>The CO2 emitted by aircraft only causes about half the climate impact of aviation. The other half comes from contrails— artificial clouds that are created by soot in the engine exhaust.

Then what does that mean?

I find it strange. Clouds happen naturally. Contrails are mini clouds (literally a cloud chamber), are we saying that all those “chemtrails” are pollution?

Or are we saying the unspent fuel particulate inside that they formed around is?

This is where this bizarro science is going off the deep end for me. As any object traveling through the atmosphere at that altitude, disrupting air, is going to form condensation and cloud trails. The more moisture in the atmosphere, the more trails. Sure there’s a little bit of unspent kerosene particles but hardly enough to even be a glycerin on a well working engine.

Are we suggesting changing flight routes and wasting more fuel (which pollutes more) to protect the ground from these 0.0000001% reduction in light cloud trails? Seriously. I want to know the science behind how this plays out.

I’m all for shutting down the black exhaust engines and cleaning up how we produce thrust. I’m all for that. This argument that clouds cause pollution is just wacky.

What about wingtips. Those cause trails (though not as pronounced as the engines turning at 12,000rpm), those contain no particulates and yet, they exist. Atmospheric science can explain a lot of what you see at 30,000ft (10,000m). This all sounds like NIMBY science posturing and pseudo-science to me.

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counters 2 days ago | parent [-]

> I find it strange. Clouds happen naturally. Contrails are mini clouds (literally a cloud chamber), are we saying that all those “chemtrails” are pollution?

Estimating radiative forcing is about measuring relative to a baseline. Here, the baseline is a world with no contrails. When you introduce contrails, you're introducing cloudy volumes predominantly made of ice crystals and occurring very high in the atmosphere. On the balance, these clouds re-emit more long-wave radiation (e.g. what' emitted by the Earth's surface) than they allow to escape the atmosphere.

Hence, these clouds have a small but positive net radiative forcing - meaning that aviation, by the way it leads to contrail formation, has at least this small radiative forcing on climate.

> As any object traveling through the atmosphere at that altitude, disrupting air, is going to form condensation and cloud trails. The more moisture in the atmosphere, the more trails.

Actually - it won't. We rigorously started studying contrail formation back in WWII when meteorologists tried to anticipate when bomber flights returning from mainland Europe might induce contrails and leave a path for intercept fighters to follow and shoot them down. As the science and understanding of vertical atmosphere thermodynamic structure and cloud microphysical structure has advanced in the ensuing 80 years, we have a much better understanding of when contrails are likely to form, versus when they aren't.

But don't take my word for it. Look up at the sky any time you hear an aircraft - sometimes you'll see a contrail, sometimes you won't. Contrails aren't a given when a jet flies high in the atmosphere.

(that's actually the entire basis for the Contrails/DeepMind team's work - avoid areas where contrails _are_ likely to form, to avoid that radiative forcing from the first part of this comment)

> Are we suggesting changing flight routes and wasting more fuel (which pollutes more) to protect the ground from these 0.0000001% reduction in light cloud trails? Seriously. I want to know the science behind how this plays out.

The science is pretty well developed at this point. You'd probably hit it in an undergraduate-level physical meteorology class. The missing detail that the Contrails team helped solve was improving forecasts of the key parameters involved here from weather models.

The whole point is that this is _another_ lever that flight planners could use to optimize their route planning. It's just one factor. It has trade-offs - although those trade-offs aren't always net negative (e.g. it's not a given that the "less contrail-y" route is also the "more fuel burn-y" one).

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reactordev 2 days ago | parent [-]

Finally some science, ok cool. So we're basing this on the baseline of no contrails what-so-ever. We measure the radiative photons? of the light refracting? through it and see it's long-wave radiation channeling properties scatter it rather than allow it through to relieve the earth's heat?

As for contrails, if they're likely to form in certain areas and not others, what's that coefficient? is it the moisture content? is it the jet fuel mixture? Since you say the Contrails program helps predict when these might occur, is that data being used to alter cruise paths?

It's been 20 years since I was a student at College so forgive me when I ask these questions. My grandfather flew bombers and my father flew fighters so I've known about exhaust and pollution from aircraft for a long time. I know the older aircraft pollute and the contrails from bombers (my grandfather had some stories) were an awful mix of frozen water vapor and diesel exhaust. Modern commercial jets are a lot more efficient.

Any route (with the exception of the SIDS and STARS) is charted across airways to the most direct route possible. Right?

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spuz 2 days ago | parent | next [-]

The link to the paper I gave is the latest research on contrails and the potential effect of contrail mitigation and answers all the questions you have asked here. The video I gave boils all that information down into an easily digestible format. It honestly baffles me that you start your reply with "Finally some science" when the very same information was already provided to you. If you want to reply to my comment without digging into the links I shared then you should specify that you haven't had a chance to look into them.

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counters 2 days ago | parent | prev [-]

> Finally some science, ok cool. So we're basing this on the baseline of no contrails what-so-ever. We measure the radiative photons? of the light refracting? through it and see it's long-wave radiation channeling properties scatter it rather than allow it through to relieve the earth's heat?

There are many different avenues you could pursue to estimate this, with different degrees of complexity and they all boil down to a radiative transfer calculation. You could start with an idealized, single-column model using a very simple approximation like a 2-stream approach, crudely parameterizing how a few bulk properties of a cloud in the column would influence the estimate of radiative balance at the top of the atmosphere. Such a model would let you explore a huge variety of factors.

To get more into the world of "real climate", you'd eventually scale up to an atmosphere model with a more complex 3D radiative transfer scheme, probably with more complex cloud microphysical representation and interactions with radiation. This still lets you do idealized experiments.

The next step up, you'd do what the DeepMind and Contrails teams have done, which is to use a standard Earth System Model ("climate model"). That just builds the complexity, and allows you to study realistic scenarios with counterfactuals (e.g. with or without additional contrail formation due to aviation). It's still going to be parameterized in some sense.

Ultimately, we'd look at real-world data top-of-atmosphere radiative estimates, which we conveniently measure from a variety of satellites (CERES would be the most well-known one). The problem is that we can't study the counter-factual very easily in the real world. We get a few isolated cases (like the aviation pause over CONUS after 9/11, or the global decrease in aviation during COVID), but there's so much inherent noise (from weather) that it's hard to infer any causality; you'd likely end up using that climate model from the last part to simulate the counterfactual in a weather scenario nudged to reproduce the real-world observations you have.

This would make a great Masters thesis, or maybe a slice of a PhD, but it's not much more complex than that.

> As for contrails, if they're likely to form in certain areas and not others, what's that coefficient? is it the moisture content? is it the jet fuel mixture? Since you say the Contrails program helps predict when these might occur, is that data being used to alter cruise paths?

It's the ambient thermodynamic and kinematic parameters. Namely, background water vapor and temperature, combined with local kinetic energy in the form of turbulence that you need to drive very local supersaturation. Given those parameters, different distributions of gases and particulates emitted from a jet engine may be more or less effective at nucleating ice - if the conditions are even right to do so.

It's worth pointing out that it's _very_ hard to capture these background conditions in weather models, hence why a major part of the DeepMind/Contrails project have been to use machine learning to try to constrain background circulation patterns most consistent with contrail develop as evidenced by high-resolution remote sensing and imagery. Some of the earliest work they did was to use these types of tools to generate data that allow us to better parameterize contrail formation in numerical weather/climate models, which is why they're able to turn this into a predictive and actionable problem.

Otherwise, there's an 80-year literature on this topic. I encourage you to read it. Kärcher's review paper in Nature in 2018 is a good starting point (https://www.nature.com/articles/s41467-018-04068-0).

> Any route (with the exception of the SIDS and STARS) is charted across airways to the most direct route possible. Right?

I'm not an expert in aviation route planning, but from personal experience I know that many other factors come into play with routing (namely avoiding weather, limiting time spent extremely remote / far away from airport or other infrastructure, traffic / network congestion, etc).

	▲	reactordev a day ago \| parent [-]
		This is why I love HN. My god man, thank you. I’ll have to look into this more.