The data is very limited and largely circumstantial, however there is some evidence that - in the immediate aftermath of 9/11 - the grounding of virtually all commercial air traffic over the continental US resulted in slightly lower temperatures. Some researchers suspect the reduction in water vapor being injected into the upper atmosphere by all those grounded aircraft was responsible.
That the average ground temperatures dropped slightly is pretty much certain, cause and effect is rather more questionable (particularly with a single data point).
Besides, "practical" and H2 powered aircraft don't belong in the same sentence given current technology. There is simply no practical way to contain the volume of hydrogen necessary for long range flight - the density of even liquid H2 is far less than Jet A - and keeping a large volume of cryogenic fuel on a commercial aircraft wouldn't leave much room for passengers and cargo (you're not going to be putting it in the wing).