Climate change of ancient Earth has been determined primarily through the study of plate tectonics and the reconstruction of the locations of the Earth's landmasses, oceans, and waterways as well as geologic evidence of atmospheric CO2. Much of the paleoclimatic reconstruction of the
Earth's ancient climate has been accomplished through the mapping of past positions of the continents and plotting of the distributions of rock types that form in specific climate regions. Certain formations, such as coal, need certain conditions in which to form. Coal needs abundant rainfall and generally forms in tropical rain forest or temperate forest conditions, which provide the necessary biomass, heat, and moisture.
In general, sedimentary rocks form in areas where water is present. Through a knowledge of rock types and the physical conditions necessary for formation, it is possible to step back in time and reconstruct the physical conditions that must have existed on the Earth at the time the rock was formed. By mapping the past distribution of thousands of rock types, scientists have been able to map the distribution of ancient climatic belts. Over the last 2 billion years, the Earth's climate has been constantly changing between hot and cold conditions, as shown in the figure.
The Earth's most ancient climates include the following geologic time span:
Precambrian eon 570 MYA-4.6 billion years ago Phanerozoic eon 0-570 MYA Palaeozoic era 240-570 MYA
Mesozoic era 65-240 MYA
Within this geologic time span, the Earth experienced a wide range of climate variability. At a period around 635 million years ago, many scientists believe the Earth was completely frozen over into what clima-tologists refer to as Snowball Earth. After that, 300 million years ago the planet became Hothouse Earth (also referred to as the mid-Cretaceous Greenhouse World).
The Precambrian—the oldest period on Earth—covers about 85 percent of the Earth's history, but because of the extreme time span, there is not much reliable evidence. Scientists have found evidence of the following two major periods of glaciation: one at 2.3 to 2.7 billion years ago and another at 0.9 to 0.6 billion years. The three intervals during this time period that scientists do have information on and that have provided some insight to the Earth's past climate are next
Average Global Temperature (°C)
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Average Global Temperature (°C)
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The graph illustrates the way climate has varied over geologic time from hot to cold; today, the Earth is in an interglacial period.
discussed: (1) the Faint Young Sun Paradox, (2) Snowball Earth, and (3) Hothouse Earth.
The inability to find more evidence for early glaciation has always stumped scientists because they expected to find evidence supporting abundant glacial events. By studying the evolution of stars in the universe, astronomers have been able to recreate the history of the Sun. The models they have developed indicate that the early Sun was about 30 percent less bright than it is today. These calculations presented a mystery for climatologists, because a decrease of just a few percentage points in the Sun's present strength would cause all the water on Earth to freeze, even with all the CO2 in the air and the greenhouse effect. If all the water in the oceans, lakes, and streams froze today, their high albedo (reflectivity) would make it difficult to melt the ice. What presents such a mystery to scientists is that with such a weak Sun, even if the Earth's greenhouse gas levels were then what they are today, the Earth should have remained completely frozen for the first 3 billion years of its existence. Yet scientists have not been able to find any evidence to support the Earth ever having been completely frozen. Geological evidence for this time has been partly found in sedimentary rocks, and sedimentary rocks are formed from running water, not frozen water. Evidence of a continued presence of life on Earth during this time also does not support the possibility of a planet frozen because of too weak a Sun. This question remains a mystery.
There is one theory concerning the Earth being in a nearly frozen state 635 million years ago that climate scientists debate—an episode referred to as Snowball Earth. The term Snowball Earth describes the coldest state in which a planet can exist. In order for this to happen, the global mean temperature would have to be -74°F (-50°C). Most of the solar radiation would be reflected back into space by the high albedo of the snow and ice covering the planet.
There is evidence that supports this hypothesis during this later time period. Evidence exists in sedimentary rocks containing mixtures
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of coarse, unsorted boulders and cobbles mixed with fine silts and clays. These unsorted deposits are characteristic of ice and glacial deposition, and they are found on almost all the continents on Earth. Based on evidence found in sedimentary rocks, it has also been proposed that between 550 and 850 million years ago, two to four of these separate ice house incidents may have occurred.
The biggest debate by scientists on this subject concerns the geographic location of the continents. If some of the continents were located in the Earth's equatorial region (the Tropics), this supports the hypothesis that the Earth could have been entirely frozen. If, however, all the continents were located at high (polar) latitudes, they could all have been frozen, but the Tropics could still have remained unfrozen, meaning the entire Earth did not necessarily freeze, although large portions could have. Scientists have done much work to determine the geographic location of the continents.
Research conducted by Paul F. Hoffman (a field geologist) and Daniel P. Schrag (a geochemical oceanographer) of Harvard University has helped to answer many of the questions surrounding this notable climate event. One of the initial enigmas was the occurrence of glacial debris found near sea level in the Tropics. This evidence contradicted evidence seen today—glaciers near the equator now survive only at 16,404 feet (5,000 m) above sea level or higher. Even at the coldest segments of the last great ice age, glaciers did not form lower than 13,123 feet (4,000 m) in elevation. What Hoffman and Schrag found, however, was not only glacial debris near sea level but that it was mixed with unusual deposits of iron-rich rock. What made this odd was that those rocks should have been able to form only in an environment that had little or no oxygen in its atmosphere or oceans. According to scientific evidence, however, the Earth's atmosphere at that time should have closely resembled that of today. Even more puzzling was that there were also deposits of rocks that could have formed only in warm water found in the rock layers that formed just after the glaciers receded. This presented a puzzle: If the Earth were cold enough to ice over completely, how did it warm up again, especially to such extremely hot conditions? In addition to that conundrum, the carbon isotopic signature in the rocks hinted at a prolonged drop in biological production, leaving scientists to conclude that there had been a dramatic loss of life at that time in the Earth's history. Hoffman and Schrag make sense of these enigmas, however, in their field studies, as reported in an article they presented in the journal Science in January 2000.
Based on his discoveries and work in 1964 concerning the magnetic orientations of mineral grains in glacial rocks, W. Brian Harland of the University of Cambridge believed that the Earth's continents had all clustered together near the equator. Because he realized that glaciers must have covered the Tropics, Harland was the first geologist to propose the concept that the entire Earth had experienced a great ice age event. While Harland was busy with his research and trying to figure out just how glaciers could have survived the tropical heat, physicists were beginning to develop the first basic mathematical models of the Earth's climate system. In particular, Mikhail Budyko of the Leningrad
Geophysical Observatory discovered a way to explain this enigma. He developed a series of equations that described the way solar radiation interacts with the Earth's surface and atmosphere to control climate. As snow and ice accumulate on the Earth's surface, their high albedo cools the atmosphere and stabilizes and perpetuates their existence. What Budyko referred to as ice-albedo feedback is the same mechanism that helps modern polar ice sheets grow.
An interesting thing occurred with his experiments, however. His climate simulations found that the ice-albedo feedback can get out of control, which is what happened to cause Snowball Earth. When ice formed at latitudes lower than 30° north and south of the equator, the Earth's albedo began to rise at a faster rate because direct sunlight was striking a larger surface area of ice per degree of latitude. This caused the feedback to become so strong in his simulation that surface temperatures dropped severely, which quickly caused the entire Earth to freeze over.
At first Budyko was puzzled at his results, reasoning that if the entire Earth had frozen over, then it must have killed all life on Earth. Yet when scientists examined rocks that were 1 billion years old, they found microscopic algae that resembled modern forms, leading them to believe that life did not cease during this time. Budyko, along with other scientists, was also hesitant to take his model too seriously because he also thought that if the Earth had entered a runaway freeze, it would not be able to pull itself out of it.
The scientific attitude toward these questions began to change in the 1970s, however, when communities of organisms living in places once thought too harsh to allow life to survive were discovered. Seafloor hot springs today support microbes that thrive on chemicals instead of sunlight. This clued in Budyko and other climate scientists to the fact that during Snowball Earth, the volcanic activity that feeds hot springs would have continued to function and could readily have supported life.
The explanation for why the runaway freeze stopped was also answered with newly discovered evidence—it all hinges around CO2. In 1992, Joseph L. Kirschvink, a geobiologist at the California Institute of Technology, determined that during Snowball Earth, the planet's shifting tectonic plates continued to produce volcanoes above subduction zones, releasing CO2 to the atmosphere. While the CO2 was collecting in the atmosphere, there was no rainfall to erode rocks and bury carbon (because the water was frozen), thereby allowing CO2 levels to become extremely high. CO2 also entered the oceans through subsea volcanoes and vents. Slowly over the years, atmospheric CO2 built up and increased the radiative forcing due to the greenhouse effect. Eventually, temperatures at the equator reached the melting point, and the dark surface melt waters caused more solar radiation to be absorbed, soon melting and exposing even larger areas of meltwater. These feedbacks started the process of melting back the ice holding the planet in its grip and conceivably took only a few thousand years to recover from Snowball Earth. Therefore, Snowball Earth was ended by a large-scale intensified greenhouse effect—a large-scale global warming event.
scientists have found several pieces of evidence that support the existence of snowball Earth, such as the following.
• global distributions of glacial deposits on all continents
• land areas that would have been near the earth's equator at the time have glacial deposits on them
• evidence of flooding and water flow exists where land would have been pushed down under the weight of glaciers then uplifted when the heavy ice melted and water flowed off of it
• glacial marine deposits occur in areas where the warmest surface parts of the ocean are evidence of snowball earth also brings up the issue of rapid climate change and its importance to life on earth today. As evidenced from this event, co2 plays a critical role in the earth's climate. As humans continue to affect the climate by heating the atmosphere with greenhouse gases, rapid climate change is a serious possibility—one that could have far-reaching ill effects on humanity and the environment.
EViDENCE FOR I SNOWBALL EARTH
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The Phanerozoic is composed of three eras—the Palaeozoic, Meso-zoic, and Cenozoic. It was during this time that the landmasses of the Earth came together in the supercontinent called Pangaea. Scientists believe that during the Palaeozoic there was increased volcanic activity and that atmospheric CO2 in the early part of this period may have been high based on evidence geologists have found in carbonate minerals from the time. Some estimates have been made that the atmospheric concentration of CO2 may have been 10 times greater in some locations than it is today.
During the mid-Cretaceous period, 90 to 120 million years ago, the Earth was much warmer than today. Often referred to as Greenhouse World, the Earth's warmth extended even to the high (polar) latitudes. This evidence is supported by the abundance of fossil records of plants and animals at polar locations that are found only in warm environments. For instance, corals were discovered far from the equator. Warm-water animals and plants have also been found in polar locations. Scientists have determined that breadfruit trees, a species of tropical vegetation native to the Malay Peninsula and western Pacific islands, grew even in Greenland.
In scientists' efforts to reconstruct past continental positions and ocean currents, they have determined that the Earth's geography and ocean currents were different during the Hothouse Earth period. They have also determined that CO2 levels were much higher—up to four times higher than today. This Hothouse Earth followed the Snowball Earth conditions mentioned previously, so the shift was due to both the rising atmospheric CO2 concentration and the melting of the vast ice sheets.
According to geologists at the University of North Carolina, this period of intense warming triggered the greatest mass extinction in the Earth's history about 250 million years ago, as opposed to the theory that the mass extinction was caused when a meteor hit the Earth. Instead, they claim that volcanic eruptions and global warming were the causes of the mass extinctions during the Permian. As a result of their research, they have determined that the climate turned exceedingly hot because
Following Snowball Earth, the Earth warmed up dramatically and became much warmer than it is today during the Mid-Cretaceous period 90 to 120 million years ago. Vegetation even grew in the polar regions. (Nature's Images)
of a large increase in volcanic activity that released huge amounts of CO2 and methane into the atmosphere, thereby causing rapid global warming. Also harmful to life on the planet was the enormous amount of hydrogen sulphide that entered the atmosphere, which damaged the ozone layer and killed the majority of life-forms.
According to Christopher Poulsen, a paleoclimatologist at the University of Michigan, as the ice melted from the ice-covered areas and the atmospheric CO2 concentration rose, the Tropics became much more arid, the vegetation became stressed, and the ecosystem was replaced by desert. One of the most important things he learned from his research is that the Tropics are very susceptible to large climate changes.
Another episode occurred 55 million years ago when methane was released from wetlands and turned the Earth into another hothouse. Methane is a very powerful greenhouse gas; it is 23 times more effective than CO2. This event is referred to as the Paleocene-Eocene Thermal Maximum (PETM). During this time, the Earth's surface warmed 9°F (5.4°C) in a few hundred years. The Earth warmed up so much that the Arctic Ocean even reached a temperature of 73°F (23°C). Sea surface temperatures rose 8.3-13°F (5-8°C). Scientists support the theory that the entire ocean depths heated up, not just the surface. In addition, the chemical composition had also changed and become harmful. Oxygen content was drastically reduced, causing major die-offs of deep-sea foraminifera.
Climatologists consider this period one of the most significant examples of Earth's sudden global climate change. The period is an example of what an extreme, rapid global warming event causes. It also coincides with a major extinction of both ocean and land species. This major global warming period lasted for about 200,000 years. Life on land was replaced with animals that could survive the extreme temperatures. These animals were mainly smaller versions of the mammal groups that exist today.
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