Climate change theories

Earth’s climate is a complex system in constant flux. An understanding of how those changes occur has emerged in recent decades. This insight has allowed for a better understanding of Earth’s history and has helped to mitigate the dangers of modern climate change.

Climate Change Theories

An understanding of Earth’s climate as a dynamic system originated in the eighteenth century from discoveries in geology and paleontology. Continued research in the twentieth century provided an increasingly complex image of how Earth systems are interrelated. An understanding of future anthropogenic (human-caused) climactic shifts is thus based on an understanding of Earth’s climate mechanisms, which are derived from matching historical data with the geologic record.

Methodology

Climatology employs many methods, including tree ring analysis, ice core sampling, sediment core sampling, and satellite observation. Tree ring analysis, or dendroclimatology, works by examining the rings of trees, which depict the growth of a tree during its growing season. Rings are wider under favorable growing conditions and are narrower under less favorable growing conditions. Dendroclimatology works only to the point at which preserved trees can be found; these trees have provided high-resolution data as far back as 11,000 years ago.

Another common method in climatology is to examine ice cores from glaciers. Ice cores are shafts of ice pulled from an ice cap by drilling into that cap with a core drill. Cores are commonly taken from Greenland and Antarctica, but cores also can be taken from mountaintop glaciers. Additionally, ice cores trap air; however, that air is not always the same age as the ice because rates of compression of snow to ice can be low. The oldest data obtained are less than 800,000 years old, but speculation exists that samples could be obtained from 1.5 million years ago. Given the age of ice caps, the oldest records are in Antarctica. Ice cores also can catch useful proxies, such as iridium, pollen, and volcanic ash.

Another common method for climate data collection is sediment drilling from lakes and sea floors. Like their icy counterparts, these sediment cores retain similar data, but they also can record fossil data. Seafloor cores can theoretically go back 200 million years.

Terrestrial rocks go back to about 3.5 billion years, with some isolated instances 1 billion years older. Analysis of their constituent materials and minerals can give an idea of the environment in which they were formed. Ancient sedimentary rocks can give ideas of water levels and temperature, while other stones record the shores and flows of lakes and rivers. Along with these markers, the fossil record also often gives an idea of ancient Earth conditions.

To study the current climate, networks such as the Earth Observation System (EOS) of the National Aeronautic and Space Administration gather various sorts of data. The EOS includes more than fifteen satellites, all designed to monitor different aspects of Earth’s environment. The satellite Aqua observes the water cycle; CloudSat measures cloud altitude and properties.

Milankovitch Theory

The Milankovitch theory is one of the most comprehensive long-term climate change theories available. The theory considers how Earth’s movement affects climate. The theory, discovered by a Serbian engineer during World War I, considers the processes of orbital eccentricity, obliquity, and precession, allowing for scientists to explore the impact of these processes’ individually and in combination.

Earth’s orbit is not a perfect circle; it is an ellipse that varies in eccentricity in time. Changes in the eccentricity of the earth from gravitational interactions with other planets mean that, on occasion, the earth is narrower, which results in greater difference in seasons, and at other times, the earth is more circular, which leads to a lesser difference in seasons. These changes occur because of Earth’s proximity to the sun.

The effect of the variance of eccentricity can be great; at its most elliptical, the earth receives about 23 percent more radiation at its perihelion (the point closest to the sun) than it does at its aphelion (the point farthest from the sun). The eccentricity ranges between about 0.0034 and 0.058. This process takes about 413,000 years. By interaction with other cycles the process combines to create a cycle of about 100,000 years.

The obliquity of the ecliptic (the tilt of Earth’s axis) varies between about 22.1 to 24.5 degrees in about 41,000 years. When the obliquity is greater, the difference in solar radiation received is also greater, meaning a greater difference in the seasons. When the obliquity is lower, less difference occurs between the seasons. It is thought that low obliquity favors ice ages. The cycle now is heading toward its lowest obliquity and, therefore, toward an ice age. It is expected to reach lowest obliquity in about 8,000 years; however, anthropogenic climate change has mitigated this trend.

The axis of Earth’s orbit also precesses in a cycle of about 26,000 years. When the earth’s axis tilts toward the sun at perihelion, the hemisphere pointing toward the sun has a greater difference in seasons that year. When the axis is not pointing toward the sun at a solstice, but instead does so at an equinox, less difference in the seasons occurs between the hemispheres.

The orbital ellipse itself also precesses and operates with changes in eccentricity to alter the length of the seasons. Orbital inclination was not an original part of Milankovitch’s work but has since been added. Earth’s orbit is pulled up and down relative to the plane of the solar system by the gravitational effects of Jupiter. Orbital inclination works on a time scale of about 100,000 years and is thought to have an effect because of the dust clouds and other debris altering the amount of light reaching the earth.

The Milankovitch theory does have its weaknesses: Some trends have been observed but not predicted, and some trends have been predicted but not observed. On the whole, however, the theory fits the observed periodicities of climate.

Cosmic Rays

Another space-based phenomenon that could affect terrestrial climate is cosmic rays. Though disputed, there seems to exist evidence for such a proposition.

Cosmic rays ionize the atmosphere and cause increased cloud cover, resulting in a lower temperature. This would be most noticeable on a long-time scale as the earth moves through the spiral arms of the galaxy and their supernovae density. Earth moves through a spiral arm about once every 135 ± 25,000,000 years, causing flux in cosmic rays with a period of 143 ± 10,000,000 years, matching the cycle of 145 ± 8,000,000 years. An absence in ice ages on earth between 2 billion and 1 billion years ago coincides with a drop in star formation, giving the idea further credence.

Greenhouse Gases and the Carbon-Oxygen Cycle

Another theoretical framework for climate shifts is examination of the carbon-oxygen cycle, which works in tandem with the Milankovitch cycle and is used to explain some points in which the climatic reaction far exceeds the change in the cycle. Because carbon is a greenhouse gas, it has great influence on the climate. Disruption of this mechanism is one of the main causes of anthropogenic climate change.

A good example of how the carbon-oxygen cycle figures into climate is the Azolla event of 49 million years ago. At that time, the Arctic Ocean was warm and closed off, such that it started to collect a layer of freshwater. It is thought that the fresh-water plant Azolla grew and covered the Arctic Ocean. After the Azolla started to die, it sank to the ocean bottom; the freshwater and seawaters did not mix, which sequestered the carbon. It is estimated that this single phenomenon could have caused the 80 percent estimated drop in atmospheric carbon dioxide, dramatically reducing the greenhouse effect and turning the climate from a much warmer one to a far cooler one. In addition, this is the leading scientific theory used to explain the gradual warming of the Earth between the pre-industrial era and the twenty-first century.

Greenhouse and Icehouse Earth

Greenhouse Earth is characterized by high temperatures, which can sometimes reach the polar regions. Eighty percent of Earth’s history has featured this climate. The rest of the climate has been Icehouse Earth. (Today’s Earth, more specifically, is in an interglacial era. Interglacials make up 20 percent of the time in the average glacial period.)

A third state, called Snowball Earth, occurred when the entire planet was believed to have frozen over, though there is some debate about whether or not there was an open or seasonally open ocean near the equator. The state was most likely ended by buildup of volcanic greenhouse gases. It is thought that the transition of icehouse to greenhouse was caused by plate tectonics.

Continental Drift

An important factor in determining climate is continental placement because continents shift the workings of various cycles. It is thought that the closing of the Isthmus of Panama, the opening of the Drake Passage, and the opening of the Tasmanian Gateway helped Earth’s climate transition from “greenhouse” to “icehouse” by changing the flow of ocean currents. The surrounding seas were open to circulation around Antarctica and less able to circulate at the equator. In the south, this led to the creation of the Antarctic Circumpolar Current, which kept warm water from Antarctica, allowing it to freeze. This led to increased albedo, which led to more freezing. A positive feedback loop like this also is thought to have led to Snowball Earth.

Impact

Increased greenhouse gases has caused a drop in the amount of ozone, a gas that shields the planet from ultraviolet radiation. When the temperature drops, as in an ice age, ozone is depleted. When depleted, the stratosphere gets still colder, further depleting the ozone. Ozone also cools the troposphere. Additionally, low-altitude ozone is a greenhouse gas. Greenhouse gases are the widely-accepted reason for the Earth's changing temperatures of the twentieth and twenty-first centuries.

An understanding of climate change not only illuminates historical processes, but also gives an idea of what occurs with anthropogenic climate change. Understanding Earth’s climate provides warning of climate change and also aids in finding solutions to related climate concerns.

Principal Terms

albedo: the reflecting power of a substance

carbon-oxygen cycle: the process by which oxygen and carbon are cycled through Earth’s environment

cosmic ray: high-energy subatomic particles that are produced by phenomena in space, such as supernovae

eccentricity: the departure of an ellipse from circularity; less circularity means greater eccentricity

El Niño: an eleven-year weather cycle in the Western Hemisphere that creates alternating wet and dry periods

greenhouse effect: the process by which some gases trap heat on Earth

obliquity: the angle of tilt between the earth’s rotational axis and an axis perpendicular to the plane of its orbit

precession: a change of the axis of rotation in a rotating body or system

proxies: traces of ancient environments that reveal details, such as climatic data, about those environments

sedimentary rock: rock formed by the repeated deposition of sediment in a body of water or by the layering of material on land

Bibliography

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