Milankovitch Cycles in Paleoclimate

Milankovitch Cycles in Paleoclimate

Milankovich cycles are cycles in the Earth's orbit that influence the amount of solar radiation striking different parts of the Earth at different times of year. They are named after a Serbian mathematician, Milutin Milankovitch, who explained how these orbital cycles cause the advance and retreat of the polar ice caps. Although they are named after Milankovitch, he was not the first to link orbital cycles to climate. Adhemar (1842) and Croll (1875) were two of the earliest.

The major cycles of the Earth's orbit

The influence of these cycles on insolation (INcident SOLar radiATION) at different latitudes has been calculated by Berger (1991), and Laskar (1993). Below is Berger's solution for 65 degrees north latitude from the present to 1 million years ago. In the Northern Hemisphere, peak summer insolation occurred about 9,000 years ago when the last of the large ice sheets melted. Since that time Northern Hemisphere summers have seen less solar radiation.

Spectral analysis is used to examine the frequency distribution of these oscillations and their effect on climate. The figure below shows an evolutive spectra of Berger's insolation solution over the last 6 million years. With this method one can see how the strength of the orbital frequencies (0.024 = 41 ky; 0.043 = 23 ky; 0.052 = 19 ky) varies over time.

Phase Differences at Milankovitch Frequencies

The phase difference between two paleoclimatic time series is used to interpret processes that link Milankovitch-cycle-driven insolation changes with Earth's climate (Imbrie et al., 1993).

What is Phase?

The following figure shows three examples of the phase between two time series. In the top figure, two time series have different amplitudes but are exactly in phase (Phase=0). In the middle diagram, two time series are exactly out of phase (Phase=180). The bottom diagram shows the general case where one time series leads or lags a second time series. The magnitude of the lead or lag is the phase angle and can be positive or negative.


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