Agence France-Presse: US researchers are studying the steam from ships, condensation trails of airplanes and volcanic eruptions as they try to understand how and even if the fledgling science of geo-engineering could slow global warming. But where some researchers are forging ahead with the new science of tinkering with the atmosphere to change the climate, many others are warning that geo-engineering is untested, potentially dangerous and distracting the world from reducing greenhouse …