The G20 is going to be all about the Paris Agreement. And that will make for some potentially very isolating moments for the United States.

When Donald Trump pulled the US out of the Paris Agreement last month, leaders worldwide warned that the country would become an outcast, the black sheep of the world. “America first,” it seemed, was increasingly becoming synonymous with “America alone.

The EU was particularly outspoken in its disapproval of Trump’s actions, and the first major fallout from Trump’s decision will likely arrive this Friday, when Trump  joins leaders from 18 other countries and the EU at the G20 summit in Hamburg.

German Chancellor Angela Merkel, the meeting’s host, has said the summit’s key focus will be climate change — and she has promised to press Trump on the issue. Addressing German parliament last week, she said she expected the talks to be “difficult.”

“Ever since the decision by the United States to leave the Paris climate agreement, we’ve become more determined than ever to make it a success,” Merkel told parliament on Thursday. “We’ve got to tackle this existential threat and we can’t and won’t simply wait around until everyone on the planet has been convinced that there is enough scientific proof that it is happening. The Paris Agreement is irreversible and non-negotiable.”

She didn’t mention Trump by name, but her words were clearly aimed at the American president, with whom Merkel had a strained relationship even before his withdrawal from the Paris Agreement. Trump has called her country “very bad” because of its trade policies, and has even criticized Merkel personally.

The G20 conference typically puts out a statement in which countries announce their position on issues — and Merkel will push for all leaders — besides Trump —  to sign onto a climate action plan and a statement affirming a strong commitment to the Paris Agreement. This will be a very profound demonstration of just how out-of-step the United States is with its European allies.

But other countries that rely heavily on fossil fuels or have a tense relationship with Merkel may choose to “side” with the US, and refuse to sign on to that statement. Saudi Arabia, Russia, Turkey and Indonesia all might distance themselves from the agreement during the summit, Lisa Friedman reports for The New York Times. Some observers would see that as an early sign of unraveling climate momentum following US withdrawal.

“It’s pretty clear that the other 19 countries are committed to moving ahead with Paris. No one else is joining president Trump and withdrawing,” said Alden Meyer, director of strategy and policy at the Union of Concerned Scientists. “That’s not to say that there aren’t elements of the [G20’s climate action plan] that aren’t controversial with some of these countries.”

Paris Agreement withdrawal a final straw

Part of Merkel’s get-tough rhetoric on Trump is for the benefit of German voters. The country goes to the polls in September, and Merkel’s chief opponent, Martin Schulz of the center-left Social Democratic Party, has accused Merkel of not taking an aggressive enough stance toward the US president. “The German chancellor must sometimes dare to be in conflict with the American president,” he told the newspaper Welt am Sonntag last week. “Up to now, she has only done that in abstract terms.”

But Merkel’s words also reflect a growing acknowledgement in Europe that the US is no longer the ally it once was. It is especially significant that climate change is the issue to which many European leaders are pointing as the source of his rupture. French President Emmanuel Macron said last week that he hoped the US would “return to reason” on climate change, and European Council President Donald Tusk said that, on climate, Europe would speak with one voice.

For many who follow international progress in the fight against climate change, the conclusion that the US is a climate antagonist has been a long time coming. The United States’ retreat from Europe in other areas — respect for free trade and NATO, for instance — are recent phenomena that have fueled transatlantic tensions. Not so with climate: Over more than a decade, the US has foiled climate negotiations again and again. Before the Paris Agreement there was the Kyoto Protocol, negotiated in the late 90s, which the US also refused to participate in — a decision for which the Bush administration also faced substantial blowback.

Not up for negotiation

This latest climate push by European leaders comes at a critical time for the Paris Agreement. A new article in the journal Nature by the group Mission 2020, headed by former UN climate commissioner Christiana Figueres, offers one analysis indicating the urgency of climate action. Global emissions have leveled off and are no longer rising, the authors write. But if they do not begin to fall by 2020, the Paris Agreement’s target of containing global warming to 2 degrees Celsius will become impossible to hit.

“There will always be those who hide their heads in the sand and ignore the global risks of climate change,” the authors write. “But there are many more of us committed to overcoming this inertia. Let us stay optimistic and act boldly together.”

European leaders — who are also aiming to work more closely with China — are now signaling that the rest of the world will move on together to strengthen their commitment to the Paris Agreement, even if the US is no longer on board. Moves by Saudi Arabia, Russia, Turkey and Indonesia to back away from the agreement could throw a stumbling block in the way of that plan.

At a news conference later on Thursday, after Merkel’s speech to parliament, she and Macron underscored that they would welcome the US to the table. “It’s pointless to isolate one country,” Macron said. Europe disagrees with Trump on climate, but, Macron said, “it’s always preferable to come up with joint statements.”

But when it comes to climate, Europe will, at the G20, attempt to set the terms of debate.

Because of waiting periods built into the Paris Agreement’s legal structure, the US will not be fully out of it until November 2020. Up until that time, Trump can reconsider his plans to pull out, and European and UN leaders hope he does. But Merkel’s statement that the Paris Agreement is “irreversible and non-negotiable” indicates little appetite in Europe for a new climate deal of the sort Trump alluded to in June, which he said the US would “begin negotiations to reenter either the Paris accord, or a really entirely new transaction with terms that are fair to the United States.” (For what it’s worth, the US has also done nothing to follow up on Trump’s pledge of renegotiation.)

So, until the US comes to the table, Merkel, Macron and EU leaders are making clear that the US will now face chillier relations on the continent — and, importantly, the country’s lonely position as the only country to enter, then quit, the Paris Agreement is to blame.