The exit of the United States, the world’s largest economy and second-largest greenhouse gas polluter would not dissolve the 195-nation pact, which was legally ratified last year, but it could set off a cascade of events that would have profound effects on the planet. Other countries that reluctantly joined the agreement could now withdraw or soften their commitments to cutting planet-warming pollution.
“The actions of the United States are bound to have a ripple effect in other emerging economies that are just getting serious about climate change, such as India, the Philippines, Malaysia and Indonesia,” said Michael Oppenheimer, a professor of geosciences and international affairs at Princeton, and a member of the Intergovernmental Panel on Climate Change, a United Nations group that produces scientific reports designed to inform global policy makers.
Once the fallout settles, he added, “it is now far more likely that we will breach the danger limit of 3.6 degrees.” That is the average atmospheric temperature increase above which a future of extreme conditions is considered irrevocable.
The aim of the Paris agreement was to lower planet-warming emissions enough to avoid that threshold.
“We will see more extreme heat, damaging storms, coastal flooding and risks to food security,” Professor Oppenheimer said. “And that’s not the kind of world we want to live in.”
Foreign policy experts said the move could damage the United States’ credibility and weaken Mr. Trump’s efforts to negotiate issues far beyond climate change, like negotiating trade deals and combating terrorism.
“From a foreign policy perspective, it’s a colossal mistake — an abdication of American leadership ” said R. Nicholas Burns, a retired career diplomat and the under secretary of state during the presidency of George W. Bush.
“The success of our foreign policy — in trade, military, any other kind of negotiation — depends on our credibility. I can’t think of anything more destructive to our credibility than this,” he added.
But Mr. Trump’s supporters, particularly coal state Republicans, cheered the move, celebrating it as a fulfillment of a signature campaign promise. Speaking to a crowd of oil rig workers last May, Mr. Trump vowed to “cancel” the agreement, and Stephen K. Bannon, Mr. Trump’s chief strategist, has pushed the president to withdraw from the accord as part of an economic nationalism that has so far included pulling out of the Trans-Pacific Partnership, a multilateral trade pact, and vowing to renegotiate the North American Free Trade Agreement.
Coal miners and coal company executives in states such as Kentucky and West Virginia have pushed for Mr. Trump to reverse all of President Barack Obama’s climate change policies, many of which are aimed at reducing the use of coal, which is seen as the largest contributor to climate change.
In a May 23 letter to Mr. Trump from Attorney General Patrick Morrisey of West Virginia and nine other state attorneys general, Mr. Morrisey wrote, “Withdrawing from the Paris agreement is an important and necessary step toward reversing the harmful energy policies and unlawful overreach of the Obama era.” He added, “The Paris Agreement is a symbol of the Obama administration’s ‘Washington knows best’ approach to governing.”
Although the administration has been debating for months its position on the Paris agreement, the sentiment for leaving the accord ultimately prevailed over the views of Secretary of State Rex W. Tillerson and Ivanka Trump, the president’s daughter and close adviser, who had urged the president to keep a seat at the climate negotiating table.
Other countries have vowed to continue to carry out the terms of the Paris agreement, even without the United States.
President Xi Jinping of China, the world’s largest greenhouse gas polluter, has promised that his country would move ahead with steps to curb climate change, regardless of what happens in the United States.
During a telephone call in early May with President Emmanuel Macron of France, according to the Chinese Foreign Ministry, Mr. Xi told the newly elected French leader that China and France “should protect the achievements of global governance, including the Paris agreement.”
But the accord’s architects say the absence of the United States will inevitably weaken its chances of being enforced. For example, the United States has played a central role in pushing provisions that require robust and transparent oversight of how emissions are monitored, verified and reported.
Without the United States, there is likely to be far less pressure on major polluting countries and industries to accurately report their emissions. There have been major questions raised about the accuracy of China’s emissions reporting, in particular.
“We need to know: What are your emissions? Where are your emissions?” said Todd D. Stern, the lead climate negotiator during the Obama administration. “There needs to be transparent reporting on countries’ greenhouse gas emissions. If the U.S. is not part of that negotiation, that’s a loss for the world.”
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