[JackFr]

That is the difficulty. When your conclusions are based on models which seem to be infinitely malleable or at least have many "estimated" parameters, the models become unfalsifiable.

If you've got a conclusion which is invariant under all possible evidence, it stops being science.

[jhaywood]

The same criticisms are leveled at the standard model of physics. You're right that it's hard for a theory to be falsified if it can be molded to fit any data. But it still does need to be consistent with past measurements and future measurements and what we know of the physical principles that drive the climate.

It also isn't that difficult to demonstrate the underlying principles that are in action. The greenhouse gas effect is fairly easy to measure experimentally (it was first experimentally measured in 1859).

Modelling the climate accurately is a very hard problem but we don't have an untouched completely controllable alternate climate on which to perform experiments. It's still better than throwing up our hands and declaring understanding impossible.

[jtheory]

Climate change models must predict future climate-related stats to be falsifiable, and those that don't make accurate predictions must be rejected; that's science. The future continues to arrive, so climate change models continue to be falsified (or not).

It's certainly difficult to do; that is not the same as saying they are producing no valid & valuable information.