Comment by js2 | Hacker News Reader

Comment by js2 | Hacker News Reader

Parent Full thread

Q: If forecasts can’t get next week’s weather right, how can we trust predictions for decades or centuries from now?

A: Weather and climate are not the same. Weather is individual, day-to-day atmospheric events; climate is the statistical average of those events. Weather is short-term and chaotic and is thus inherently unpredictable beyond a few days. Climate is long-term average weather and is controlled by larger forces, such as the composition of the atmosphere, and is thus more predictable on longer timescales. For the same reasons, a cold winter in one region does not disprove global warming.

As an analogy, while it is impossible to predict the age at which any particular man will die, we can say with high confidence that the average age of death for men in industrialized countries is about 75. The individual is analogous to weather, whereas the statistical average is analogous to climate.

https://www.climatecommunication.org/questions/predictions/

OK, I'm going to play devil's advocate. That answer doesn't really seem all that good since it doesn't address the predictability at all.

To use the human lifespan argument, yes we can say the average lifespan for men is 75, but that's looking backwards. How good would we be at predicting future life expectancy? I'd say we're probably pretty bad at it.

Also, yes, we're trying to predict the average temperature of the earth with climate models. However, that average is determined by the climate in a number of different areas.

Someone please explain this to me.

I'll take a stab :)

You ask "how good are we at predicting future life expectancy?"

Unfortunately I can't do an analysis right now, so hopefully this qualitative discussion will help answer it for you.

If you look at historical life expectancy, grouped by cohort, you'll see a relatively smooth function with easily identifiable trends. So, group everybody into the year they were born, and graph their average lifespan.

Past trends do not guarantee future performance, however a smooth function implies an underlying order to these observations. This is the core thesis everything else stems from then; we assume there is a relationship between the year someone was born, and their average lifespan.

Now a function relating just the year they were born to their average lifespan is almost certainly too simplistic. Where they were born, their socioeconomic status, and so much more will affect that number. The problem we have is that many of these parameters are hidden to us. Even worse, we can't even say for sure what all the parameters are! What we can do, however, is estimate the effect of these parameters, and even estimate the effect of parameters we don't know exist.

This process is inherently fuzzy, as statistics and modelling from less than perfect knowledge must be, but the models that have been created have proved to have great predictive power. The existence and general profitability of the life insurance industry is evidence to that.

We use models all the time, and for the most part this goes unquestioned. Regardless of your model of the sun and planets, it better predict the sun rising tomorrow, or else it's got some pretty big gaps. If someone told you they predicted the sun would not rise tomorrow, you'd be rightfully discredulous. But you'd be as equally discredulous if someone told you it was impossible to predict the future, so we really don't know if the sun will come up or not.

We test our models on their predictive power. Even if our models were bad, we don't simply throw them away because they are not perfect. We work to refine them and make them more accurate. Asimov's essay on the relativeness of 'wrong' is well worth the read.

So, life expectancy is a decent model, and is constantly being improved upon as we learn more about the world. Something may come along and cause us all to die, or live forever, but that remote possibility is no reason to throw our hands in the air and say the whole exercise is pointless.

Similarly, our climate models may be inaccurate or not account for some unknown future event, but that is not a reason to stop refining them, nor a reason to say "we can't know the future so this is pointless."

I appreciate the response.

I understand that you can model something and constantly improve it, but that still doesn't inspire confidence in the model's predictive ability.

Maybe a better answer would be: "Yes, weather forecasts are often wrong, but climate modeling from 10 years ago accurately predicted today's global temperatures with a margin of error of +/- 0.5%."

I'd say that's a worse answer, because while it explicitly answers the question, it doesn't address the flawed assumption in the question itself: predictive failure in micro implies predictive failure in the macro.

ofc, answering the question as well would still be useful.

Can you provide any evidence that "climate modeling from 10 years ago accurately predicted today's global temperatures with a margin of error of +/- 0.5%." I don't believe that statement. Perhaps you were saying wouldn't that be great proof if it were true.

I'm not claiming that's true, but if it were, it would be strong evidence that the modeling is accurate.

Right, you might say that our models predict the sun will rise tomorrow with a high level of confidence but you would have to give a slightly lower level of confidence for the sun rising the day after tomorrow and still lower one million years from now. All models have diminishing predictive power.

I agree with you, however our model of the solar system gives us pretty good confidence about macro level events even into the distant future.

To the specific example of the sun rising - for the sun to stop rising, the earth has to either become tidally locked about the sun, or the earth or sun must be destroyed.

One article[0] says:

Scientists have reliable data on the Earth's rotational speed, based on observations of the sun's position in the sky during solar eclipses, going back some 2,500 years. Although the rotational rate hasn't declined smoothly, over that period the average day has grown longer by between 15 millionths and 25 millionths of a second every year. Even at the faster rate, it will take 140 million years before the Earth's rotation slows enough to necessitate a 25-hour day.

The real crux of this matter is predictive power. We can get a rough understanding of the predictive power of a model by asking the question "at what point does the expected error in our prediction make that prediction meaningless?" For example, how accurate does a population model need to be for us to be willing to use it to predict population distribution in 50 years? It's fair enough to say that our models are not capable of predicting the population distribution in 1 million years, but no model could and it's unlikely we would be able to utilise such a prediction even if it were made.

As I said before, the existence of a profitable life insurance industry is evidence that we are good at making lifespan predictions at the individual level, in aggregate. Every time a policy is written, it's like a wager is being made between the insured and the insurer. Yes the insurers lose some bets, and some years they may even lose a lot of them, but it's clear that the odds are stacked in their favour, for otherwise they would be unable to offer insurance at all.

If you were to bet on what the climate of this planet will look like next year, how would you make your prediction? If you were to offer odds, how would you structure the book to make sure you win?

Yes, the absolute accuracy of climate models is going to diminish over the next 1, 5, 10 years, so the error associated with a specific global average temperature prediction (for example) will increase. That doesn't mean the actual temperature is just going to fluctuate wildly! Next year the global average temperature may decrease. It is extremely unlikely that in 10 years time the average temperature will have decreased, even if there is drastic intervention. Any bookmaker looking to make a profit would be offering odds on how much the temperate increases, not merely on if it increased or not.

Yes, predictive power is going to decrease as time goes on. No, that doesn't mean models are useless, nor that we can't have predictions that become more likely as time goes on.

[0] http://www.popsci.com/jessica-cheng/article/2008-09/ive-hear...

How good would we be at predicting future life expectancy? I'd say we're probably pretty bad at it.

The fact that businesses selling life insurance make money over the aggregate, despite sometimes losing money in the individual, seems to imply we're decently good at it.

Only in the short-term.

Another analogy: we can't look at a single atomic of Uranium and tell when it's going to decay - but that doesn't stop us knowing enough about the bulk nucleonics of Uranium to be able to build sophisticated reactors.

Thank you, that helped quite a bit. It makes sense, as predicting when a single individual will die (a single area will have <x> weather), is different than, the observed trend is males live to avg. 75 years. Makes sense to the layman!

Reply on news.ycombinator.com