Tempers flared in Copenhagen today as protesters were beaten back by police outside the Bella Center, where the international climate talks were taking place. The police used tear gas to subdue the protests, and they arrested around 250 people.

But the turmoil wasn’t limited to the outside of the Bella Center. The chairwoman of the conference, Connie Hedegaard of Denmark, announced that she was stepping down and handing the reins to Lars Locke Rasmussen, the Danish prime minister. “With so many heads of state and government having arrived it’s appropriate that the Prime Minister of Denmark presides,” she said, calling the move procedural.

Amid the upheaval, some negotiators have grown bearish about the prospects for a climate agreement. “There are many details that aren’t getting resolved and I’m not sure two days is enough time,” said Tarja Halonen, the president of neighboring Finland. “The atmosphere here has declined.” But she remains “cautiously optimistic” that President Obama can breathe new life into the talks when he arrives on Friday.

Of course, Obama’s encouragement will mean little if the United States Senate is unable to pass a meaningful domestic climate bill. That’s why Sen. John Kerry (D-Mass.) took to the conference floor this morning in an effort to persuade delegates that American climate legislation is indeed forthcoming, echoing the message his colleague Barbara Boxer delivered two days earlier. But he also stressed the importance of commitments by China and other countries in order to convince reluctant U.S. senators that America will not bleed jobs by acting alone.

China, however, remains critical of any agreement that will allow an international team to inspect and verify its emissions regulations. That could be a major sticking point as negotiations move toward the finish line.

Despite all these hiccups, UN climate chief Yvo de Boer is emphasizing that there’s still plenty of room for optimism. “I still believe it’s possible to reach a real success,” he said. “The next 24 hours are absolutely crucial.”

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