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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.
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.
Continue reading here: Thermohaline circulation and climate
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Above the equator, winter officially begins in December. But in many areas, January is when it really takes hold. Atmospheric scientist Deanna Hence explains the weather and climate factors that combine to produce wintry conditions at the turn of the year.
How does the Earth’s orbit influence our daylight and temperatures?
As the Earth orbits the sun, it spins around an axis – picture a stick going through the Earth, from the North Pole to the South Pole. During the 24 hours that it takes for the Earth to rotate once around its axis, every point on its surface faces toward the Sun for part of the time and away from it for part of the time. This is what causes daily changes in sunlight and temperature.
There are two other important factors: First, the Earth is round, although it’s not a perfect sphere. Second, its axis is tilted about 23.5 degrees relative to its path around the Sun. As a result, light falls directly on its equator but strikes the North and South poles at angles.
When one of the poles points more toward the Sun than the other pole, that half of the planet gets more sunlight than the other half, and it’s summer in that hemisphere. When that pole tilts away from the Sun, that half of the Earth gets less sunlight and it’s winter there.
Seasonal changes are the most dramatic at the poles, where the changes in light are most extreme. During the summer, a pole receives 24 hours of sunlight and the Sun never sets. In the winter, the Sun never rises at all.
At the equator, which gets consistent direct sunlight, there’s very little change in day length or temperature year-round. People who live in high and middle latitudes, closer to the poles, can have very different ideas about seasons from those who live in the tropics.
There’s an old saying, “As the days lengthen, the cold strengthens.” Why does it often get colder in January even though we’re gaining daylight?
It depends on where you are in the world and where your air is coming from.
Earth’s surface constantly absorbs energy from the Sun and stores it as heat. It also emits heat back into space. Whether the surface is warming or cooling depends on the balance between how much solar radiation the planet is absorbing and how much it is radiating away.
But Earth’s surface isn’t uniform. Land typically heats up and cools off much faster than water. Water requires more energy to raise and lower its temperature, so it warms and cools more slowly. Because of this difference, water is a better heat reservoir than land – especially big bodies of water, like oceans. That’s why we tend to see bigger swings between warm and cold inland than in coastal areas.
The farther north you live, the longer it takes for the amount and intensity of daylight to start significantly increasing in midwinter, since your location is tilting away from the Sun. In the meantime, those areas that are getting little sunlight keep radiating heat out to space. As long as they receive less sunlight than the heat they emit, they will keep getting colder. This is especially true over land, which loses heat much more easily than water.
As the Earth rotates, air circulates around it in the atmosphere. If air moving into your area comes largely from places like the Arctic that don’t get much sun in winter, you may be on the receiving end of bitterly cold air for a long time. That happens in the Great Plains and Midwest when cold air swoops down from Canada.
But if your air comes across a body of water that keeps a more even temperature through the year, these swings can be significantly evened out. Seattle is downwind from an ocean, which is why it is many degrees warmer than Boston in the winter even though it’s farther north than Boston.
How quickly do we lose daylight before the solstice and gain it back afterward?
This depends strongly on your location. The closer you are to one of the poles, the faster the rate of change in daylight is. That’s why Alaska can go from having hardly any daylight in the winter to hardly any darkness in the summer.
Even for a particular location, the change is not constant through the year. The rate of change in daylight is slowest at the solstices – December in winter, June in summer – and fastest at the equinoxes, in mid-March and mid-September. This change occurs as the area on Earth receiving direct sunlight swings from 23.5 N latitude – about as far north of the equator as Miami – to 23.5 S latitude, about as far south of the equator as Asunción, Paraguay.
What’s happening on the opposite side of the planet right now?
In terms of daylight, folks on the other side of the planet are seeing the exact opposite of what we’re seeing. Right now, they’re at the peak of their summer and are enjoying the largest amounts of daylight that they’re going to get for the year. I do research on Argentinian hailstorms and Indian Ocean tropical cyclones, and both of those warm-weather storm seasons are well into their peaks right now.
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But there’s a key difference: The Southern Hemisphere has a lot less land and a lot more water than the Northern Hemisphere. Thanks to the influence of the southern oceans, land masses in the Southern Hemisphere tend to have fewer very extreme temperatures than land in the Northern Hemisphere does.
So even though a spot on the opposite side of the planet from your location may receive exactly as much sunlight now as your area does in summer, the weather there may be different from the summer conditions you are used to. But it still can be fun to imagine a warm summer breeze on the far side of the Earth – especially in a snowy January.
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