Climate changes due to phase shift of the seasonal component of Earth's surface temperatures

Climate changes have been investigated by using the seasonal oscillation of monthly averaged temperatures recorded at 106 stations covering the whole USA. We found that the phase of the seasonal component of temperature is occasionally destabilized by the precession which is locally changing the balance between direct insolation and energy received by the Earth. This local effect can be described through a nonlinear circle-map, indicating that the process can be associated to an intermittent transition from quasi-periodicity to chaos. The net effect results in a cumulative phase-shift, in the last century, of about 1.4 days towards earlier seasons, in agreement with the phase shift expected from precession. Quite surprisingly, the local intermittent dynamics is also modulated by a periodic component of about 18.6 years due to the nutation of Earth.