"Welp, it's the end. Apocalorgy, anyone?
After the attacks, the U.S. government temporarily grounded every plane in the nation (including crop dusters and those little planes that tow ad banners -- no, that is not a joke). Inbound flights that were already in the air were rerouted to other countries, and some airports stayed closed until as late as October, because as we all know, terrorists have only ever killed people in airplanes.
The problem with that is, unbeknownst to most, contrails (those white vapor trails that form out of airplane exhaust) act as mirrors, reflecting heat away from the Earth's surface. Those thousands and thousands of vapor trails have actually had a cooling effect that has counteracted global warming to some extent for years -- each airborne plane is sort of like Captain Planet zipping around with a UV shield tied to his boots.
But, unlike Captain Planet, only assholes want to hurt planes.
With so many planes grounded all at once, the atmosphere suddenly found itself without the mirror it had grown accustomed to over the past half century, and the temperature in the United States increased by 2 degrees in just three days. That may not sound like much, but consider this: Global warming typically only causes about a 0.5-degree increase every year. Grounding all of the planes caused a 2-degree increase, 400 percent of the average annual rate, in three goddamned days.