The glacial cycle of temperature and greenhouse gases
Over the past 800 kyr, there have been eight glaciations characterized by large ice sheets on the continents and globally lowered temperatures as reflected in the Antarctic record. The glacial periods are separated by shorter and warmer interglacial periods, of which the present interglacial period, the Holocene, is one. Each glacial period lasted for 40-100 thousand years, whereas the interglacial periods are shorter, typically 5-40 thousand years.
This glacial cycle is known to be closely linked to periodic variations in the Earth's orbit, described by the so-called Milankovich theory (Read more at Wikipedia - opens in new window). The orbit of Earth's motion around the sun is described by three so-called orbital parameters that change over thousands of years due to the gravitational drag of the other planets in the Solar system. As the orbital parameters change, the tilt and orientation of the Earth's rotational axis and the elongation of its elliptical orbit change. As a consequence, Earth experiences variations in the amount and distribution of energy received from the Sun, and the glacial periods coincide with periods of low energy input.
However, the variations are rather small and cannot account for the large differences between glacial and interglacial temperatures. The fact that small differences in incoming energy result in large temperature differences can be explained by a mechanism involving the greenhouse gases in the atmosphere.
Generally, the pre-industrial greenhouse gas concentrations are seen to co-vary with the temperature. During glacial periods the greenhouse gas concentrations are low whereas in the interglacial periods, the concentrations are higher. It is commonly accepted that global temperatures and greenhouse gas concentrations are coupled through a so-called positive feedback mechanism. For example, at the termination of a glacial period, a small temperature increase caused by increasing solar energy input leads to a rise in the greenhouse gas concentrations due to gas release from oceans and continents. The increased greenhouse gas concentration, in turn, leads to a rise in temperatures through an increased greenhouse effect. The two processes thus reinforce each other. Similar but opposite mechanisms are believed to be active during glacial initiations.
When examined in more detail, the methane profile is seen to relate rather closely to the Greenland climate profile with very abrupt changes following the Greenland stable isotope record (i.e. Greenland temperature). For this reason, the natural methane concentration variability is believed to show a mainly continental response and to be closely coupled to Northern Hemisphere climate variability. CO2, on the other hand, reacts more closely to the Antarctic climate signal and is therefore believed to be more tightly coupled to ocean influenced Southern Hemisphere temperature variations.
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