What is the mystery of the ice age disaster?

We believe in the existence of interglacial period. In the past 350 million years, there have been many extremely cold ice ages on earth, and the interglacial period is a relatively warm "valley" between the "peaks" of the cold ice age. As we know, in the temperate zone of the earth, snow flutters in winter and melts in spring. At present, the poles are covered with ice and snow. This means that according to the climate history of the earth, we are currently in a cool period. Dinosaurs lived for 250 million years. When they walked leisurely on the earth, it was much hotter than it is now. At that time, green trees grew near the North Pole. It is speculated that the last ice age ended 654.38+200,000 years ago. About 20,000 years ago, on a sunny day, hippos strolled in Hertfordshire, southeast England. /kloc-in the 9th century, the discovery of this jungle animal skeleton attracted the attention of scientists, and it was also one of several factors that prompted scientists to start studying the past climate change of the earth. At some point in the earth's past, our cool north latitude was close to the tropical temperature; On another occasion, the climate was very cold, and snow and ice covered most parts of North America, even new york and Illinois in the south.

Later, geologists' eyes shifted from hippos in Hertfordshire in the19th century to puzzling stones-fossils, especially oyster shell fossils all over England, northern Europe and parts of North America. Where did these various fossils come from? William Baekeland, a famous British geologist, thinks that they may have been deposited after the flood recorded in the Bible. But others quickly gave a more scientific answer. The first person to associate the Ice Age with fossils (stone piles) was the Swiss scientist Jean-Louis Agassi, who was a zoologist at first and later became one of the founders of modern geology. From 65438 to 0846, he moved to the United States and became a professor at Harvard University. His thoughts influenced a generation of scientists. While studying the glaciers in the Swiss Alps from 65438 to 1930, he noticed that the Alps had shrunk recently and found oyster shell fossils everywhere in England. Therefore, he concluded that glaciers once covered far more areas than the Alps and northern regions. Later, as geologists continued to dig, more such fossil-bearing strata were discovered. This means that glaciers have moved south several times along Europe and North America, and there is a long time before each new movement begins. A new understanding of the earth's past was born: the earth experienced many ice ages.

Most of the evidence left by the ice age is incomplete, and the topography of the earth's surface is constantly changing, which makes the evidence unrecognizable. But after the past century and a half, especially since the 1920s, people have learned how to identify the general pattern of the ice age cycle. A large-scale ice age began in the Middle Carboniferous, that is, 325 million years ago, and lasted until the Permian 260 million years ago. The ice age was followed by a warm period, when dinosaurs were in a period of prosperity. In the following 35 million years, the ice age became common, once every 654.38 million+years on average. This figure is an estimate, and there may be a small discrepancy. This incomplete time scale means that scientists can have many explanations, which also opens the door for the existence of different theories and arguments.

When scientists in different fields begin to seriously think about why the ice age happened, they can't ignore an obvious starting point, that is, the global average temperature will inevitably change greatly in different historical periods of the earth, and the most basic reason for this change is the amount of solar energy reaching the earth's surface. It is understood that even in the19th century, the earth's orbit around the sun was not as stable as the street we walked. It was not until the 1920s that Yugoslav mathematician Milutin Milankovi accurately calculated three important factors that affected the Earth's movement in space. First of all, the earth runs in an elliptical orbit, not a perfect circle-its orbit is more like an egg's ellipse than a baseball's circle. In addition, this elliptical orbit is a bit abnormal-the orbit will gradually change from ellipse to circle and then to ellipse, with a period of 654.38+ million years. Second, the earth is inclined, and the inclination angle with the vertical varies between 24.5 ~ 21.5 (currently in the middle of these two extremes), and the change period is 4 1 10,000 years. Third, the earth also makes a spiral rotation around the earth's axis, just like a gyro that swings up and down. This is called precession. It maintains a cycle of 22000 years, with a small jump every 1.9 million years.

Milankovic spent nearly 30 years trying to reveal the relationship between these three anomalies and the emergence of the ice age through a set of equations. He believes that when the precession period and tilt period reach the extreme value, the solar energy reaching the ground will be greatly reduced and the glacier will begin to expand again. Although scientists have some doubts about the 654.38+ million-year period that affects the earth's elliptical orbit around the sun, they still think it is reasonable. The change of the earth's orbit from ellipse to circle to ellipse is less than 0.3%, which is insignificant relative to the cosmic scale. However, people know that the earth's atmosphere is easily affected by extremely small factors, so even with the most advanced computing technology, it is difficult to predict the long-term weather conditions in areas less than 300 miles (482 kilometers). Therefore, some scientists are willing to accept the fact that although the change of the earth's orbit is only 0.3%, it may cause great changes in the global climate.

However, Milankovic's equation is still just a theory. The evidence supporting this theory finally appeared in 1976. At that time, researchers found that there were some special sediments at the bottom of the ocean, which recorded the changes of seawater temperature in the past thousands of years and could be called indicators of seawater temperature. It turns out that the sediment contains some shells called foraminifera, and the chemical composition of the shells of foraminifera changes with the temperature of seawater, which is oxygen. The ratio of the common isotope O- 16 to the heavier isotope O- 18 in the shells of this animal changes with the change of seawater temperature. When the earth gets cold, the temperature of seawater is lower, and the content of O- 16 in foraminifera shells is less, because glaciers formed on the ground will capture this isotope during the cold period. The discovery of sediments and subsequent laboratory determination are arduous, but this kind of work is also fruitful. The determination of the deepest sediments obtained by deep-sea drilling shows that the ocean temperature during the Cretaceous period was nearly 20 degrees higher than our current temperature. This is a huge change. It is found that the change is not very drastic but very obvious, which is consistent with the gradual cooling that began before 1 15000 years ago (when England was actually tropical). This cooling continued until the peak of the last ice age 6,543.8+0.5 million years ago, when the ice covering the northern part of new york was 654.38+0 miles thick-finally, the melting of the ice exposed Long Island, and the water after the melting of the ice turned the land into an ocean.

Western scientists drilled into the depths of Greenland's polar ice sheet, and the ice core samples taken from it and the ice core samples taken by Russian scientists in Antarctica for a long time confirmed and spread the discovery of the seabed. The ratio of oxygen isotopes is once again used as a measurement standard, because ice forms different ice layers, just like tree rings. Many more detailed data have been obtained from Greenland and Antarctica, which is helpful to determine the cold and warm periods of the earth in the past 25 million years. Even if all the evidence supports Milankovic's theory, after decades of research, some scientists increasingly feel that this theory can only provide 80% of the reasons for the ice age, as if things are not over yet.

1979, a Swiss physicist named Hans Ershi went to Greenland and joined the team led by Chester Langway of new york State University. By crushing the ice sample, the gas in the bubbles captured by ice thousands of years ago was collected. Erjie found that 12000 years ago, when the world began to warm, it was 65433. When these results were made public, the new verification work was carried out in deep-sea sediments, and it reached the same result. Carbon dioxide seems to be a gas that enhances the solar cycle effect of the earth's atmosphere.

How does this mechanism work? Many famous scientists have solved this problem from different directions. We now know that it is the "greenhouse effect". In recent news, we often hear arguments about global warming. In fact, the "greenhouse effect" makes it possible for life to exist on the earth. The focus of recent debate is whether global temperature rise will lead to global climate out of control. We also know that one of the most important factors in this process is the increase of carbon dioxide content in the atmosphere.

Why does the carbon dioxide content in the atmosphere rise or fall without human intervention to break the balance? Many scientists believe that there are different reasons in different periods of earth's history. For example, Cretaceous warming may be the result of the rapid expansion of vegetation on the earth's surface. This kind of vegetation uses carbon dioxide in the air, but it will release carbon dioxide. With the prosperity of new plant species, the total content of carbon dioxide is increasing. In another period, the number of marine plants increased greatly, which may absorb carbon dioxide in the atmosphere and capture it underwater, leading to the cooling of the climate, eventually breaking the balance and triggering a new ice age.

Similarly, some people speculate that the movement of the earth's plates and the continents they create may affect the climate. Today, the Mexican warm current brings warm equatorial seawater from the Atlantic Ocean to England, bringing relative warmth to the "green and beautiful land" at high latitudes. Perhaps it was the emergence of the Central American continent 2.5 million years ago that stopped the flow of water between the Pacific Ocean and the Atlantic Ocean and triggered the ice age in the northern hemisphere. 654.38+0.5 million years ago, the separation of Antarctica and South America may have led to the latest global cooling.

Another controversial theory is based on the process of rock erosion, which was discovered by American chemist harold urey (who won the Nobel Prize in 1934). In Yuri's reaction, silicate-containing stones will consume carbon dioxide in the atmosphere during erosion. If they are buried and thrown out of the ground by volcanic eruptions after countless years, they can release carbon dioxide back into the atmosphere. American climatologists Maureen Raymond and William Ruddiman believe that the ice age may be related to the large-scale mountains formed when the Himalayas and Andes rose, and then these mountains absorbed carbon dioxide from the atmosphere when they were eroded. However, this is not enough evidence to explain global warming, because scientists believe that human burning fossil fuels will release a lot of carbon dioxide into the atmosphere like volcanic eruptions.

In the past few years, a new theory has been published almost every year, at least in scientific journals. From 65438 to 0997, Richard A. Muller of the University of California at Berkeley and Gordon J. McDonald of the International Society for Applied Systems Analysis applied Milankovitch Cycle to computer models. It has been proved that 30 thousand tons of cosmic dust fall to the surface of the earth every year without being noticed by us. To use a vivid phrase in the 1960s, these cosmic dust are mixed with "flying dust". Muller and MacDonald speculate that the earth passes through a special cosmic dust belt every 654.38 million+years. Because its axis is tilted, the amount of cosmic dust falling to the earth's surface has increased to a crisis level. Two other researchers, Stephen J. Cottenkamp of the Carnegie Institution in Washington and Stanley F. Demotiron of the University of Florida, tested this conjecture with another computer model. They announced in 1998 that the increase of dust in the universe is not caused by the inclination of the earth, but by the orbit shape of the earth around the sun, which is closer to Milankovic's original equation. According to the science news report of1May, 1999, Kenneth A. Farley of California Institute of Technology found that sediments do show that cosmic dust increases three times every 65,438+million years, but the model predicts that the amount of cosmic dust should decrease at some point. "This is really strange." Farley concluded.

1999 A new space-oriented theory has emerged, which can certainly explain the sharp increase of cosmic rays. These rays constantly bombard the earth, but in a more concentrated situation, they will lead to a significant increase in the density that may be covered. Cosmic ray bombardment can be measured by carbon-14 radioactive decay technique. As reported in Discovery magazine in April 1999, this new theory was put forward by Henrik Svenns Mark of Danish Space Research Society, who can provide evidence that "only one of the two factors" can lead to the increase of cosmic ray activity in the past ice age.

Of course, the newly proposed theory about the causes of the ice age needs to be discussed. Some scientists have become impatient with some strange theories, complaining that they are not as good as strict computer models, and thinking that people from different disciplines pretend to be experts is pure speculation. The nature of this subject inevitably attracts people from different fields. Milankovic's periodic theory is widely accepted, although scientists still disagree about its importance. In any case, astronomers are in a safe position in this debate, and so are evolutionary biologists, because life forms existing in a specific period reflect—and sometimes may affect—climate change. Geologists and chemists tend to cooperate more closely, just like the process of obtaining and analyzing sediment samples from the deep sea. Although team work in one professional field can sometimes support or even confirm team work in another professional field, it is inevitable that there will be conflicts between them. From the geographer's point of view, something meaningful may conflict with the evolutionary data, and vice versa.

In fact, many scientists are skeptical about completely solving the mystery of the ice age. Many theories have been put forward, some of which have been proved to be correct. Of course, if the Milankovic cycle is as important as many scientists think, then in the next 2000 years or so, there should be some kind of answer, and we will see the beginning of a new ice age. But there is still a problem, because human activities lead to the release of excessive greenhouse gases such as carbon dioxide, which leads to global warming and may lead to the disorder of the whole process. If so, we may usher in a period of melting polar ice and snow, rather than the expected ice age. Even this is not brand new. After all, in the dinosaur era of the past 200 million years, there was no sign of an ice age. Even though there have been many ice ages in the past 35 million years, they have not appeared on time. Perhaps the reasons for the ice age are varied and complicated, so there is no timetable. At least when the whole history of the earth is more an agreement on the needs of human order and less about other things, we can only look at it this way.