Most democratic mayors can only do so much. They are charged with setting their city's agenda – but that agenda is usually only as good as the accompanying city council. During the conference, Parker pledged to lower Houston's emissions by 10% over the next two years.

"We can keep each other accountable," Tau said when asked about the role of this global gathering, as if the mayors present were AA sponsors. And maybe he is on to something there. Running a city has traditionally been about answering to residents, pounding the pavement and visiting public works projects, much like Tau's predecessor is said to have done. But as the world becomes more interconnected and cities grow beyond anyone's wildest dreams, mayors may increasingly seek new means of consolidating their power base.

Mark Watts, C40 executive director, has little doubt that one day, mayors will rule the world. "Mayors," he said, "make decisions every day that affect people's lives."