The idea is that there's a mostly random high frequency medium amplitude component (weather), a somewhat predictable medium frequency high amplitude component (

The idea is that there's a mostly random high frequency medium amplitude component (weather), a somewhat predictable medium frequency high amplitude component (seasons), and a low frequency low amplitude component (climate). So if you observe for long enough, the higher frequency parts mostly average out to zero and you can get an idea of the low frequency component. Even without being able to predict the other components. (See also, http://en.wikipedia.org/wiki/Fourier_transform http://en.wikipedia.org/wiki/Fourier_transform )