Ernest Moniz says COP 21 is already a “substantial success.”

By Vivienne Walt ~ PARIS

At the tail end of the U.N.’s mammoth climate-change negotiations outside Paris, U.S. Energy Secretary Ernest Moniz told Fortune the 11-day talks had already produced “substantial success” and that the conference known as COP 21 would prove a turning point in tackling global warming.

Secretary Moniz, who is a nuclear physicist, also believes the administration could move ahead without Congress, despite its many critics. The Republican-controlled U.S. Senate has made it clear they will not ratify a binding treaty on climate change; so far, the U.S. has committed itself to reducing carbon emissions by about 26% by 2025.

On Wednesday, Secretary of State John Kerry announced the U.S. would double its grants to poor countries to work against climate change, from the current $430 a year to $860 a year. Still, that’s a fraction of the billions countries say they need to move away from their dependence on fossil fuels. The overall aim of COP 21 is to cap global warming at a maximum of 2 degrees Celsius warmer than it was before the Industrial Revolution 150 years ago, a level that scientists believe is the outer limit of what the planet can reasonably manage.

Kerry’s announcement was designed to mollify the deep divisions among delegates: Leaders of poor countries—most vocally Indian Prime Minister Narendra Modi—have said repeatedly that rich countries bear the blame for 150 years of polluting the planet, and therefore should shoulder the financial responsibility for solving the problem. The U.S.’s new funding “comes at a key point in the Paris climate negotiations,” says Alden Meyer, director of strategy and policy at the Union of Concerned Scientists. “The prize of an ambitious, comprehensive, and effective long-term climate agreement is within our grasp, if compromises can be found on the remaining crunch issues.”

Those ‘crunch issues’ include the unresolved differences evident Wednesday when the U.N. published its 29-page draft agreement, a prelude to the final one that 195 countries are expected to sign on Friday. Racing against the clock, activists gathered outside the closed-door negotiations. “Don’t compromise our future,” said one sign, as dozens of activists staged a protest sit-in the conference center on the edge of Paris.

Against that backdrop, Secretary Moniz sat down with Fortune. The following is an abridged version of the conversation.

Read this story on FORTUNE Magazine.