countries will try to impose tariffs on U.S. imports – paving the way for fresh trade wars.

The EU is currently debating a package of proposals, known as the EU New Green Deal, that would include a so-called carbon border adjustment, aimed at preventing companies from dodging emissions reduction efforts in the 27-nation bloc by manufacturing goods in places without stringent measures.

Germany, which currently holds the EU's rotating presidency, said it was "highly regrettable" that the U.S. had left the pact. "It's all the more important that Europe, the EU and Germany lead by example," said government spokesman Steffen Seibert, citing the EU's goal of becoming the first climate-neutral continent by 2050.

Mahowald said she worries that with the U.S. out, China, which initially agreed to emission curbs in a two-nation agreement with the Obama administration, and other nations may decide they don't have to do as much to cut carbon pollution.

While the Trump administration has shunned federal measures to cut emissions, Seibert noted that U.S. states, cities and businesses have pressed ahead with their own efforts. In addition to condemnation from abroad, environmental and public health groups in the U.S. criticized Wednesday’s withdrawal.