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?
Variations in the Earth's orbit through time have changed the amount of solar radiation received by the Earth in each season (Figure 3). Interglacial periods, shown as the periods of higher temperature (shaded in yellow) in the Dome Fuji ice core from Antarctica, 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 1.8 million years). Since the middle Quaternary, glacial-interglacial cycles have had a frequency of about 100,000 years. 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. 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:
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