The moon and the sun influence Earth’s climate at different times of year, a new study suggests. Researchers have analyzed month-to-month changes in the temperature gradient between the poles. The changes varied every 11 years in winter — meaning the sun was the driving force with its 11-year solar cycle — and 18.6 years in summer — meaning the moon dominated then. Basil Davis, of the École Polytechnique Féderal de Lausanne in Switzerland and Simon Brewer of the University of Wyoming in Laramie describe the work in an upcoming Quaternary Science Reviews.[Science News 6/11/11]
A unified approach to orbital, solar, and lunar forcing based on the Earth’s latitudinal insolation/temperature gradient
Basil A.S. Davis and Simon Brewer
Abstract: Widespread empirical evidence suggests that extraterrestrial forcing influences the Earth’s climate, but how this could occur remains unclear. Here we describe a new approach to this problem that unifies orbital, solar and lunar forcing based on their common control of the Earth’s latitudinal insolation gradient (LIG). The LIG influences the climate system through differential solar heating between the tropics and the poles that gives rise to the latitudinal temperature gradient (LTG), which drives the Earth’s atmospheric and (wind driven) ocean circulation. We use spectral analysis of recent changes in the Earth’s LTG to support earlier work on orbital timescales (Davis and Brewer, 2009) that suggests the climate system may be unusually sensitive to changes in the LIG. Identification of LIG forcing of the LTG is possible because the LIG varies according to seasonally specific periodicities based on obliquity in summer (41 kyr orbital and 18.6 yr lunar cycle), and precession (21 kyr orbital cycle) and total solar irradiance (11 yr solar cycle) in winter. We analyse changes in the Northern Hemisphere LTG over the last 120 years and find significant (99%) peaks in spectral frequencies corresponding to 11 years in winter and 18.6 years in summer, consistent with LIG forcing. The cross-seasonal and multi-frequency nature of the LIG signal, and the diffuse effect of the LTG driver on the climate system may account for the complexity of the response to extraterrestrial forcing as seen throughout the climatic record. This hypersensitivity of the LTG to the LIG appears poorly reproduced in climate models, but would be consistent with the controversial theory that the LTG is finely balanced to maximise entropy.