> As for contrails, disturbing the atmosphere is going to cause some freezing (clouds) at that altitude, at that temperature. How do you suggest we mitigate that? Fly lower and burn more fuel? Fly less and tell people to take the train or that their package will arrive next week?

See this article: https://www.technologyreview.com/2024/03/12/1089620/how-rero...

Here are some highlights, in case the paywall is a problem for some.

First, how much do contrails matter?

> This cirrus-forming phenomenon could account for around 35% of aviation’s total contribution to climate change—or about 1% to 2% of overall global warming, according to some estimates

> A small fraction of overall flights, between 2% and 10%, create about 80% of the contrails. So the growing hope is that simply rerouting those flights could significantly reduce the effect, presenting a potentially high leverage, low cost and fast way of easing warming.

Breakthrough Energy, Google Research, and American Airlines conducted a test:

> They employed satellite imagery, weather data, software models, and AI prediction tools to steer pilots over or under areas where their planes would be likely to produce contrails. American Airlines used these tools in 70 test flights over six months, and subsequent satellite data indicated that they reduced the total length of contrails by 54%, relative to flights that weren’t rerouted

Avoiding the contrail prone regions would use more fuel, so the question is how much would it cost to avoid those regions?

> A new study published in Environmental Research: Infrastructure and Sustainability explored this issue by coupling commercial tools for optimizing flight trajectories with models that simulated nearly 85,000 American Airlines flights, both domestic and international, under various weather conditions last summer and this winter.

> In those simulations, the researchers found that reducing the warming effect of contrails by 73% increased fuel costs by just 0.11% and overall costs by 0.08%, when averaged across those tens of thousands of flights. (Only about 14% of the flights needed to be adjusted to avoid forming warming contrails in the simulations.)

There are also a couple of other factors to consider, such as:

> There are also some thorny complications that still need to be resolved, like the fact that cirrus clouds can also reduce warming by reflecting away short-wave radiation from the sun.

> The loss of this cooling effect would have to be tallied into any calculation of the net benefit—or, perhaps, avoided. For instance, Shapiro says the initial strategy might be to reroute flights only during the early evening and night, which would eliminate the sunlight-reflecting complication.

Another one is the the increased fuel will add to CO2 to the atmosphere. CO2 stays in the atmosphere a lot longer than water, so it is possible that reducing contrails would be a net positive in the short term but a net negative over the long term.