Glacial-Interglacial cycles

Figure 2. Comparison between summer ice coverage from 18,000 years BP (based on CLIMAP) and modern day observations. Note that when more water is locked up in ice, more land is exposed due to lower sea levels.

Large, continental ice-sheets in the Northern Hemisphere have grown and retreated many times in the past. Times with large ice-sheets are known as glacial periods (or ice ages) and times without large ice-sheets are interglacial periods. The most recent glacial period occurred between about 120,000 and 11,500 years ago. Since then, the Earth has been in an interglacial period called the Holocene (Figure 2). Glacial periods are colder, dustier and generally drier than interglacial periods. These glacial-interglacial cycles are apparent in many marine and terrestrial paleoclimate records from around the world.

What is the cause of glacial-interglacial cycles?

Figure 3. (Top) Solar radiation varies smoothly through time with a strong cyclicity of ~23,000 years, as seen in this time-series of July incoming solar radiation at 65°N (Berger and Loutre, 1991). (Middle) In contrast, glacial-interglacial cycles last ~100,000 years and consist of stepwise cooling events followed by rapid warmings, as seen in this time-series inferred from hydrogen isotopes in the Dome Fuji ice core from Antarctica (Kawamura et al., 2007). (Bottom) Atmospheric CO2 measured from bubbles in Dome Fuji ice shows the same pattern as the temperature time-series (Kawamura et al., 2007).

Interglacial periods tend to occur during periods of peak solar radiation in the Northern Hemisphere summer. As you can see in Figure 3, however, full interglacials occur only about every fifth peak in the precession cycle. The full explanation for this observation is still an active area of research. Non-linear processes such as positive feedbacks within the climate system must also be very important in determining when glacial and interglacial periods occur.

Another interesting fact shown in Figure 3 is that temperature variations in Antarctica are in phase with solar radiation changes in the high northern latitudes. Solar radiation changes in the high southern latitudes near Antarctica are actually out-of-phase with temperature changes, such that the coldest period during the most recent ice age occurred at about the time the region was experiencing a peak in local sunshine. This means that the growth of ice sheets in the Northern Hemisphere has an important influence on climate worldwide.

Why do glacial periods end abruptly?

Notice the asymmetric shape of the Dome Fuji temperature record, with abrupt warmings shown in yellow preceding more gradual coolings (Figure 3). Warming at the end of glacial periods tends to happen more abruptly than the increase in solar insolation. There are several positive feedbacks that are responsible for this. One is the ice-albedo feedback. A second feedback involves atmospheric CO₂. Direct measurement of past CO₂ trapped in ice core bubbles show that the amount of atmospheric CO₂ decreased during glacial periods (Figure 3), in part because more CO₂ was stored in the deep ocean due to changes in either ocean mixing or biological activity. Lower CO₂ levels weakened the atmosphere's greenhouse effect and helped to maintain low temperatures. Warming at the end of the glacial periods liberated CO₂ from the ocean, which strengthened the atmosphere's greenhouse effect and contributed to further warming.

Some important datasets related to glacial/interglacial cycles:

• Berger and Loutre (1991), calculated incoming solar radiation for the last 5 million years
• Peltier (1994), ice sheet topography since the last glacial maximum
• Petit et al. (1999), stable isotopes and trace gases from the Vostok ice core
• Lisiecki and Raymo (2005), benthic δ¹⁸O records used as a proxy for global ice volume
• Siegenthaler et al. (2005), carbon dioxide from the EPICA Dome C ice core in Antarctica
• Spahni et al. (2005), methane and nitrous oxide from the EPICA Dome C ice core in Antarctica
• Jouzel et al. (2007), stable isotopes from the EPICA Dome C ice core in Antarctica
• Kawamura et al. (2007), stable isotopes and trace gases from the Dome Fuji ice core

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