Its tough at the top. Twenty of the 100 highest-grossing U.S. law firms have started practices advising companies on climate change, according to Bloomberg. The attorneys help clients finance clean-energy projects and lobby Congress, typically billing $500 to $700 an hour.

The move into climate-change law is gaining traction as Congress considers a mandatory carbon market to curb greenhouse gas emissions.

“Since the elections last November, climate change has had a higher profile as a political issue,” said Paul Gutermann, co- leader of Washington-based Akin Gump’s group, which comprises 50 of the firm’s 1,023 attorneys.

Gutermann’s team is helping clients including PG&E Corp. push U.S. lawmakers to establish a market that uses so-called carbon credits to penalize heavy polluters financially.

Baker & McKenzie, a Chicago-based firm with 3,335 lawyers, was a pioneer, creating a climate-change group a decade ago. It became profitable after two years, said Richard Saines, who heads the U.S. part of the practice.

“We saw this as one of the key international-law issues that would affect U.S.-based multinationals,” Saines said. “And that is now the case.”

I bet he is laughing all the way to the bank.