Activist, environmentalist, author and former frequent magazine writer BILL MCKIBBEN has been warning readers about climate change's potential catastrophic effects since 1989 with the publication of his book The End of Nature. Last year he co-organized Step It Up 2007, a simultaneous 50-state protest against government inaction against global warming. He speaks about environmentalism Tues., MARCH 11, at the Holy Trinity Episcopal Church to coincide with the publication of the anthology The Bill McKibben Reader. Free. 7 p.m. 515 E. Ponce de Leon Ave., Decatur. 404-377-2622. www.billmckibben.com.