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This is a top.

This is also a top, but it is a little different from the top above.

Yes, you are right, in fact, this is our common home of mankind-the earth. However, this comfortable and warm home seems to have opened the "hell mode" in recent decades. In the latest sixth assessment report of the United Nations Intergovernmental Panel on Climate change (IPCC), Climate change 2021: the foundations of Natural Science, UN Secretary-General Ant ó nio Guterres put forward a "red warning for mankind". The speed of climate warming is greatly accelerated and extreme weather phenomena occur frequently, all of which are challenging life on Earth again and again. So what is driving the change in the climate system?

Climate change 2021: the Foundation of Natural Science for nearly a hundred years of climate change, people's main focus is on the factors of human activities. But if climate change is involved in a geological period on a time scale of ten thousand to one million years, it can be explained by Milankovich theory. This theory involves the changes of three astronomical factors: eccentricity, precession and inclination of the earth's orbit.

1. Milankovitch cycles cycle (Milankovich cycle) at the beginning of the 20th century, the Serbian civil engineer and mathematician Milutin Milankovi Milutin Milankovi accurately calculated the changes of the earth's eccentricity, orbital inclination, precession and 65 °N summer solar radiation over the past few million years, and believed that the changes in the three elements of the earth's orbit caused changes in the radiant energy obtained by the earth. It leads to periodic changes in the latitude and seasonal configuration of the top of the earth's atmosphere and solar radiation, which leads to changes in the global climate.

Milankovich proposed that the variation of 65 °N summer solar radiation is the main reason for driving the Quaternary glacial cycle, that is, the trigger driving mechanism of a single sensitive region: the high latitude climate change signal in the Northern Hemisphere will be amplified, transmitted and then affect the global climate change. Among them, the variation of solar radiation at 65 °N in summer is mainly affected by three parameters of the earth's orbit: eccentricity, inclination of the earth's axis and precession. So what will happen to these three parameters, and how do they cause global climate change?

Figure Earth orbit parameters (Pisias and Imbrie,1986) 2. Earth orbit parameters will cause changes in the distance and position of the sun and the earth. Eccentricity (Orbital eccentricity) the shape of the earth's orbit is not a positive circle, but an ellipse, which mathematically quantifies the degree of ellipse "flattening" by "eccentricity". The minor axis of the elliptical orbit is the perihelion axis, and the long axis is the distal axis. The aphelion axis is often stable, and the orbit change from circle to ellipse or from ellipse to circle is always accomplished by the perihelion axis, that is, the short axis. The average eccentricity of the earth's orbit is 0.028 (currently 0.017). There are two kinds of variation periods: 400,000 years and 100,000 years, which are called long and short periods of eccentricity, respectively. When the short half axis shortens with the increase of eccentricity, the seasonal change will be intensified. But the direct climate effect caused by eccentricity is small because the earth is too far from the sun. The difference between perihelion and aphelion of the distance between the earth and the sun causes little change in the amount of radiation, and the eccentricity mainly affects the earth's climate system by regulating the variation of the climate precession.

Diagram of the eccentricity of the earth's orbit and its periodic variation (Williams et al., 1998, etc.) precession (Axial precession) precession, also known as precession, refers to the slow and continuous change of the rotation axis of one celestial body relative to the inertial coordinate system in space under the action of the gravity of other celestial bodies. Seen from space, the earth looks like a top, and the direction of the top is gradually changing. The motion of this gyro is caused by the tidal forces exerted by the sun and moon on the solid earth. For example, the direction of the earth's rotation axis gradually drifts, tracking the top of its swing, sweeping out a cone in a cycle of about 25,800 years.

The direction of precession (clockwise) and the diagrams of precession (P), nutation (N) and earth rotation (R) the concrete manifestation of precession is the change of the earth's equatorial plane and ecliptic plane, which are also known as equatorial precession and ecliptic precession respectively. The influence of equatorial precession is mainly represented by the continuous westward movement of the vernal equinox at a rate of about 51 "per year. This effect can be directly observed by the difference between the regression year and the sidereal year, as well as the change in the relative position of the North Pole and the North Star. For example, after 12,000 years, Vega will become the new North Star. The influence of the ecliptic precession is shown by the eastward movement of the vernal equinox at a rate of about 0.1 "per year, and the slow change of the yellow-equinox angle. Equatorial precession and ecliptic precession are collectively referred to as total precession.

Precession itself does not change the annual amount of incident radiation received by the hemisphere, only the time of reception. When the summer of a hemisphere is at perihelion, the hemisphere experiences a short, hot summer and a long and cold aphelion winter, that is, seasonal enhancement. On the contrary, the hemisphere has long and cool summers and short and warm winters, which are not seasonal. Of course, the heat and cold here, warm and cool are relative.

Precession is the only orbital parameter that should be paid attention to on the time scale of the century, and the season can advance 1.4 days in a century. In recent centuries, it has caused delays in the start of cold spring, long summers and winters in the northern hemisphere. The amplitude of the periodic climate cycle of the precession is controlled by eccentricity, which has a great influence on the climate in the middle and low latitudes and little influence on the polar region.

The third obliquity is the tilt angle of the earth's rotation axis. The inclination angle is the angle of the earth's axis relative to the orbital plane. In the solar system, the earth's orbital plane is the ecliptic, so the earth's axis obliquity is also known as the yellow-red cross angle, which is expressed by ε. The gravitational interference of the moon and massive planets causes the obliquity of the earth's axis to wobble slightly, with a range of 2.4 °, slowly changing from 22.1 °to 24.5 °in a period of about 41000 years.

At present, the inclination of the earth's axis is about 23.44 °(23 °26'). Although the tilt of the earth's axis is in the same direction throughout the year, as the earth moves around the sun, the hemisphere originally facing the sun will gradually change into a hemisphere away from the sun, and vice versa. This effect is the main reason for seasonal change. The sun-facing hemisphere has relatively long hours of sunshine every day, and the closer the angle of sunlight to the ground at noon is closer to the vertical direction, the more energy the area gets per unit area.

When the inclination increases, the summer days are longer and the winter nights are longer in the high latitudes of the hemisphere, which leads to an increase in radiation received in summer, while less radiation is received in winter, and the annual variation of solar radiation in the high latitudes of the hemisphere becomes larger. In addition, the northern hemisphere, where land is more concentrated, is more likely to be cold in winter.

Let's imagine a scenario where summers in the northern hemisphere are near perihelion, summers are shorter, and winters are longer. At the same time, the inclination of the earth's axis is small, and the high latitudes of the Northern Hemisphere receive less radiation in summer, warm in winter and cool in summer. As a result, the accumulated snowfall in winter cannot melt in a short summer, in which case the earth's temperature will drop, which in turn will rise. In other words, this change in angle will not change the total energy from the sun to the earth, but will affect the distribution of sunlight at different latitudes. Therefore, the combined effect of eccentricity and inclination can make one hemisphere colder than the other. This is an important reason why Milankovich is concerned about the variation of summer solar radiation in the high latitudes of the Northern Hemisphere.

It seems that the slight "wobble" of the earth will cause changes in the parameters of the earth's orbit, which in turn will affect the distribution of solar radiation received by the earth, and ultimately affect global climate change.

However, due to the lack of continuous records, it is difficult to find global climate indicators, and it is very difficult to verify the Milankovich hypothesis. Milankovich theory was not confirmed until the development of core drilling technology and ice core isotope determination technology, and scholars used stable oxygen isotopes to interpret Quaternary climate change.

Figure Vostok time series and solar irradiance as well as CO2, atmospheric isotope temperature, CH4, atmospheric δ 18O and 65 °N solar irradiance over time. However, these techniques had not been developed while Milankovich was alive, so his theory was difficult to verify. What underpins Milankovich is his belief in science. When Milankovich entered the prison camp, he wrote in his diary:

"that lonely room, far from the hustle and bustle of the world, is simply tailor-made for scientific research. In my suitcase, there were my notes on the problems of the universe, as well as paper and pen, so I began to write and calculate. When I stopped writing, I looked around the room late at night, and suddenly wondered where I was. Prison seems to be just a hotel where I stay on my way through the universe."

The top of the earth may never stop, because in the vast universe, there is always someone guarding its rotation with life.

references

[1] Pisias N G, Imbrie J. Orbital Geometry, CO2, and Pleistocene climate [J]. Oceanus, 1986, 29 (4): 43-49.

Wu Huaichun, Zhang Shihong, Feng Qinglai, et al. Theoretical basis, research progress and prospect of cyclic stratigraphy [J]. Geoscience-Journal of China University of Geosciences, 2011: 409428.

[3] Imbrie J. Astronomical theory of the Pleistocene ice ages: A brief historical review [J]. Icarus, 1982, 50 (2-3): 408422.

[4] Hays J D, Imbrie J, Shackleton N J. Variations in the Earth's Orbit: Pacemaker of the Ice Ages [J]. Science, 1977, 194 (4270): 1121-1132.

[5] Petit J R, Jouzel J, Raynaud D, et al. Climate and atmospheric history of the past 420000 years from the Vostok ice core, Antarctica [J]. Nature, 1999 (6735): 429-436.

This article comes from the official account of Wechat: stone popular Science Studio (ID:Dr__Stone), author: Zhixing

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