With a younger generation increasingly focused on the problem of rising global temperatures, prominent environmental activist and author Bill McKibben is launching a new grassroots movement to mobilize older Americans to combat climate change and to work on related social justice issues. McKibben, 60, has partnered with co-founders Akaya Windwood, a 65-year-old nonprofit consultant, and Vanessa Arcara, a millennial who worked at 350.org, to mobilize older Americans. Their new group, known as Third Act, is having a soft launch this week and a full-fledged rollout early next year. “Third Act is for people like me — that is to say...