Croll corrected some of the conceptual mistakes that plagued Adhémar's work, but he also added important innovations of his own. First, Croll incorporated recent findings by the French
astronomer Leverrier demonstrating that the shape of earth's orbit changed slowly but consistently over time. As this graphic illustrates, the shape of the earth's orbit varies from nearly circular (eccentricity approaching 0.00) to more elliptical
(eccentricity=0.06). These variations occur at a frequency of 100,000 years (which Leverrier demonstrated) and 400,000 years (which later scientists discovered). Croll understood that variations in orbital eccentricity had a small impact on the total
amount of radiation received at the top of earth's atmosphere (on the order of 0.1%), but that the eccentricity cycle modulated the amplitude of the precession cycle. During periods of high eccentricity(a more elliptical orbit), the effect of
precession on the seasonal cycle is strong. When eccentricity is low (more circular), the position along the orbit at which the equinoxes occur is irrelevant because all points on the orbit become, in effect, perihelia.
As Croll's research
indicated, variations in the precession and eccentricity cycles altered the amount of insolation received at different times of the year, creating variations in the distribution of solar energy received from season to season. But other scientists
argued that these variations were too minor to explain the tremendous climatic oscillations that occurred as the earth alternated between glacial and interglacial periods. Croll's response to these scientists represented his third major innovation:
the concept of climatic feedback mechanisms in the earth's climate system.
Croll believed that the variation of winter insolation was the critical variable explaining climate change. When the precessional cycle placed the earth at its aphelion during the Northern Hemisphere’s winter, the northern winters would be significantly colder if this occurrence was coupled with a period of high eccentricity. Croll reasoned that snow would then begin to accumulate to a greater degree, eventually creating large snowfields and glaciers. As reflective snow and ice covered more of the Northern Hemisphere’s land area, the earth would absorb less solar radiation. The climate would cool further as glaciers and ice sheets reflected a great deal of solar energy back into space. Croll also speculated about another feedback effect, this one involving the position of warm currents in the Atlantic Ocean. As the northern latitudes cooled, the strength of the trade winds would increase, drawing them southward towards the equator. This in turn reduced the strength of the Gulf Stream as warm currents turned south rather than north as they flowed towards the bulge of Brazil.
Thomas G. Andrews
NOAA Paleoclimatology Program
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