David Battisti had arrived in Cambridge, Massachusetts, expecting a bloodbath. So had many of the other scientists who had joined him for an invitation-only workshop on climate science in 2007, with geoengineering at the top of the agenda. We can't take altering the atmosphere seriously, he thought, because there's no way we'll ever know enough to control it. But by the second day, with bad climate news piling on bad climate news, he was having second thoughts. When the scientists voted in a straw poll whether to support geoengineering research, filled with fear about the future, Battisti voted in favor of it. While the pernicious effects of global climate change are clear, efforts to reduce the carbon emissions that cause it have fallen far short of what's needed. Some scientists have started exploring more direct and radical ways to cool the planet, among them pouring reflective pollution into the upper atmosphere and growing enormous blooms of algae in the ocean. Schemes that were science fiction just a few years ago have become earnest plans being studied by alarmed scientists determined to avoid a climate catastrophe. In Hack the Planet, Science magazine reporter Eli Kintisch looks more closely at this array of ideas and characters, asking if these risky schemes will work and just how geoengineering is changing the world.