Many changes in the Earth's climate that control relative sea level are caused by variations in the amount of incoming solar energy, which in turn are caused by systematic changes in the way the Earth orbits the Sun. These systematic changes in the amount of incoming solar radiation caused by variations in Earth's orbital parameters are known as Milankov-itch cycles, after the Serbian mathematician Milutin Milankovitch, who first clearly described these cycles. These changes can affect many Earth systems, causing glaciations, global warming, dramatic sea-level
changes, and changes in the patterns of climate and sedimentation.
Astronomical effects that influence the amount of incoming solar radiation include minor variations in the path of the Earth in its orbit around the sun and the inclination or tilt of its axis, causing variations in the amount of solar energy reaching the top of the atmosphere. These variations are thought to be responsible for the advance and retreat of the Northern and southern Hemisphere ice sheets in the past few million years and the associated huge sea-level changes. In the past 2 million years alone, the 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. The Earth rotates around the sun following an elliptical orbit. The shape of this elliptical orbit, called its eccentricity, changes cyclically with time over a period of 100,000 years, alternately bringing the 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 2 million years, suggesting that this is the main cause of variations within the present-day ice age.
The 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 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.
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. Presently the precession of the equinoxes is such that the Earth is closest to the Sun during the Northern Hemisphere winter. This precession changes with a double cycle, with periodicities of 23,000 years and 19,000 years.
Because each of these astronomical factors acts on a different time scale, their effects are combined in a more complex cycle. These factors interact in a complicated way, known as Milankovitch cycles. Using the power of understanding these cycles, it is possible to make predictions of where the Earth's climate is heading, whether into a warming or cooling period, and whether sea levels will rise or fall, or if some regions may experience desertification, glaciation, floods, or droughts.
Present data shows that temperatures were about 3-5°F (2-3°C) cooler at the height of the glacial advances 12,000 years ago than they are today and that temperatures may warm an additional 3-4°C by the year 2100. If this warming occurs as predicted, then large amounts of the glacial ice on Antarctica and Greenland will melt, raising sea levels dramatically. Many scientists predict sea levels will rise at least a foot (0.3 m) by 2100, others predict more. The sea-level rise will likely continue past the year 2100, with at least 16 feet (5 m) over the next few centuries. When this happens, most of the world's large port cities will be partly to largely underwater and world civilizations will have needed to find ways to move huge populations to higher ground. There is a current debate about how much humans are contributing to global warming and the consequent sea-level rise. Most data suggest that human-induced warming is about or slightly less than 2°F (1°C) over the past 100 years, but that warming is superimposed on the longer-term cycles described above. What is not known is how these long-term natural cycles may change. Warming may continue, or the natural cycles may reverse, or other sudden catastrophic cooling events may occur, such as a volcanic eruption on the scale of Tambora in Indonesia in 1815 that lowered global temperatures by about 2°F (1°C).
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