Chinese President Xi Jinping, center, President Obama, right, shake hands earlier this month as U.N. Secretary-General Ban Ki-moon looks on during a joint ratification of the Paris climate change agreement at the West Lake State Guest House in Hangzhou in eastern China’s Zhejiang province. (How Hwee Young/Pool Photo via AP)

Earlier this year, Republican presidential nominee Donald Trump sparked the ire of scientists and climate activists when he vowed to “cancel the Paris climate agreement” once in office. Now, hundreds of U.S. scientists have addressed this threat in an open letter warning of the perils of such action.

Signed by 375 members of the National Academy of Sciences, the letter explicitly refers to “the Republican nominee for President” and notes that “such a decision would make it far more difficult to develop effective global strategies for mitigating and adapting to climate change. The consequences of opting out of the global community would be severe and long-lasting – for our planet’s climate and for the international credibility of the United States.”  

The letter, published online Tuesday, coincides with Climate Week in New York — a gathering of governments, businesses, scientists and activists hosting dozens of events aimed at advancing the conversation around fighting climate change —  as well as a gathering of the United Nations General Assembly. And while organizers have noted that they did not intentionally time the letter’s publication this way, it nevertheless has added to ongoing conversations this week about the world’s climate future.

During his opening address to the general assembly, in fact, UN Secretary-General Ban Ki-Moon reiterated the importance of bringing the Paris Agreement into full force this year. In order to take effect, the agreement must be ratified by at least 55 countries representing 55 percent of global greenhouse gas emissions. Currently, 60 countries representing 48 percent of global emissions, including the U.S., have signed on. Thirtyone of these joined en masse Wednesday.

Experts expect even more countries may jump on board. But without continued U.S. cooperation in the future, the letter writers argue, the entire agreement could be in jeopardy.

“The U.S. withdrawal from the Paris Agreement would really set back all of our efforts to deal with climate problems,” said Kerry Emanuel, a professor of atmospheric science at MIT and one of the letter’s organizers. “American leadership is such an essential thing trying to solve climate problems.”  

He added that “if the administration that withdraws from the Paris Agreement is also going to withdraw from efforts to develop clean energy, then the U.S. will lose its leadership position in developing clean energy, and that’s perhaps one of the worst things that will happen.”

Ben Santer, another organizer and a climate scientist at Lawrence Livermore National Laboratory, pointed out that a withdrawal could send a poor message to other countries that have not yet committed to the agreement, suggesting that climate change “isn’t a real problem, this isn’t a serious problem — you shouldn’t care about it either.”  

Santer said such outcomes could fundamentally shake the international community’s ability to meet the most difficult task laid out in the Paris Agreement,  — the goal of keeping global temperatures within 1.5 degrees Celsius of their pre-industrial levels.

Prior to the Paris Agreement, the international community had focused on a goal of keeping temperatures within 2 degrees Celsius of pre-industrial levels. At Paris, the threshold was narrowed to “well below” 2 degrees, and the agreement also explicitly mentioned 1.5 degrees as an aspiration. Ways of meeting this more stringent goal has been one of the central conversations at this year’s Climate Week.

“The US is the second largest global emitter of global warming gases but its actions still rank first in terms of influence on the course of action taken by other countries,” said Michael Oppenheimer, a climate expert at Princeton University, in an email to The Washington Post. “If we withdraw, or even slow our implementation of the EPA regulations, other countries will take note and their own ambition in tackling climate change will likely decrease. Then the prospect of attaining 1.5 or 2 degrees or even 3 degrees would fade like a pipe dream.”  

On Monday, Oppenheimer participated in a panel event hosted by nonprofit climate science and policy institute Climate Analytics discussing both the importance and the feasibility of reaching the 1.5-degree target. There, he and other presenters noted that the difference between 1.5 and 2 degrees has considerable consequences in terms of climate impacts.

At the event, Carl-Friedrich Schleussner, a physicist with Climate Analytics and researcher at the Potsdam Institute for Climate Impact Research, pointed to a recent paper he co-authored, suggesting that the gap between 1.5 degrees and 2 degrees could mean substantial differences in the number of extreme heat and precipitation events around the world, water availability in certain regions such as the Mediterranean and even the ability of coral reefs to adapt to their changing environment.

And while scientists have debated how close we actually are to reaching that 1.5-degree threshold, many experts have indicated that it’s still theoretically possible to meet the mark. The problem is that recent analyses suggest the carbon-cutting pledges submitted by individual nations to the Paris Agreement are still not enough to get us there. This means that it’s imperative for nations to not only remain committed to the accord, but to actually step up their game even more.

“At present, a 1.5 [degree] limit appears to be a very difficult to achieve, not impossible but requiring focused efforts by governments and individuals to rapidly reduce utilization of fossil fuels through a combination of technological and lifestyle changes on an unprecedented scale,” Oppenheimer said by email.  

And therein lies the importance of continued U.S. involvement and leadership in the agreement.

“I believe scientists have a responsibility…to set the record straight and to speak out clearly and forcefully and say this is what we know we with confidence, this is why we should care about it and here are the likely outcomes if we do nothing,” said Santer, the letter organizer. “And the likely outcomes are very serious for all of us.”  

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