“Cities are where climate change problems originate, and therefore that’s where the solutions are,” said Michael R. Bloomberg, the former mayor of New York City who is co-chairman of the Global Covenant of Mayors for Climate and Energy and the United Nations’ Secretary General’s special envoy for cities and climate change.

A number of cities have made substantial progress. In Miami Beach, where $400 million has been invested to deal with flooding, roads have been elevated and sea walls have been constructed. Susanne M. Torriente, the city’s chief resiliency officer, said the city had also recently completed its greenhouse gas inventory and now would aim to reduce its emissions, regardless of federal policy.

Whatever policies the Trump administration adopts, she said, “won’t really be a big change for us.”

Republican mayors also govern some cities that are especially vulnerable to climate change. James C. Cason, the mayor of Coral Gables, Fla., is working to protect the city from some of the flooding it is already experiencing and to prepare it for more flooding that will most likely accompany rising sea levels. Florida has a Republican governor, Rick Scott, who has questioned the cause and extent of climate change, but that has not stopped Mr. Cason and other Republican mayors in South Florida from making pragmatic decisions on the issue.

Cities have also seen lots of benefits from networks — the Compact of Mayors, which has been signed by more than 120 American cities, C40 Cities Climate Leadership Group, Climate Mayors and others — most notably, information sharing and solutions, and reaffirming commitments to each other and to their citizens, as many mayors did in a recent letter to the president-elect.

Depending on their state, however, cities are somewhat limited in what they can do from a legislative or regulatory standpoint.

Some policy experts and state officials maintain that state governments, if they are willing to act on climate or energy policy, are where the measurable progress is made.

In most states, governors and legislatures have the authority to regulate the two biggest sources of emissions: power plants and transportation.