After warning Congress in 1988 that climate change posed a serious threat to the planet, he has spent much of the past quarter-century trying to persuade policymakers to take bold action to curb global carbon emissions.

In an e-mail Monday, Hansen wrote that he decided to step down “so that I can spend full time on science, drawing attention to the implications for young people, and making clear what science says needs to be done.”…

Climate activist Bill McKibben, co-founder of 350.org, said Han­sen, 75, decided to step down so he could engage in lawsuits and protests full time. Han­sen was arrested in February in Washington in a protest against the Keystone XL pipeline plan. He participated in the demonstration on a day off from his government job.