Prediction Test — A simple linear + seasonal model
(net(t) = a + b·t + c·cos(2πt/12) + d·sin(2πt/12)) is fit to each 10° block
using the first 250 months. The remaining months are predicted and compared to actual
observations. High RMSE months indicate nonlinear events: El Niño/La Niña, volcanic
aerosols, or abrupt cloud regime changes.