Climate change denial will switch from being a litmus test for major Republican politicians to a liability in the near future.

At least that's the hypothesis that Todd Stern, the United States envoy on climate change, shared with a packed auditorium at Yale Law School in New Haven on Tuesday.

"We have all seen in recent years the abruptness with which hot-button issues can suddenly become the stuff of consensus," Stern told students, faculty and members of the public. "I doubt, even a year from now, whether major political candidates will consider it viable to deny the existence of climate change."

Stern's visit to Yale comes three weeks before a midterm election that has serious implications for U.S. involvement in climate treaty talks taking place in Lima this year and Paris in 2015.

Republicans and Democrats are fighting fiercely for control of the Senate. If the Senate falls into the hands of a Republican majority—as many analysts predict—the body charged with ratifying treaties would be led by Senator Mitch McConnell of Kentucky, who seeks to block EPA climate change regulations.

Stern said that mounting public pressure would eventually force American politicians' hand on global warming. The People's Climate March in New York City last month, he noted, was a start. Hundreds of thousands of people flooded the streets of Manhattan calling for action—any action—on climate change. The event took place 48 hours before world leaders, including Stern, gathered at the United Nations to lay the groundwork for climate treaty talks.

"It's a basic rule of politics that politicians listen to the voice of potential voters," he said. "When politicians come to believe that not listening could be detrimental to their political health, they listen."

It is unusual for a diplomat to talk about politics—even indirectly. And although Stern chose his words discreetly, the message was clear: Americans must get engaged in the political fight and persuade politicians that they need to shift gears.

But the climate movement today still isn't strong enough to solve the Obama administration's immediate problem: Whichever way the midterms go, it would be nearly impossible to get Congress on board with a climate treaty. So the administration favors an approach put forward by New Zealand that would not require emission reduction commitments to be legally binding at the international level, Stern told the Yale audience.

Each country would plot its own emissions course, ideally developing domestic climate regulations to give their pledges legal force. There would, however, be a legally binding accountability system, an "obligation to submit a schedule for reducing emissions" and report, review and update that schedule, Stern said.

That ought to be easier for the more hesitant nations to swallow, but it might not satisfy those who are more eager for a fixed set of emission reductions. This is especially true for developing countries, which argue that because rich nations bear historic responsibility for climate change, they must also bear the brunt of solving it.

Stern noted that one of the key obstacles to climate treaty success is the dated classification of countries. When the UN climate regime started in 1992, nations were split into two categories—developing and developed countries—which allowed emerging economies such as China and India to skirt binding carbon cuts that might slow their growth. But this two-decades-old division doesn't reflect today's realities, Stern said. (China, for example, the world's second largest economy, is still considered a developing nation.) The dividing line can best be erased through a system like the one he outlined, which would be based on every country doing its "best as it sees fit."

Stern also noted that a robust global fund of at least $100 billion for adaptation and mitigation activities in developing countries is vital to the success of any climate treaty.

While some "are sure to disapprove" of the more flexible approach to emissions reductions, Stern said, "we would counsel against that kind of orthodoxy." A Paris agreement that mandates binding emissions targets would never be accepted by some major countries, he said. Others would "low-ball" their commitments.

"A punitive system is just not going to be agreed to," he asserted. Better, he said, to go for "a stable, durable, rules-based climate agreement with legal force that is more ambitious than ever before, even if not ambitious enough."

Transcript of Todd Stern's speech:

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