If there is one phrase in the global climate debate that sets my teeth on edge it is this: the claim by so many greens that the “science of global warming is settled.”Really? “Settled?” The essence of science is that it is never settled. Science is a frontier, not a town. That the science of astronomy was ‘settled’ is what the Inquisition said to Galileo. It is what the Ptolemaeans said to Copernicus.Far from being settled, real science (as opposed to religious dogma) is always being challenged — and often being changed. That is especially true when it comes to the science of complex systems. The NY Times, whose editorial pages bang on endlessly about settled climate science, brings us this story about unsettled science: cutting salt intake, it now appears, may not help people avoid heart trouble after all. New and better randomized trials say that the benefits from low sodium diets, while real, appear to be too small to have much impact on heart health. Was this because the diets don’t really work or because people can’t stick to them?Nobody knows and it’s likely the controversy will be around for a while. That’s what real science is like.Greens need to oversell the science because the messy, complicated truth is not enough to sell the world on a complex and extremely dubious policy agenda. But science is messy and the science of complex systems is messier still.That is the inconvenient truth.Oh, and by the way: that settled science about eight glasses of water a day? Forget about it, says a new study in the British Medical Journal.Funny thing, science.