New figures on the cost of contrail avoidance

Here’s a new paper from the Breakthrough contrails team, at long last giving an operational picture of what it’d cost to avoid contrails.

We’ve long known that moving a few percent of flights up or down by about 2000’ in response to humidity conditions could cut the climate impact of aviation approximately in half. That’s because when contrails seed artificial cirrus clouds, those cirrus clouds trap outgoing infrared light just like CO₂ does. 

But now we have a high-confidence assessment of the fleet-level cost of that intervention.

Given that all contrails represent ~2% of global warming, this indicates that ~1% of global warming is addressable today at under $1/tCO₂eq. I don't know of any other climate intervention with such a low upfront cost and such a high probability of success.

So why aren’t all airlines doing this yet?

You’d think convincing airlines to avoid contrails would be easy. It’d cost them about $5 per flight to cut their climate roughly in half, vastly cheaper than any low-carbon fuel. But no airline has yet committed to it. 

A pessimistic take is that contrails are a ‘sincerity test’ for the world’s willingness to engage in the most cost-effective climate action, and the world is flunking so far. 

Cynics often point out that corporate action on climate change is mostly marketing. Ken Caldeira’s “Breakeven Year” paper suggests that this almost has to be the case; since the physical time constants of the earth system are so long, action to mitigate climate change today won’t break even in terms of climate damage avoided until everybody working now is long retired. So the incentives for climate action today can only be either long-term altruism, or near-term marketing. The fact that  no airline yet avoids contrails suggests which one it actually is.

But trying to be charitable, there are a few sincere reasons people might have to hesitate on contrail control. 

For one, people might want a higher standard of proof for unconventional climate strategies like contrail avoidance – and we aren’t yet able to quantify things enough precision. After all, it takes some complicated math to equate the climate impact of cloud that lasts 8 hours with a gas that stays around for centuries, and the results aren’t always crystal clear. The most recent report of the Intergovernmental Panel on Climate Change estimates that contrails might warm the Earth anywhere from one-third to nearly double the central estimate.

Another factor could be the proximity to geoengineering. Any scheme for avoiding contrails implicitly credits  changes to the earth’s albedo, which some see as a stepping stone to more intrusive geoengineering projects. And the most cost-effective approach to contrail avoidance doesn’t eliminate all contrails, rather just the few percent that are most warming, deliberately leaving a smaller fraction of contrails that actually cool the planet. So even though avoiding harmful contrails is certainly “less geoengineering, not more”, it may break a seal on geoengineering activities that some would prefer to keep intact. 

There’s also the fact that many efforts to avoid climate change are in fact proxies for other issues that aren’t directly related to warmer temperatures. In all likelihood more people will die of air pollution this century than from warming. More land species will go extinct from agriculture-related deforestation than from warming. Since many other approaches to address climate change can’t help but address these other concerns as well, you could imagine someone coming from a sincere place in deemphasizing contrails, since contrails only address warming, and not other externalities they care more about. 

Maybe these reservations would have been worth thinking about if contrail avoidance was going to cost about the same as other climate interventions, high $10s or $100s per ton CO2 equivalent. But at $0.10 to $1? Hopefully this paper moves the needle to where airlines and flight planners see contrails avoidance for what it is, an opportunity too good to pass up.