@matt_black I am trying to place the source of my disquiet here. I am not sure I have distilled it down completely yet.

There is an issue that this is a summary metric of a complex area. By necessity, that means it has accuracy problems. If you treat maximising as the main goal, you will certainly miss better solutions. But, I think everyone who has given it more than 10 seconds thought agrees on that.

There is an issue about whether it is better to drive populations to act by using predigested simplistic data rather than overwhelming them with accurate data. This seems to be a "Why not both?" question to me. Pique interest with the simple data, and link to the scientific bases. See Wittgensteins Ladder and Lies-to-children.

But that is my opinion. What is the factual question for Skeptics.SE?

There is an issue about whether it is better to drive populations to act by using fear rather than hope. I imagine there might be a carefully Skeptics.SE question about the science behind that, but it has very little to do with this particular measure.

@matt_black I am trying to acknowledge and incorporate what I think you are implying with this line: improvements that have occurred recently, and the Footprint measure doesn't reflect that. Therefore the Footprint measure is both numerically wrong and also unfit for purpose (because it is promoting fear over hope). Is that right? Am I reflecting your argument accurately, or putting false words in your mouth?

Think along the lines of the GDP - it has lots of problems and is actually a really bad measure of things, but it gets used a lot since it is fairly easy for people to understand and policy measures to manipulate it are also fairly easy to understand as well.

Likewise, something like ecological footprints can be used to set national level agendas and can very coarsely be broken down to account for national contributions. Once you know that national contribution you can then proceed to say something like, "There's not much you can do other than diplomacy" or "Improving public transportation will reduce the countries carbon footprint." However, the understanding with the sustainability community is that these are really coarse measures and ...

... there is a lot of criticism aimed at them as a result when you start looking at things with a narrow focus.

@Oddthinking You are right that there are two different cancers here: one about how best to motivate the public (or politicians) to act for the good of the environment; the other about whether metrics like this have any scientific credibility. For this question I'd prefer to stick with the scientific credibility.

The underlying problem in doing good for the environment is that it is complicated. Some "obvious" actions (eg reduce air miles in transported food) don't stand up to detailed analysis. but those are the ones campaigners concentrate on because they play well in the public consciousness.

Eat more beef. I mean.... its good for cow numbers, eat more kangaroos too if you don't want them extinct

This, I think, leads to bad actions. Perhaps the public is only motivated to do anything if we give them simplistic messages (or just lie). That is a separate question (and I don't think it is true). Scientists sometimes need to say "its complicated". The overshoot metric is simplistic and I suspect based on bad science. This question was meant to be about that. I'd far rather people looked at gapminder or ourworldindata than relied on dodgy metrics that misdirect them on the state of the world.

@daniel ???

@matt_black There are lots of overly simplified proxy metrics that we use all the time. @anonymous suggests GDP. I'd include oven thermometers, bathroom scales, bank balances, megapixels, and the percentage of laptop batteries. They can be oversimplified AND based in science AND misleading AND useful all at once.

@Oddthinking Oven thermometers, battery charge meters bathroom scales given the purposes they are used for. They are not accurate but they are good enough to support good decisions. GDP is not perfect, but it is useful. OTOH, the meters scientologists use to diagnose your mental state are not based on science, are bullshit and are designed to mislead. Which is the overshoot metric?

I'm not against simple, approximate metrics. Bathroom scales weighing to the nearest kg or two, no problem; scales giving reading in g, misleading and useless; scales that completely miscalibrated and deliberately read 30kg too high to encourage you to lose weight, I don't think they help.

@matt_black [Off-topic rant: I discovered my bathroom scales deliberately hide their terrible inaccuracy, by including a hysteresis function. If a new reading is close enough to an old reading, it just displays the same value as last time. I would do 2-3 samples, to average them out, but they were always the same, which I attributed to being reliable, rather than the truth: super unreliable.]

@matt_black The Scientology E-meters are a good example. They clarified to me what you were concerned about. I am comfortable that Global Footprint is based on better science than that. But many people would argue that bathroom scales (even if accurate) are misleading - they measure the wrong thing, that is merely correlated with the right thing. I would put Global Footprint in the middle with that.

[I fully acknowledge this is me stating my opinion, without support, so I do not intend it to be convincing to you, and this doesn't belong in an answer.]

I am trying to work out if there is a way of properly expressing this issue in the question.