Climate and the causes of its change

The weather is changeable, but we can still predict the future weather situation and development trend of an area after we understand how it is formed and fully grasp the historical and realistic observation data as soon as possible. If the climatic factors in a region (such as temperature, rainfall, humidity, wind, air pressure, etc. ) Long-term comprehensive, we can find that it remains relatively stable on the whole. Therefore, China people who have been farming for generations in the Yellow River Basin can sum up the 24 solar terms with certain regularity. The synthesis of weather conditions in an area for many years is what is usually called climate. Weather is a comprehensive expression of meteorological elements in a short period of time in an area.

Because of the different latitude and ground conditions, the climate in different parts of the earth has its own characteristics. The climate can be divided into several regions. Climatic zone is the largest unit of climate division. Because the most important factors controlling climate are solar radiation and atmospheric circulation, the climate zone is distributed along the latitude zone. At present, the world is generally divided into four categories: tropical (roughly in the Tropic of Cancer, between 25 north and south latitudes), subtropical (between 25 and 35 north and south latitudes), temperate (between 35 and 60 north and south latitudes) and frigid (between 60 north and south latitudes and the polar regions), with a total of seven climatic zones. Climatic zones can also be divided into several climatic types.

For a long time, with the passage of time, the climate and climatic zones in different geological periods are changing. If we take the temperature as a sign, we can see that the global temperature has fluctuated greatly in the evolutionary history of the earth. Glaciers used to cover not only polar regions and high latitudes, but also middle and low latitudes, which is called ice age. The period between the Great Ice Age and the relatively warm climate is called interglacial period. During the Great Ice Age, the climate was not always as cold, but it still changed from time to time, and it could be divided into several small ice ages and relatively warm small interglacial periods.

In the past, the changes of the earth's temperature and warmth were mainly understood by studying the geological and historical data that can indicate the climate characteristics. Before Neoproterozoic (800-600 million years ago), there were not many materials to be verified, but traces of glacier activity were found. It has been found that there have been three major glacial periods since Neoproterozoic: the first glacial period occurred in Neoproterozoic (which can be subdivided into two glacial periods); The second ice age occurred at the end of Paleozoic; The third ice age began in the early Quaternary. At the coldest time, 20% ~ 30% of the global continent is covered by glaciers. About 1000 years ago, the Quaternary glaciers began to retreat, and now they have shrunk to cover only about 10% of the mainland area. The general trend is warming, and people call this warming period the post-glacial period. During the Quaternary Ice Age, the climate changed many times, and glaciers sometimes expanded, but there were still fluctuations (Figure 4-6).

Figure 4-6 Cenozoic Climate Change History

Why did the climate change so much that there was even an ice age? It was once thought that the increase or decrease of sunspot activity led to the emergence of the ice age. Of course, the change of solar radiation intensity will have an impact on the climate on the earth. It has been observed many times that the increase of sunspots, that is, when the activity is strong, will cause climate anomalies. However, it may be difficult whether the amount of solar radiation can be changed enough to lead to an ice age. Some people think that it is because of the change of the inclination angle of the earth's rotation axis that the irradiation angle of the earth relative to the sun changes, thus increasing or decreasing the amount of radiation from the sun. Among them, Milankovic, an astronomer from Serbia, is an outstanding representative. According to the change of solar radiation in the mid-latitude area since 700,000 years, he found that the tens of thousands of years' cycle of the earth's climate change is consistent with the three orbital elements of the earth's movement [eccentricity of the orbit, the angle between the ecliptic plane and the equatorial plane, and the precession of the orbit perihelion (also called precession)]. Others believe that the interval between the three major ice ages is roughly equivalent to the time when the solar system orbits the center of the Milky Way. It is believed that when the solar system enters an area with dense interstellar dust, the sun's radiation is blocked by these dust, which reduces the heat gained on the earth, thus resulting in the Ice Age. In addition, it is also possible that if the earth is hit by a huge meteorite, a large amount of dust will enter the atmosphere, blocking the radiation of the sun and the climate will turn cold (see chapter 12 for details). The movement of the earth's lithosphere causes drastic changes in the distribution of land and sea, and biological activities, especially human activities, have an important impact on climate change. The relevant chapters of this book will be discussed separately.

Think about a problem

1. Why can there be such an atmosphere on the earth?

2. What are the factors that affect the earth's climate?

3. What is the development trend of the earth's climate now?

Bibliography for further reading

1. Zhou Shuzhen. 1996. Meteorology and climatology. Beijing: Higher Education Press.

2. Barry HG, happy KJ. (1976), 1984. Atmosphere, weather and climate. Beijing: Higher Education Press.

3. Zhu Kezhen. 1979. Zhu Kezhen's collected works (related chapters). Beijing: Science Press.

4. Kobe E. 1997. Liu Dongsheng et al. 2004. Climate process and climate change. Beijing: Science Press.