The U.N.'s climate chief said on Friday she will step down in July, at the end of a six-year term, and praised governments for reaching a 195-nation deal in Paris in December to shift the world economy from fossil fuels to cleaner energies.
Christiana Figueres, a 59-year-old Costa Rican, said she would not accept any extension of her term as head of the Bonn-based U.N. Climate Change Secretariat after what she called the historic Paris Agreement.
"We now move into a phase of urgent implementation," she wrote in a letter to governments, which agreed a goal in Paris to cut greenhouse gas emissions to net zero by 2100 by shifting from fossil fuels to renewable energy such as wind or solar power.
"The journey that lies ahead will require continued determination, ingenuity and, above all, our collective sense of humanity and purpose," she wrote in the letter, dated Feb. 12 and made public on Friday.
Figueres, a former Costa Rican climate negotiator, took over the U.N. job at a low point in 2010 after a summit in Copenhagen the year before collapsed in acrimony between rich and poor.
She patiently worked to build trust among governments with radically different interests, ranging from the United States and China to small island states worried by rising seas or OPEC nations fearing a loss of export revenues.
Nicholas Stern, of the London School of Economics who wrote a 2007 study about the economics of climate change, said Figueres had an "outstanding ability to see where we need to go as a world and to bring people together".
Her successor is likely to come from a developed nation - Figueres succeeded Yvo de Boer, of the Netherlands.
Separately, Hela Cheikhrouhou, head of the U.N.'s Green Climate Fund set up to channel money to developing nations to help them cope with climate change, also said on Friday she would stand down in September after a three-year term.
The fund has so far attracted about $10 billion in pledges from 43 nations, after repeated delays. The cash will help developing nations cut emissions and adapt to impacts such as desertification, heat waves, floods and rising seas.
Earlier this week, former French Foreign Minister Laurent Fabius, main host of the Paris summit, also quit a post presiding over U.N. climate negotiations. French Environment Minister Segolene Royal will take over the job, overseeing talks until the next annual talks in Morocco in November.
(By Alister Doyle, With additional reporting by Valerie Volcovici in Washington)