President Trump is expected to announce Thursday afternoon whether the USA will be withdrawing from the Paris accord - the historic global agreement, reached in 2015, to set targets for reducing greenhouse gas emissions and limiting the rise in average global temperatures.
Tesla and SpaceX boss Elon Musk said he would have "no choice" but to leave White House-backed business councils if Trump pulls out. Economist Marc Hafstead of Resources for the Future says if economic growth picks up, leaving the Paris deal may mean overall USA emissions drop only by 10 percent. Withdrawal would take effect a year after notification, which can be given at any moment.
The UNFCCC has almost universal membership and provides the underlying framework for worldwide cooperation to combat climate change. Its air is scarcely breathable in some regions; air quality in the U.S.is generally good. Presidents have unilaterally exited Senate-ratified treaties before, but it's rare and controversial.
Before taking office, Trump repeatedly dismissed climate change as a hoax, and suggested that Obama-era climate regulations put the U.S.at a competitive disadvantage.
The EU and China, at a summit in Brussels on Friday, plan to reaffirm their commitment to Paris no matter what Trump says.
But the accord has a number of detractors. The leaving party does not have pay a price for opting out of the deal.
The White House had signaled earlier in the week that withdrawal was likely, but Trump has been known to change his mind at the last minute on such major decisions.
And then, of course, there's the argument advanced by candidate Trump.
The ongoing debate within the Trump administration over whether to leave the Paris deal, as Trump pledged to do on the campaign trail, has become the subject of media fascination, intense corporate lobbying, and global vertigo.
Trump has said he will announce his decision on the climate deal later Thursday.
Trump had previously expressed his refusal to accept the science of climate change.
All but a very few scientists say the overwhelming majority of warming is man-made, as do dozens of scientific academies and professional societies. And he's already overturned several Obama-era efforts to reduce emissions.
Unlike its predecessor, the Kyoto Protocol, the Paris deal holds the same level of stringency in the contract for all the parties.
CEOs from 24 leading United States firms, including tech giants Apple, Facebook, Google, and Microsoft, and food maker Unilever also urged him to stay.
The Paris agreement is created to set the world on a path towards keeping the warming of the planet "well below" a 2 degrees Celsius (3.6 degrees Fahrenheit) rise above pre-industrial temperatures. It also sets out a goal of reaching a limit of 1.5 degrees Celsius if possible.
And the agreement relies on voluntary cuts in emissions, which is seen by some critics as a major weakness. Forty Democratic senators sent Trump a letter urging him to stay in, saying a withdrawal would hurt America's credibility and influence on the world stage. Climate change is real.
The doubters are in the minority.
The researchers, who are monitoring the so called Larsen C ice shelf, said on Thursday there was just 12km of ice left before a crack broke off a piece of ice two times the size of Juncker's home country, Luxembourg.
A Pact In Name, Or In Deed? The various Paris pledges by each nation were not actually enough to achieve that target. CBS News has learned the president has told his advisors he will withdraw the US from the agreement.
In his speech, Mr. Trump also criticised India and China for "unfair" terms.
"The U.S. has successfully bent its greenhouse gas emissions curve", Kate Larsen, of the economics research team Rhodium Group, told Christopher.
Still, Cohn said that the carbon levels agreed to by the prior administration "would be highly crippling to the USA economic growth", and if the president had to choose between limiting carbon and economic growth, "growing our economy is going to win".
But while the Paris accord isn't synonymous with US emissions cuts, that doesn't mean Trump's decision on the accord is meaningless.
Co-chair of the congressional caucus, the Sustainable Energy and Environment Coalition, Representative Paul Tonko said: "Pulling out of the Paris agreement would be a global embarrassment for the United States at the hands of this President", adding that even Shell and Exxon have supported the agreement.
The think tank Climate Interactive has calculated that the planet would warm about.3 degrees Celsius (.5 degrees Fahrenheit) more by the year 2100 if the US leaves the deal and continues on a "business as usual" greenhouse gas emissions trajectory - but that the warming of the planet could be even worse if a USA departure leads other countries to follow.
On April 30, Germany established a new national record for renewable energy use with 85 percent of all electricity produced in the county coming from renewable sources. It was the first legally binding global climate deal.