Variability of the Earth’s orbit: The astronomical theory - Ice Ages and Interglacials: Measurements, Interpretation, and Models - page 240

9

Variability of the Earth's orbit:
The astronomical theory

9.1 INTRODUCTION

A history of the origination and evolution of the astronomical theory (often
called the ''Milankovitch theory'') was first provided by Imbrie and Imbrie (1979).
Bol'shakov (2008) provided an interesting and valuable review of the various
contributors to the astronomical theory of ice ages, emphasizing the early innova-
tive ideas of James Croll who realized that orbital variations did not change total
solar input to the Earth very much, but rather changed the distribution of solar
energy by latitude. In addition, as Bol'shakov said: ''Croll was the first to
introduce Earth positive feedbacks that enhance insolation variations as a climatic
influence.''
It is well established that the Earth has undergone a long series of ice ages
interspersed with interglacial warm periods over the past 3 million years. The ice
ages were not continuous and were heavily interpolated by rather sudden large
swings in temperature, up and down. Nevertheless, aside from short-term fluctua-
tions, the data display quasi-periodic variations that span many tens of thousands
of years.
Quasi-periodic variations in Earth's orbital parameters change peak solar
intensity in summer at higher latitudes with periods of many tens of thousands of
years. The fact that peak solar input in summer to high latitudes and data on past
climate variations both show quasi-periodic variations with comparable time spans
suggests that the two may be inherently coupled. Spectral analysis (see Section
10.3) supports this viewpoint to some extent. It has been widely presumed that
this variability has a significant effect on the ability of surface ice and sea ice at
higher northern latitudes to withstand the onslaught of summer heat. It has
been theorized that during time periods when peak solar intensity in summer at
higher northern latitudes is lower than average, the lower solar input may trigger
feedback processes that lead to the spreading of ice cover and the start of ice