How are CO2 emission effects and high altitude effects of air travel compared to each other?
The CO2 is a greenhouse gas which causes a net heating effect as it retains additional solar radiation, the exact magnitude of this effect is still highly debated. The high altitude effects you've mentioned are often referred to as "jet exhaust shading" and it's estimated, based on some temperature data from the week after the 9/11 attacks in the US, that it causes roughly 4°C of cooling, this is because the jet exhaust contains water which condenses out as high albedo clouds, and sulfur compounds that absorb solar radiation as well as the CO2.