A climate signal of ~1500-yr quasi-periodicity (e.g., Bond events) has been found from the Arctic to Antarctica, in palaeoclimatic data derived from a range of environmental proxy records. Solar and lunar forcing have individually been suggested as the cause of this millennial-scale signal, although some contend that it is little more than stochastic resonance within the climate system. Also debated is whether this climate signal is forced by the ocean-atmosphere dynamics of the North Atlantic or the tropical Pacific, as well as relevant climatic teleconnections. The cause of this cycle is elusive as no known solar cycle of this length exists, whilst forcing due to lunar gravitation has been dismissed as being too weak.

The most likely cause of any periodic climate signal is an astronomical one, as demonstrated in Milankovitch cycles. This thesis explores a potential astronomical cause of millennial- and centennial- scale periodicities and variability, based on a combination of solar and lunar forcing. Conceptual and trigonometric models, physical models of insolation, solar irradiance, and gravitation, and astronomical data were used in this exploration. Identified as a possible cause of the ~1500-yr climate cycle is precession. Precession changes the timing of Earth’s seasons relative to our calendar and external reference points, such as the fixed stars and the closest point in Earth’s orbit to the Sun (the perihelion). The primary actors in precession are the Sun and Moon through their gravitational influence on the rotating Earth. Significant climatic impacts of precession are seen in the ~21-ky Milankovitch precessional cycle, the result of two interacting precessional cycles: equinoctial (associated with the seasonal/tropical year) and apsidal (associated the perihelion and anomalistic year). This thesis investigates precession at a high-frequency scale, enabling a more detailed tracking of the moving seasons relative to the moving perihelion at sub-Milankovitch scales through-out the last 5,500 yrs.

This research found a statistically-significant, strong positive correlation between solar irradiance reconstruction derived from Antarctic 10Be ice-core data and a normalised, chronologically-anchored model of superimposed astronomical cycles that emulates the ~1500-yr climate cycle. Pronounced millennial-scale signals were observed in Earth-Moon distances and gravitation data. Maximum forcing occurs at perihelion, close to lunar perigee and Bond events occur at these points during the range of the astronomical data. Previously identified potential components of the ~1500-yr climate signal, viz the 209-yr Suess de Vries cycle and a 133-yr cycle, can be clearly seen in the astronomical and physical data. Modelled, high-frequency, perigee-perihelion interaction by this research reinforces these results, reproducing and explaining the variability of millennial-scale climate signals. Such variability is also supported through the movement relative to the tropical year, in conjunction with mapping of Metonic lunation and perihelion positions.

Supported by multiple lines of evidence, the results of this thesis suggest that the Sun and Moon act together through gravitation and insolation to produce millennial-, centennial-and decadal- scale climate signals through tidal forcing of Earth’s atmosphere and ocean. Key mechanisms and components are precession, perihelion, perigee, lunation, and nutation (wobble of Earth’s axis). Key inferences from these results are that astronomical forcing influences radiocarbon chronological variability, such as marine reservoir values, variability and time lag in radiocarbon data, and also suggest that the 209-yr SdV cycle is caused by combined solar and lunar forcing rather than previously inferred solar variability.

Advisors: Assoc Prof Patrick MossProf Fred Menk (UON), Dr Lynda Petherick

Project members

Alison Kelsey

Alison KELSEY

PhD candidate - graduate 2018