> No, it's not! If you look at NGOs and charities, you find massively different numbers, even for flights with the same characteristics.

Aside from those two sentences, nothing you've written contradicts anything I'm saying, and it's mostly orthogonal to the point.

It is obviously true that a lot of factors can affect the weight of an airplane, and the CO2e impact of a flight will vary based on that weight. And weight is one of many factors, as you mention at the end. But just because something is complicated and may seem overwhelming does not mean that the academic community engaged in researching the topic doesn't understand it.

It isn't possible to know a priori what the impact of a specific flight will be, and it isn't practical to determine the exact ratio of impact of a first-class to economy passenger on one specific flight. But nobody is making that claim, since that isn't how people think about systems in any large-scale domain like this. Instead, we look at large data sets and examine statistical relationships.

Your comment seems to be imply that because the system is high-variance with many variables, it is impossible to understand it in useful ways. That is clearly not the case. If it were, we wouldn't be unable to make decisions about virtually anything in the world outside of extremely narrow domains, and rational policy-making would be impossible. The number of variables in this domain is nothing compared to public health, for example. We can talk about the effectiveness of condoms in preventing infection even though there are a million variables affecting it with numerous outlier cases. In the same way, we can talk about the impact of flying first-class. (In fact, we can do better re: flying because there we have better analytical modeling in addition to the datasets.)

You claim that one would find massively different numbers among researchers. That hasn't been my experience as I've sought ought out detailed information online. Here are just a few references as a starting point:

* 2008 Union of Concerned Scientists: https://www.ucsusa.org/sites/default/files/legacy/assets/documents/clean_vehicles/greentravel_report.pdf -- Chapter 2 is on air travel and very high-level, but Appendix B provides more detailed numbers across various parameters. It only considers CO2, not CO2e.

* 2009 academic study, Oxford prof: https://www.eci.ox.ac.uk/research/energy/downloads/jardine09-carboninflights.pdf -- finds that on average, there's a 2x difference in per-passenger emissions between the most dense and least dense seating configurations.

* 2013 World Bank study: http://documents.worldbank.org/curated/en/141851468168853188/pdf/WPS6471.pdf -- outlines analytical models that include the different variables you're mentioning (and many more), and examines variance in the results as those parameters are adjusted.

The results from all of these studies are consistent across broad parameters (in particular the impact of seating configuration). In fact, it's clear that my back-of-the-envelope calculation of a 69% increase due to first-class is pretty much the lower bound.

[Note, I said in my original answer that the impact of radiative forcing is still being understood, so I'm not talking about that. I intentionally said that CO2 impact is well understood at the start and called out CO2e as a distinct notion for that reason.]

> I'm going to make a bold claim: the CO2e per passenger kilometer can vary by at least one order of magnitude depending on the flight characteristics (airline, weather, route, direction, location, aircraft, etc) - before you take class into account!

That's not a bold claim at all; it's just irrelevant. A cross-country flight on an old plane with a couple layovers in bad weather that has to refuel unexpectedly and is only half-full will have an order of magnitude worse impact on the climate than the best case non-stop scenario. But if one asks, "what is the impact of flying first-class vs economy?", one is not asking "what is the impact of flying first-class in the best possible case vs flying economy in the worst possible case?".