Christiana Figueres’ announcement that she will step down this summer as head of the U.N. Framework Convention on Climate Change has sparked speculation about who might fill her shoes and help set the Paris climate agreement in motion.

The coming months will see countries nominate potential replacements for Figueres, to be vetted and assessed by U.N. Secretary-General Ban Ki-moon with consultation with parties. Already, though, observers are floating names from U.N. senior climate adviser János Pásztor to Youba Sokona, the Malian vice chairman of the Intergovernmental Panel on Climate Change, as possible candidates.

The executive secretary has huge responsibility and no authority, making it one of the most difficult jobs in the world.

Experts said whoever replaces Figueres will face steep challenges. On the one hand, the candidate must be steeped enough in the issue of climate change to oversee implementation of the agreement that parties reached on Dec. 12, 2015. On the other, he or she must possess enough diplomatic ability to serve as the public face of the U.N. climate process and to coax and prod parties to voluntarily increase the ambition of their targets before 2030.

“The executive secretary has huge responsibility and no authority, making it one of the most difficult jobs in the world,” said U.N. Foundation Vice Chairman Timothy Wirth.