Weather and climate forecasting confront different problems. I can't predict the behaviour of a single gas module but I can predict the behaviour of the gas as

Weather and climate forecasting confront different problems. I can't predict the behaviour of a single gas module but I can predict the behaviour of the gas as a whole as the randomness averages out. Climate forecasting is a lot more complex a system but the daily variability, at least, averages out in the same way. (Another analogy some people use is that we can predict that winter is colder than summer even if some days in the season aren't etc. this isn't really an argument for modeling but I suppose it's easy to grasp.)