The U.N.'s lead climate negotiator is being hailed a "hero" by climate activists, after saying Friday she would step down from her post this summer.

"Christiana Figueres is a climate hero," said Dan Lashof, the chief operating officer for NextGen Climate America, the environmental advocacy group founded by billionaire Tom Steyer.

Figueres is considered the key in bringing together 196 countries to agree to a nonbinding climate change pact in December in Paris. Achieving the deal is a key pillar of President Obama's second-term climate change agenda.

"Overcoming many obstacles, she shepherded the crucial Paris climate talks to a successful conclusion," Lashof said. "Ms. Figueres' vision was critical to the progress made in Paris — and we wish her the best in the next phase of her campaign for a healthy and just future for our climate."

The deal set countries on the path to reduce greenhouse gas emissions and invest in clean energy technologies to avert to worst effects of climate change. Many scientists blame the manmade emissions for raising the Earth's temperature, resulting in more severe weather, drought and sea-level rise.

A key piece in the U.S. meeting its obligations under the Paris deal are far-reaching climate rules for power plants, which 29 states and dozens of trade and industry groups are opposing in the D.C. Circuit Court of Appeals. Figueres' resignation comes just a week after the Supreme Court placed a halt on the rules, called the Clean Power Plan, marking a significant blow to Obama's climate agenda.

On Friday, states began filing initial briefs with the appeals court against the Clean Power Plan, in preparation for oral arguments in early June on the merits of their case.

The Louisiana Public Service Commission, the state's utility regulator, was the first to submit a "statement of issues" against the climate plan that will be detailed in subsequent briefs and arguments. The issues include whether the Clean Power Plan exceeds the EPA's authority under the Clean Air Act and whether the rule "usurps authority" granted to states under the Constitution.

A consolidated brief from all 29 states, with briefs from dozens of others opposing the rule, are expected to provide more substantive case documents into the evening.

In the weeks to come, the Obama administration will be fighting it out with the states through the briefing process.

The court has slated two days of oral arguments, June 2-3, since so many groups are suing the administration.