When the Paris climate talks opened earlier this month, many observers assumed the terrible costs of failure meant some kind of deal was assured.

Todd Stern, the chief U.S. climate change negotiator, was not one of them.

"When seeking legislation or agreement, there are people that are more interested in preventing things from happening," Stern told Reuters in an interview upon his return to Washington this week. "Blocking is easier than getting things through."

When the Paris talks started on Nov. 30, Stern sensed some countries were ready to settle on a "minimalist" agreement that would leave key issues to be resolved in the future.

He saw an opportunity to break through that reluctance by aligning the United States with a loose assembly of rich and poor nations that shared the long-term goal of getting the earth's average temperature to settle at 1.5 degree Celsius above pre-industrial levels by 2100.

In 2011, the European Union formed a loose alliance with small island states, a few Latin American countries and the 48 least-developed countries that would push for more ambitious goals in a 2015 climate agreement and challenge the north-south divide that has for decades hampered negotiations.

The U.S. decision to join the alliance added "rocket fuel" to the group, Stern said, breaking through the solidarity of a bloc of developing countries that clung to old positions that would make a new climate agreement hard to reach.

By the end of the talks on Dec. 12, 195 countries agreed on a pact that commits both rich and poor countries to curbing their greenhouse gas emissions and transitioning away from fossil-fuel dependence.


Stern sensed trouble well before arriving in Paris.

One month earlier, in Bonn, Germany, the last round of negotiations before the Paris summit ended on a decidedly negative note, making a final deal hard to imagine, he said.

Stern got reports from his team that the G77, the main bloc of developing countries, including China and India, was retreating to old, divisive positions, maintaining that rich countries should shoulder all of the burden of combating climate change, especially on finance.

For a Paris deal to be successful, rich countries had expected emerging economies to take on more.

That was when Stern decided the United Sates needed to counteract that negative "hardline tack" in Bonn by joining an alliance of rich and poor "progressive" countries pushing for a more "ambitious" deal, which would come to be known in Paris as the High-Ambition Coalition.

Stern first raised the idea of the United States joining at a dinner with other ministers in Paris on Nov. 8. The group met again in between the two weeks of the Paris summit on Dec. 6, where Stern said the United States would formally join.

In Paris, the group created a theatrical united front, marching together into plenary sessions and holding press conferences. With Brazil, which allies itself with emerging economies China, India and South Africa, joining the group late in the second week, the negative tone seen in Bonn changed.

"There was real sense of dynamism and force in this thing by that point," Stern said. "I think it was tactically and diplomatically quite important for the negotiations."

By Thursday of the second week of the talks, Stern said, it felt like pieces were falling into place for a deal and that by Saturday morning, "countries had basically made their peace" with the shape of a final agreement.

That goodwill also got the countries over a final hump, when what has been described as a drafting error - but with crucial legal implications for the United States - was discovered in the text and required last-minute agreement to fix.

Stern said Chinese negotiators were instrumental in averting a crisis.

"There was a lot of goodwill by parties to get past this and sign off on the agreement," he said.

At the end, Stern says, the United States got most of its key asks in the Paris agreement.

He said the Paris deal changes the architecture of previous climate agreements by moving beyond a world where only rich countries shoulder the burden of paying to solve climate change.

The agreement also creates a crucial "transparency" regime that requires both rich and poor countries to monitor, report and verify their emission cuts, but gives developing countries "flexibility" to do so.

It requires governments to review their targets in the next four years and decide if they can "update" them to make them stronger.

Stern said countries will need to come up with more detailed guidelines of how the transparency regime will work at next year’s U.N. climate summit in Marrakech.

"This came together in a really significantly more robust sense than a lot of people would have expected," he said.

After the Paris deal was concluded, the U.S. Congress voted to lift a 40-year ban on crude-oil exports and Indian and South Korean officials confirmed that they will continue to use coal-fired power plants.

Stern said regardless, the Paris agreement puts all countries on the same path toward lower-carbon development.

"The path does not involve the end of fossil fuels tomorrow," he said. "What it involves is a longer-term movement toward clean energy."

(Reporting by Valerie Volcovici; editing by Bruce Wallace and Dan Grebler)