Rising greenhouse emissions may tip off aliens that we are a rapidly expanding threat, warns a report for Nasa It may not rank as the most compelling reason to curb greenhouse gases, but reducing our emissions might just save humanity from a pre-emptive alien attack, scientists claim. Watching from afar, extraterrestrial beings might view changes in Earth's atmosphere as symptomatic of a civilisation growing out of control – and take drastic action to keep us from becoming a more serious threat, the researchers explain. This highly speculative scenario is one of several described by scientists at Nasa and Pennsylvania State...