. in response to a 2012 ruling by the u.s . court of appeals district of columbia circuit requiring the epa to make a final determination on whether aircraft ghg emissions cause or contribute to air pollution , which may reasonably be anticipated to endanger public health or welfare , the epa announced in september 2014 that it is in the process of making a determination regarding aircraft ghg emissions and anticipates proposing an endangerment finding by may 2015