According to scientists, climate change is real. Humans have, are and will continue to affect this planet's weather with their CO2 shenanigans. This includes WW2, where there was a demonstrable human-created invention that had a measurable impact on the weather.
That invention is the bomber. There were many, but the one you're probably thinking of looks something like the Flying Fortress. This four-engine flying tank was the workhorse of the bombing war, and its contrails were responsible for a 0.8 °C decrease in air temperature around the bases from which it flew to go blast the hell out of the Nazis.
But you said cooler! I know. I know what I said. Contrails, long suspected of having some of the same effects as naturally occurring cloud cover, can cause both a decrease and increase in temperature. Unfortunately for us, it's mostly the latter that happens. [New Scientist, Image: Image: Popperfoto/Getty]