They have neatly divided their world — Bill is the academic theorist, Bob the legal mind and political pragmatist — but their work is intertwined.

“I tend to have lots of crazy ideas, and I run them by Bob first,” Bill said by phone from the Acela train between Boston and New Haven, Conn. He described himself as “an academic economist” who has stayed out of policy debates, although his ideas have not.

Bob agreed. “Bill’s work is about what needs to be done and how soon, using the tools of economic analysis,” he said over a recent lunch in Washington. “My work is: How do you convert that into a legal and regulatory policy?”

The two have a friendly rivalry, but both believe that cutting carbon pollution is crucial to protecting the environment and the economy from the risks posed by climate change. They also agree on the best way to do it: A Bill-style carbon tax, they say, would be far more effective and efficient than a Bob-style regulation.

Their story starts in Albuquerque, where their father, the grandson of a wealthy Santa Fe merchant, started the ski resort at the top of Albuquerque’s Sandia Peak and with a partner built the city’s iconic tram up the granite cliffs to get there. A specialist in energy and Native American law, Robert Nordhaus Sr. won a Supreme Court case giving Apache tribes the authority to levy fees on the oil companies that drilled on their native land.