The orders of magnitude more nodes and inputs mean that there are orders of magnitude more opportunities for mistakes to be made in the models. It also increases the likelihood of failing to find a confounding variable. The upper bound on system complexity is factorial. I shouldn't need to explain how big that is when you start talking about many orders of magnitude.
That is why it is so difficult to predict the weather. But you don't need to predict the weather to predict changes to the global climate.
Just like you don't need much of a model to predict that if a rogue planet passed through the solar system, existing orbits would be perturbed. Calculating the precise perturbations would be complex, but not predicting their existence.
Likewise, it's difficult to predict exactly how more heat in the atmosphere will change local weather. But it's not hard to predict that heat will build up if the gases build up.
Just because you can make a simplification doesn't mean you have achieved understanding. That understanding might be correct, or it might not. Even if you think you know the error bars are small, that doesn't mean that they are.
Given that the earth has had temperature excursions of +4C and -4C in the past 100k years it seems that there are some kind of forces that keep the temperature within that range. What are they? Climatologists are saying "tipping points" and whatnot, but if you believe the very long climate history as measured by proxies you would have to admit that there certainly SEEMS to be something that stops the temperature from rising further, and something that stops it from falling further. What are those mechanisms?
That's what a planetary climate is: an energy equilibrium. But if a forcing or feedback changes, the equilibrium will change.
The history of Earth's climate shows the limits of these changes, but that doesn't mean there are guard rails keeping us safe. It's just a reflection of the physical limits of the forcings and feedbacks. Our orbit only varies so much. Our axial tilt only wobbles so much. The sun's output only varies so much. The atmosphere can only hold so much water vapor at a given temperature. Etc.
If the sun suddenly doubled in brightness, I doubt you would expect the climate to still stay within that +/- 4C range, right? Well, atmospheric gases are also a forcing, one that we know we're changing. Granted--not at that level, but change is change. Models help us think about the sensitivity to that change, but the equilibrium must shift, somehow.
Anyway, the Earth's climate could stay well within that +/- 4C range, and the warming would still cause mankind a ton of trouble if it happens too fast. Remember that these are global average energy levels...it doesn't take much to raise sea level a troublesome amount, for example, especially since societies have built right at the water line all over the world.
