Until we can travel by fireplace, Harry Potter-style, the only way to get from Los Angeles to London is by carbon-spewing jet airliner. One simple change can help: adjust the exit and entry points each nation sets for its airspace so that planes can fly in as straight a line as possible. Last year the International Air Transport Association negotiated a more direct route from China to Europe that shaved an average 30 minutes off flight time, eliminating 84,800 metric tons of CO2 annually. Unifying European airspace as a "single sky" could cut fuel use up to 12%. Pilots could also change the way they fly. Abrupt drops in altitude waste fuel, so experts are advocating "continuous descent" until the plane reaches the runway—where it could be towed instead of burning fuel while taxiing. Of course, the best way to reduce plane emissions is to fly less. At least until the fireplace is ready for takeoff.