I don't see why "weather forecasting" is similar to "brain simulation" in difficulty. The comparison would be more valid if the problem were "brain forecasting", that is to build an AI that would behave exactly the same as a particular human, but that's an even harsher measure of intelligence than the turing test that I doubt anyone would take seriously. I'm not saying the brain's easy to simulate, it's just the analogy here seems shaky. Where is the source of exponential difficulty?