For one thing, Barrett says, researchers still need to test, refine, and engineer systems that can reliably predict, with enough time to reroute planes, when and where contrails will form—all amid shifting weather conditions.
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.
In addition, any decreased warming from contrail avoidance must more than offset the added warming from increased greenhouse-gas pollution. This becomes a trickier question when we weigh whether we care more about short-term or long-term warming: not producing contrails delivers an immediate benefit, but any added carbon dioxide can take decades to exert its full warming effect and may persist for hundreds to thousands of years.
The new study, at least, found that even when additional greenhouse gases are taken into account, reducing contrails cuts net warming over both a 20-year and a 100-year timeline, though less so in the latter scenario. But that, too, would need to be evaluated further through additional studies.
Yet another open question is whether airspace constraints and traffic bottlenecks might limit airlines’ ability to regularly reroute the necessary flights.
As a next step, Breakthrough Energy hopes to work with airlines to explore some of these questions by scaling up real-world flights and observations.
But even if subsequent studies do continue to indicate that this is a fast, affordable way to ease warming, it’s still not clear whether airlines will do it if regulators don’t force them to. While the fuel costs to make this work may be tiny in percentage terms, they could add up quickly across a fleet and over time.
Still, the study’s authors assert that they’ve shown contrail avoidance could deliver “massive immediate climate benefits at a lower price than most other climate interventions.” In their view, this approach “should become one of aviation’s primary focuses in the coming years.”