Northern Hemisphere tilted toward the Sun at aphelion

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Orbital variations that lead to variation in the amount of incoming solar radiation, including eccentricity, obliquity (tilt), and precession of the equinoxes the past two million years alone, Earth has seen the ice sheets advance and retreat approximately 20 times. The climate record as deduced from ice-core records from Greenland and isotopic tracer studies from deep ocean, lake, and cave sediments suggests that the ice builds up gradually over periods of about 100,000 years, then retreats rapidly over a period of decades to a few thousand years. These patterns result from the cumulative effects of different astronomical phenomena.

Several movements are involved in changing the amount of incoming solar radiation. Earth rotates around the Sun following an elliptical orbit, and the shape of this elliptical orbit is known as its eccentricity. The eccentricity changes cyclically with time with a period of 100,000 years, alternately bringing Earth closer to and farther from the Sun in summer and winter. This 100,000-year cycle is about the same as the general pattern of glaciers advancing and retreating every 100,000 years in the past two million years, suggesting that this is the main cause of variations within the present day ice age. Presently, we are in a period of low eccentricity (~3 percent) and this gives us a seasonal change in solar energy of ~7 percent. When the eccentricity is at its peak (~9 percent), "seasonality" reaches ~20 percent. In addition, a more eccentric orbit changes the length of seasons in each hemisphere by changing the length of time between the vernal and autumnal equinoxes.

Earth's axis is presently tilting by 23.5°N/S away from the orbital plane, and the tilt varies between 21.5°N/S and 24.5°N/S. The tilt, also known as obliquity, changes by plus or minus 1.5°N/S from a tilt of 23°N/S every 41,000 years. When the tilt is greater, there is greater seasonal variation in temperature. For small tilts, the winters would tend to be milder and the summers cooler. This would lead to more glaciation.

Wobble of the rotation axis describes a motion much like a top rapidly spinning and rotating with a wobbling motion, such that the direction of tilt toward or away from the Sun changes, even though the tilt amount stays the same. This wobbling phenomenon is known as precession of the equinoxes, and it has the effect of placing different hemispheres closest to the Sun in different seasons. This precession changes with a double cycle, with periodicities of 23,000 years and 19,000 years. Presently the precession of the equinoxes is such that Earth is closest

(Opposite) Milankovitch cycles related to changes in eccentricity, obliquity (tilt), and precession of the equinoxes. All of these effects act together, and the curves need to be added to each other to obtain a true accurate curve of the climate variations due to all of these effects acting at the same time.

Butterloch Canyon, Dolomites, Italy, showing cyclic layering interpreted as reflecting different environmental conditions caused by Milankovitch cycles (CORBIS)

to the Sun during the Northern Hemisphere winter. Due to precession, the reverse will be true in ~11,000 years. This will give the Northern Hemisphere more severe winters.

Because these astronomical factors act on different time scales, they interact in a complicated way, known as Milankovitch cycles, after Milutin Milankovitch. Using the power of understanding these cycles, we can make predictions of where Earth's climate is heading, whether we are heading into a warming or cooling period, and whether we need to plan for sea level rise, desertification, glaciation, sea level drops, floods, or droughts. When all the Milankovitch cycles (alone) are taken into account, the present trend should be toward a cooler climate in the Northern Hemisphere, with extensive glaciation. The Milankovitch cycles may help explain the advance and retreat of ice over periods of 10,000 to 100,000 years. They do not explain what caused the Ice Age in the first place.

The pattern of climate cycles predicted by Milankovitch cycles is made more complex by other factors that change the climate of Earth. These include changes in thermohaline circulation, changes in the amount of dust in the atmosphere, changes caused by reflectivity of ice sheets, changes in concentration of greenhouse gases, changing characteristics of clouds, and even the glacial rebound of land that was depressed below sea level by the weight of glaciers.

Milankovitch cycles have been invoked to explain the rhythmic repetitions of layers in some sedimentary rock sequences. The cyclical orbital variations cause cyclical climate variations, which in turn are reflected in the cyclical deposition of specific types of sedimentary layers in sensitive environments. There are numerous examples of sedimentary sequences where stratigraphic and age control are sufficient to be able to detect cyclical variation on the time scales of Milankovitch cycles, and studies of these layers have proven consistent with a control of sedimentation by the planet's orbital variations. Some examples of Milankovitch-forced sedimentation have been documented from the Dolomite Mountains of Italy, the Proterozoic Rocknest Formation of northern Canada, and from numerous coral reef environments.

Predicting the future climate on Earth involves very complex calculations, including input from the long- and medium-term effects described in this chapter and some short-term effects, such as sudden changes caused by human input of greenhouse gases to the atmosphere and effects such as unpredicted volcanic eruptions. Nonetheless, most climate experts expect that the planet will continue to warm on the hundreds-of-years time scale. However, based on the recent geological past, it seems reasonable that the planet could be suddenly plunged into another ice age, perhaps initiated by sudden changes in ocean circulation, following a period of warming. Climate is one of the major drivers of mass extinction, so the question remains if the human race will be able to cope with rapidly fluctuating temperatures, dramatic changes in sea level, and enormous shifts in climate and agriculture belts.

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