"My concern is whether the transformation is going to happen fast enough to avert the worst impacts," Christiana Figueres said, referring to the global shift from carbon-polluting fossil fuels to green energy.
"Greenhouse gas emissions have to peak quickly and descend," she said in an interview to AFP, as diplomats wrapped up their first negotiating session since hammering out the landmark pact in December.
"It is a race against the clock."
Under the Paris Agreement, 195 nations vowed to hold average global warming to well under two degrees Celsius, and even 1.5 C if possible.
Barely 1 C of warming so far has fuelled a crescendo of devastating droughts, super storms and rising seas threatening the homes and livelihoods of tens of millions.
But the tally of national pledges to curb greenhouse gases still falls far short of the mark, and scientists say they must be rapidly strengthened to hit the Paris goal.
Frontline negotiators tasked with converting the political blueprint into a workable plan met in Bonn for the 10-day session, and will reconvene along side their ministers in November in Marrakesh, Morocco.
Stepping down in July, Figueres was feted by the climate diplomats in a joint work session.
During the emotional send off, she was even regaled in song to the tune of a 1970s Abba hit as a "Climate Queen".
Famously -- some would say stubbornly -- upbeat, Figures told AFP that her six-year mandate was also filled with harrowing, make-or-break moments.
"It was in response to the situation that I inherited, which was anything but optimistic."
After the Copenhagen debacle, it was an open question whether the UN could get climate talks back on track.
Each of the six end-of-year climate meets she oversaw was laced with "moments of crisis," Figueres said.
During the 2011 summit in Durban, South Africa, where the 2015 Paris deadline was set, the press was reporting that the negotiations -- deep into overtime -- had failed, she recalled.
"That's when you have to take a deep breath," she said.
