Astronomical Forcing of the Climate

Medium-term climate changes include those that alternate between warm and cold on timescales of 100,000 years or fewer. These medium-term climate changes include the semiregular advance and retreat of the glaciers during the many individual ice ages in the past few million years. Large global climate oscillations that have been recurring at approximately a 100,000-year periodicity at least for the past 800,000 years have marked the last 2.8 Ma. The warm periods, called interglacial periods, appear to last approximately 15,000 to 20,000 years before regressing to a cold ice age climate. The last of these major glacial intervals began ending about 18,000 years ago, as the large continental ice sheets covering North America, Europe, and Asia began retreating. The main climate events related to the retreat of the glaciers can be summarized as follows:

• 18,000 years ago: the climate begins to warm

• 15,000 years ago: advance of glaciers halts and sea levels begin to rise

• 10,000 years ago: Ice Age megafauna goes extinct

• 8,000 years ago: Bering Strait land bridge becomes drowned, cutting off migration of people and animals

• 6,000 years ago: the Holocene Maximum warm period

• So far in the past 18,000 years Earth's temperature has risen approximately 16°F (10°C), and sea level has risen 300 feet (91 m)

This past glacial retreat is but one of many in the past several million years, with an alternation of warm and cold periods apparently related to a 100,000-year periodicity in the amount of incoming solar radiation, causing the alternating warm and cold intervals. Systematic changes in the amount of incoming solar radiation, caused by variations in Earth's orbital parameters around the Sun, are known as Milankovitch cycles, after Milutin Milan-kovitch (1879-1958), a Serbian scientist who first clearly elucidated the relationships between the astronomical variations of the Earth orbiting the Sun and the climate cycles on Earth. These changes can affect many Earth systems, causing glaciations, global warming, and changes in the patterns of climate and sedimentation. Milankovitch's main scientific work was published by the Royal Academy of Serbia in 1941, during World War II. He calculated that the effects of orbital eccentricity, wobble, and tilt combine every 40,000 years to change the amount of incoming solar radiation, lowering temperatures and causing increased snowfall at high latitudes. His results have been widely used to interpret the climatic variations, especially in the Pleistocene record of ice ages, and also in the older rock record.

Astronomical effects influence the amount of incoming solar radiation; minor variations in the path of the Earth in its orbit around the Sun and the inclination or tilt of its axis cause variations in the amount of solar energy reaching the top of the atmosphere. These variations are thought to be responsible for the

Temperature and CO2 changes in past 400,000 years based on Antarctic ice cores

Years before present (present = 1950) in thousands
Years before present (present = 1950) in thousands

& Infobase Publishing advance and retreat of the Northern and Southern Hemisphere ice sheets in the past few million years. In the past two 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 suggest 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, 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 the Earth closer to and farther from the sun in summer and in 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. Presently the Earth's orbit is in a period of low eccentricity (~3 percent), and this yields 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.

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, 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 winters would tend to be milder and summers cooler. This would lead to more glaciation.

Proyeccion Geografia

Eccentricity cycle (100,000 years)

Obliquity cycle (41,000 years)

Normal to ecliptic

Precession of the Equinoxes (19,000 and 23,000 years)

Northern Hemisphere tilted away from the Sun at aphelion

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

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Orbital eccentricity

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100 200 300 400 500 Thousands of years ago

Tilt

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Precession of equinox

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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 because all these effects act at the same time.

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 phe-

nomenon is known as precession of the equinoxes, and it places 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 the Earth is closest 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 each of these astronomical factors acts on different timescales, they interact in a complicated way (Milankovitch cycles, as described previously). understanding these cycles, clima-tologists can make predictions of where the Earth's climate is heading, whether the planet is heading into a warming or cooling period, and whether populations need to plan for sea-level rise, desertification, glaciation, sea-level drop, 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 further complicated by other factors that change the climate of the 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 detect cyclical variation on the timescales of Milankovitch cycles; 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 inputs from the long- and medium-term effects described in this entry, and some short-term effects such as sudden changes caused by human inputs of greenhouse gases into the atmosphere, and effects such as unpredicted volcanic eruptions. Nonetheless, most climate experts expect that the planet will continue to warm on the hun-dreds-of-years timescale. But judging by the recent geological past, we can reasonably expect 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 whether the planet 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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Responses

  • rohan
    How astronomical periodicities affect climate change?
    1 year ago

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