The end of last month brought big news in the battle to rein in climate change. The U.S. Environmental Protection Agency announced that greenhouse gas (GHG) emissions from airplanes pose a threat to human health and the environment and therefore are subject to regulation under the Clean Air Act.The Act was originally passed in 1970 to combat air pollution in the form of airborne lead and mercury, sulfur and nitrogen oxides, carbon monoxide, particulates, and ground-level ozone — to name a few. It was updated in 1990 to include emissions that threaten the ozone layer, and again in 2009 to deal with emissions known to contribute to climate change.This announcement now clears the way for the EPA to develop rules to regulate aircraft emissions, much as the agency has done for emissions from cars and trucks. Aircraft are responsible for roughly 12 percent of all U.S. transportation-related greenhouse gas emissions, or a little over 3 percent of all U.S. GHG emissions.