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Trump’s Paris decision comes today, with the world watching

President Trump says he will announce “very soon” if the U.S. will leave the Paris climate agreement. (Video: The Washington Post)

This story has been updated.

For a rare moment, climate change will be the subject in the news that everybody is watching.

That’s because President Trump has stated that at 3 p.m. Thursday, Eastern time, he will give a Rose Garden address outlining whether the United States will stay in the Paris climate agreement. The Washington Post, and many other outlets, have already reported that Trump is leaning toward exiting the deal, while cautioning that he could still change his mind.

Vice President Pence’s schedule was updated late Wednesday night to note that he will be attending the announcement, but Secretary of State Rex Tillerson, according to his public schedule, will not.

Tillerson has argued for keeping the United States in the Paris accord, as has the company he previously headed, ExxonMobil.

The Environmental Protection Agency did not immediately respond to a question about whether administrator Scott Pruitt, one of the biggest opponents of the deal in the administration, will be in attendance.

The 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 international vertigo.

The agreement itself, negotiated in late December 2015 and championed by the Obama administration, presents a rare case of near-total global unanimity.

President Trump has decided to pull the U.S. out of the Paris Agreement. Here's what you need to know. (Video: Daron Taylor/The Washington Post)

It was signed by more than 190 countries and has been fully joined, in a legal sense, by 147 so far. Those countries, including the United States, collectively account for the vast majority of the world’s greenhouse gas emissions that drive climate change.

It’s also heavily backed by U.S. and global corporations, including many fossil fuel majors such as Royal Dutch Shell, ExxonMobil and BP. Large corporations, especially those operating in international markets, have had years to get used to the idea that there are likely to be reductions on carbon emissions, and they have been adapting their businesses accordingly for some time.

Withdrawing the United States from the agreement would take years due to the accord’s legal structure and language, but such a move would weaken its goals almost immediately. The United States is the world’s second-largest greenhouse gas emitter and would otherwise have accounted for 21 percent of the total emissions reductions achieved by the accord through 2030.

The real question, however, is whether other countries would also proceed to weaken their commitments — or if they would stay the course and simply shun or ignore the United States — and what that means for the continuing warming of the planet.

The Paris agreement is designed to set the world on a path toward keeping the warming of the planet “well below” a 2 degrees Celsius (3.6 degrees Fahrenheit) rise above preindustrial temperatures, an amount of warming that scientists would consider “dangerous” climate change. We’re already halfway there, with even more warming already baked in due to a time lag in the full impact of greenhouse gases on the atmosphere.

The think tank Climate Interactive has calculated that the planet would warm about 0.3 degrees Celsius (0.5 degrees Fahrenheit) more by the year 2100 if the United States leaves the deal and continues on a “business as usual” greenhouse gas emissions trajectory, but other countries stick to their pledges. And that the warming of the planet could be even worse if a U.S. departure leads other countries to follow. One major emitter that could struggle with meeting its Paris pledge is Brazil, a nation where deforestation, a key contributor to climate change, has recently risen.

Hans Joachim Schellnhuber, a physicist who founded the Potsdam Institute for Climate Impact Research in Germany, agrees that the U.S. departing from Paris could make the planet a few tenths of a degree Celsius warmer, based on the assumption that the world carries ahead with its plans while the U.S. keeps on emitting greenhouse gases as usual. But he emphasizes that that’s no small deal — especially if the goal is ultimately to hold the planet to an even more ambitious 1.5 degree Celsius (2.7 degrees Celsius) temperature rise, another target cited in the Paris agreement.

“It would mean the remaining distance between the guardrails and where we stand right now would be halved by the U.S. contribution,” Schellnhuber said. “And this is significant, because it’s a narrow escape anyway.”

However, Schellnhuber says he thinks that other countries would not follow the U.S. out of the accord, and instead are likely to keep on pushing to cut their emissions — in which case they may be able to offset a loss of the U.S. to the agreement.

Still, the diplomatic repercussions of a U.S. withdrawal would be vast, as demonstrated when European leaders last week pushed Trump to stay in the Paris climate deal at the Group of Seven meeting in Italy. Trump appeared unswayed, and a communique coming out of the meeting pointedly failed to include the United States among G-7 countries backing the agreement.