Paleo Slide Set: The Ice Ages
Photograph of James Croll from J. C. Irons (1896)
Adhemar's theory was seriously flawed, but it nevertheless contained the seed 
of a theory developed in the 1860's and '70s by Scottish scientist James Croll, 
shown here. There were three major points to Croll's theory. First, Croll argued 
that while the total amount of insolation received at a given latitude did not vary 
from year to year, the amount received in a given season for a given latitude could 
vary significantly from year to year because of changes in the earth's orbit. 
Second, he claimed that these seasonal variations were caused by two orbital 
phenomena known as precession and eccentricity. He was also aware that the tilt 
of the earth's axis changed, but he was unable to include obliquity into his 
calculations because the mathematical relationship for this aspect of the Earth's 
orbit had yet to be developed. Finally, Croll suggested that while the initial 
climatic effect of changes in the earth's orbit might be rather small, but that 
these changes were amplified significantly by climatic feedback mechanisms 
in the earth's climate system. To better understand Croll's ideas, it is helpful 
to learn more about earth's orbit and how it changes through time.

Photo Credits:
Courtesy of John Imbrie
Department of Geological Sciences, Brown University