What causes glacial–interglacial cycles?
Variations in Earth's orbit through time have changed the amount of solar radiation Earth receives in each season. Interglacial periods tend to happen during times of more intense summer solar radiation in the Northern Hemisphere. These glacial–interglacial cycles have waxed and waned throughout the Quaternary Period (the past 2.6 million years). Since the middle Quaternary, glacial–interglacial cycles have had a frequency of about 100,000 years (Lisiecki and Raymo 2005). In the solar radiation time series, cycles of this length (known as “eccentricity”) are present but are weaker than cycles lasting about 23,000 years (which are called “precession of the equinoxes”).
Interglacial periods tend to occur during periods of peak solar radiation in the Northern Hemisphere summer. 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. Nonlinear processes such as positive feedbacks within the climate system may also be very important in determining when glacial and interglacial periods occur.
Another interesting fact 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 Antarctic temperature record (black line), with abrupt warmings shown in yellow preceding more gradual coolings (Kawamura et al. 2007; Jouzel et al. 2007). Warming at the end of glacial periods tends to happen more abruptly than the increase in solar insolation. Several positive feedbacks 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 shows that the amount of atmospheric CO2 decreased during glacial periods (Kawamura et al. 2007; Siegenthaler et al. 2005; Bereiter et al. 2015), in part because the deep ocean stored more CO2 due to changes in either ocean mixing or biological activity. Lower CO2 levels weakened the atmosphere's greenhouse effect and helped to maintain lower 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
- 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
- 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
- Bereiter et al. (2015), carbon dioxide from the EPICA Dome C ice core in Antarctica