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Comment by crznp | Hacker News Reader

Their fuels burn the same as conventional fuels (that seems to be the point). The fuel needs carbon to have the right properties and when you burn it, the carbon atoms have to go somewhere. CO2 is the best option, carbon monoxide and soot are worse.

It sounds like the change is not how it burns, but how the feedstocks are generated. If the carbon is being pulled from the air (instead of the ground) at the start, then the overall process can be "low life-cycle carbon emissions relative to fuels from crude oil" (from the bioforming link).

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H2O is actually a significant greenhouse gas from jets. Water is the most significant, giving Earth +30°C of warming, but most of the time we’re maxed out on that already. Still, in the dry upper troposphere, contrails make a pretty sizable contribution to AGW. Even in a carbon-neutral system, where you emit your GGs makes a difference.

TIL. Thnx for sharing. Does that make h20 worse or better than co2 as an aviation byproduct in respect to global warming.

Long term, CO2's worse, but that's probably not the best way to look at it.

Excess H2O (water vapor) falls out of the sky as rain, so it's fairly constant (ie. "maxed out"), and its long term concentration levels are a function of CO2 equivalent gases in the atmosphere. So if you increase CO2 levels, H20 levels will naturally increase as well, and vice versa.

Water gives slightly more radiative forcing than CO2 from airplanes, and the effects of NOx are negligible. But of course the water disappears from that altitude in hours, while CO2 has a long atmospheric residence time.

https://www.nature.com/articles/s41467-018-04068-0

The article implies that the radiative forcing is from the contrail clouds, not from the water per se.

Perhaps there is an opportunity for a fuel additive that minimizes the formation of cirrus clouds, if the clouds are the source of so much of the climate impact.

A very short term difference. We would not make much of a dent (or bump, if you are into being literal) in the thermal balance of our planet if our greenhouse gas activities did not involve unearthing large quantities of carbon that had been buried away "forever" in climate timescale. Years are not the issue, the problem is that everything we unearth won't re-bury itself in centuries and millennia.

The difficulty is in catastrophe curves. While the water from a single flight doesn't hang around long, there are always aircraft in the sky, so it's constantly getting refreshed and amounts to a constant thermal load. If that additional thermal load, on top of the other greenhouse gases, is enough to tip some nonlinear system past its tipping point, then we've still got a problem.

Fortunately, it's also its own solution: if we find we need to avert a catastrophe next week, grounding all flights today might do the trick. But of course it's very hard to predict a tipping point to that level of accuracy. If we had that sort of movie-plot threat in climate change we might actually see meaningful action.

So yes, while carbon dioxide and methane are terrible, awful, no good gases we shouldn't be dumping into the atmosphere with quite such abandon, choosing to maintain water vapour at a particularly vulnerable point in the system is also something of an unforced error.

You misspelled “radiative forcing” as “thermal load” but the analysis is correct. We did in fact shut down flights on 9/11 and that’s when we noticed the cooling. The tipping points are like, well, there are millions of them and they all interact with each other, thus computer models.

I feel unforced error is bit harsh too. Climate sustainability is a series of mostly hard problems, and aviation is one of the hard ones.

Water is a far more efficient insulator than CO2 at more than double heat retention. This is mathematically tolerable due to atmospheric life cycle duration. Water vapor has an atmosphere duration of about 8 days before recapture at the surface but CO2 remains in the atmosphere for 72 days.

Release of CO2 causes problems for much longer than 72 days. The bump from all our fossil fuel burning won't go away for many thousands of years.

In a net zero scenario, we should start to see improvement in a couple centuries. The ocean overturning time is roughly 500 years, and getting carbonic acid under the carbonate compensation depth can have a huge effect, dwarfing anything we do in the terrestrial biosphere like “planting trees”.

Yes, we start to see improvement, but then that plateaus. To get back to a preindustrial state would take a very long time, as I understand it.

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