Britons vote Thursday on whether they’ll withdraw from the European Union. The latest polls show a virtual tie between the staying-in and the getting-out camps.

The effects of a possible British exit, or Brexit, on the world’s brand-new global climate-change agreement are complicated, but most advocates want to remain in the EU. (For more on why, read our explainer on what’s at stake for the climate.)

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Former New York City mayor Mike Bloomberg and outgoing U.N. climate head Christiana Figueres are in that camp, having argued that a Brexit would hamper global efforts to fight global warming, which rely on international collaboration.

“One lesson in 21 years of U.N. negotiations is this has to be done together; it cannot be done individually,” said Figueres, according to the website Climate Home. Bloomberg says a Leave vote will “leave the U.K., America and the rest of the world in a weaker position to combat terrorism, promote trade, and confront other global challenges including climate change.”

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The rest of the world is waiting to hear the verdict. The United States won’t know the results until late Thursday night.