Agence France-Presse: President Barack Obama warned Wednesday that climate change was a serious test for humankind, and told Americans small changes in their daily lives could make a big difference to the environment. Obama, who was later due to tour a plant which makes towers for wind turbines in Iowa to mark Earth Day, vowed that the United States would lead a global drive to cut greenhouse gas emissions under his watch. “Climate change presents a serious test for humankind, but it also provides …