What Changes in Seasonality Should Arise as Consequences of Milankovitch Cycles
Our lives literally revolve around cycles: series of events that are repeated regularly in the same order. There are hundreds of different types of cycles in our world and in the universe. Some are natural, such as the change of the seasons, annual animal migrations or the circadian rhythms that govern our sleep patterns. Others are human-produced, like growing and harvesting crops, musical rhythms or economic cycles. Cycles also play key roles in Earth’s short-term weather and long-term climate. A century ago, Serbian scientist Milutin Milankovitch hypothesized the long-term, collective effects of changes in Earth’s position relative to the Sun are a strong driver of Earth’s long-term climate, and are responsible for triggering the beginning and end of glaciation periods (Ice Ages).
Today, the Northern Hemisphere winter occurs near Perihelion, and NH summer occurs when the Earth is farthest from the sun. At present, the Southern Hemisphere has a tendency toward hotter summers , and with a more moderate seasonal cycle in the North, although that simple idea is complicated by differences in land distribution and thermal inertia between hemispheres. However, about 13,000 years ago, the Northern Hemisphere summer would occur when the Earth is closest to the sun, and NH winter when it is furthest from the sun (Figure 4). This would enhance the strength of the seasonal cycle. Precession varies on timescales of 19,000 and 23,000 years, and is thus important even over historical times. The precessional cycle is the key player behind the Holocene Climate Optimum, a time between ~7,000 and 5,000 years ago of particularly warm Northern Hemispheric extratropical summers, and colder tropical and extratropical winters. It is important to note that under the precessional cycle, the change in solar radiation striking the Earth is opposite in each hemisphere, unlike the case of obliquity where a higher tilt will mean more intense radiation at both poles as the planet revolves around the sun (although, obviously at the local summer summer for both poles, and thus at different points in the orbit). Furthermore, eccentricity modulates the effect of precession. For zero eccentricity, the precession angle is irrelevant.
Although Earth's orbit is never far from circular, terrestrial planets around other stars might experience substantial changes in eccentricity. Eccentricity variations could lead to climate changes, including possible "phase transitions" such as the snowball transition (or its opposite). There is evidence that Earth has gone through at least one globally frozen, "snowball" state in the last billion years, which it is thought to have exited after several million years because global ice-cover shut off the carbonate-silicate cycle, thereby allowing greenhouse gases to build up to sufficient concentration to melt the ice. Due to the positive feedback caused by the high albedo of snow and ice, susceptibility to falling into snowball states might be a generic feature of water-rich planets with the capacity to host life. In systems that do not form any giant planets and have no distant companions, there is no obvious mechanism by which large eccentricities should be induced in Earth-like planets. We therefore expect planets in such systems to have relatively circular orbits (Raymond et al. 2007).As a result, the existence of a low-temperature equilibrium climate might be a generic feature of water-rich terrestrial planets, and such planets might have a tendency to enter snowball states (Tajika 2008).
In sum, periodic variations in Earth’s orbit and rotation axis occur over tens of thousands of years, producing rhythmic climate changes known as Milankovitch cycles. The geologic record of these climate cycles is a powerful tool for reconstructing geologic time, for understanding ancient climate change, and for evaluating the history of our solar system, but their reliability dramatically decreases beyond 50 Ma.
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