A "staunch environmentalist" refers to someone who strongly supports and actively promotes the protection and preservation of the environment. Full definition
Even staunch environmentalists tend to go green only as long as it's convenient. (fivethirtyeight.com)
Although the environmental community supports wind power in general, large wind farms near populated areas tend to generate substantial local opposition, often from staunch environmentalists. (issues.org)
She became a staunch environmentalist during her college years, so when she was pregnant with her daughter, she didn't even consider disposable diapers as a possibility. (realdiapers.org)