Comment by c1ccccc1 | Hacker News Reader

Comment by c1ccccc1 | Hacker News Reader

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Thanks for replying with details. The difference for liquid water is small, but for ice it looks like it's actually about 7%, which could be significant. Looks like what's going on is that the constants used are for different temperatures. The original constant was for ice at 0C, while the new constant is for ice at -15C. I'm guessing this is probably based on the thought that more ice on the planet is near -15C than is near 0C. Ideally for the best accuracy you'd compute the heat capacity using interpolation and a lookup table, but I can see how it would be a lot harder to enforce energy conservation that way. It would be interesting to see a sensitivity analysis on how these kinds of decisions affect the outcome of the simulation.

They have a real issue of choosing physical constants which aren't really constants, some models get it more right, others get it wrong and some are just unreadable so I couldn't get a conclusion.

There are also other issues, some code is clearly untestable, the whole thing is written in incoherent merged up pieces so you might be using different constants / calculations for the same grid location from different modules. NASA GISS code looks even worse, but I couldn't link to GitHub it's only available as tar gz download. They have even worst stuff there.

And remember the entire calculation is meant to measure the effect of changing CO2 which is only 0.0042% of the atmosphere. You need very high accuracy in everything in the first place to get the real effect rather than the model drifting into an alternative climate of imaginary world with different constants. And then remember that the real system is chaotic and the only signal that they claim to get is the average over many different simulations all drifting in average temperatures, over decades. The way the code is written there's just no way it's suitable for these kinds of predictions. And actually in most cases these models weren't built to predict climate over decades.

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