When all aircraft were grounded over the USA after 9-11, the temperature over the USA spiked 1.1 degrees above expected. It then fell back when flights resumed.

It turns out that the contrails left by aircraft bounce some of the solar radiation back into space before it reaches the ground, which stops it being converted into infra-red and being trapped in the atmosphere.

So if global aviation ceased entirely, it would help global warming along quite a bit. Of course, the emissions would be gone, but their effect is long-term...