Climate change may result from both natural and human causes. The importance of human causes has been increasing during the past few decades.
Causes
The major causes of climate change are described in the following sections.
1. CO2 and Other Greenhouse Gas Variations
Many natural and human-made gases contribute to the greenhouse effect that warms the Earth's surface. Water vapor (H2O) is the most important, followed by carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and the chlorofluorocarbons (CFCs) used in air conditioners and many industrial processes.
The increasing atmospheric CO2 concentration is likely the most significant cause of the current warming. Other greenhouse gases along with other factors discussed in the following sections also contribute.
2. Human Activity and Greenhouse Gas
The world’s economy runs on carbon: the “fuel” in fossil fuels. Coal, oil, and natural gas contribute energy to nearly every human endeavor in industrialized nations, and carbon dioxide (CO2) is a by-product of burning these fuels. Immediately eliminating CO2 emissions would literally stop the industrial world. This graph illustrates how thoroughly fossil fuels and CO2 emissions are integrated into American life.
CO2 contributes more to the recent increase in greenhouse warming than any other gas. CO2 persists in the atmosphere longer and longer as concentrations continue to rise.
Other chemicals such as methane, nitrous oxide, and halocarbons also contribute to the global greenhouse effect. A number of additional chemicals related to urban pollution, such as low-level (tropospheric) ozone and black soot, can have a strong regional and perhaps global warming effect. Sulfate aerosols may have a cooling effect.
3. Reducing Other Greenhouse Gases :. Bessy’s Stomachs
Methane is the second most significant cause of greenhouse warming, behind carbon dioxide. Bessy, the science cow, and her many brothers and sisters are one of the greatest methane emitters. Bessy’s grassy diet and multiple stomachs cause her to produce methane, which she exhales with every breath. The sheer size of her herds makes a significant contribution to global warming.
4. Ocean Circulation
A. Direct Effect of Oceans on Climate
The atmospheric circulation (winds) and ocean currents carry heat from the tropics toward the poles. Many processes can alter these circulation patterns, changing the climate regionally or even over the whole world.
Interactions between the ocean and atmosphere can also produce phenomena such as El Niño, which tends to recur every two to six years. Changes in deep ocean circulation can produce longer-lived climate variations that endure for decades to centuries. The ice age cycles may have been influenced by changes in ocean circulation arising from changes in the Earth’s orbit around the Sun.
B. Effect Of Oceans On Greenhouse Gases
The oceans play an important role in determining the atmospheric concentration of CO2. CO2 gas in the atmosphere and CO2 dissolved in the ocean surface reach a balance. Changes in ocean circulation, chemistry, and biology have shifted this balance in the past. Such changes may affect climate by slowly moving CO2 into or out of the atmosphere.
A volcanic eruption may send ash and sulfate gas high into the atmosphere. The sulfate may combine with water to produce tiny droplets (aerosols) of sulfuric acid, which reflect sunlight back into space. Large eruptions reach the middle stratosphere (19 miles or 30 kilometers high). At this altitude, the aerosols can spread around the world.
A massive volcanic eruption can cool the Earth for one or two years. The 1982 El Chichon eruption and the 1991 Pinatubo eruption caused the globally averaged surface temperature to cool less than 1°F.
6. Solar Variations
The Sun is the source of energy for the Earth’s climate system. Although the Sun’s energy output appears constant from an everyday point of view, small changes over an extended period of time can lead to climate changes. Some scientists suspect that a portion of the warming in the first half of the 20th century was due to an increase in the output of solar energy.
Learning how the Sun changed before modern instruments were available is not easy, but it appears that changes in the output of solar energy have been small over the last million years, and probably even longer.
8. Orbital Variations
Slow changes in the Earth’s orbit lead to small but climatically important changes in the strength of the seasons over tens of thousands of years. Climate feedbacks amplify these small changes, thereby producing ice ages.
Eccentricity
Earth’s orbit oscillates very slightly between nearly circular and more elongated every 100,000 years. This cycle is evident in the glacial/interglacial cycles of roughly the same period.
Tilt
The Earth spins around an axis that is tilted from perpendicular to the plane in which the Earth orbits the Sun. This tilt causes the seasons. At the height of the Northern Hemisphere winter the North Pole is tilted away from the Sun, while in the summer it is tilted toward the Sun. The angle of the tilt varies between 22° and 24.5° on a cycle of 41,000 years. When the tilt angle is high, the polar regions receive less solar radiation than normal in winter and more in summer.
Wobble
There is a slow wobble in the Earth’s spin axis, which causes the peak of winter to occur at different points along the Earth’s elliptical orbital path. This variation in the seasons occurs on an approximately 23,000-year cycle.
9. Land Use Changes
When humans transform land from forests to seasonal crops or from natural to urban environments, the regional climate system is altered. For example, clear-cut hillsides are significantly warmer than forests. Urban environments are also islands of heat produced by industry, homes, automobiles, and by asphalt’s absorption of solar energy. Land use changes are not likely to have a large, direct effect on global average temperature.
Changing uses of the land are also associated with changes in the usage and availability of water, as well as the production of greenhouse gases. Deforestation can significantly increase the amount of atmospheric CO2, which warms the planet.
Amplifiers
Factors that can amplify or reduce the effect of the causes of change are known as "feedbacks." Some of the key feedbacks are described in the following sections. These feedbacks consist of interconnected processes in which a change in one leads to a change in another, which ultimately leads to further changes in the first.
1. Aerosols
Small particles in the air (aerosols) may have warming or cooling effects, depending on their characteristics. Sulfate (SO4) aerosol, for example, is light-colored and reflects sunlight back into space. The cooling effect of volcanic aerosols from the Mt. Tambora eruption of 1815 caused North America’s “year without a summer” in 1816. Sulfate aerosol is also produced by fossil fuel burning.
Black soot, which is a familiar component of urban smog and smoke from wild fires, has the opposite effect. The dark particles absorb the Sun’s energy in much the same way that dark asphalt roads become warm on sunny days.
Aerosol concentrations change for many reasons, including volcanic eruptions, spread of fires, increased windiness, drying of damp soils, changes in industrial processes, and more. Accurately projecting the extent and effect of aerosols is one of the major challenges in modeling the future of climate change.
2. Clouds
Like aerosols, clouds can either warm or cool the Earth, depending on their density and altitude. Their behavior demonstrates the intricate interactions at work within the climate system.
Very small differences in clouds may produce large feedbacks. An increase in high, thin clouds produced by greenhouse warming would further increase the warming. This is because high, thin clouds are relatively effective in trapping infrared radiation (heat) while allowing the Sun’s energy to pass through. In contrast, an increase in thick, low clouds could lessen the warming because these clouds reflect sunlight efficiently.
Changes in clouds result from changes in the distribution of water vapor, temperature, and winds. The effects of global warming on these factors are complex and not well understood.
In addition, aerosols may also play a role in cloud formation. Tiny aerosol particles can “seed” clouds by providing the “nuclei” around which cloud droplets are formed. High concentrations of some aerosol types may affect the character of clouds by causing many tiny droplets to form rather than a few big ones. Clouds with more tiny droplets reflect more solar energy and tend to produce less rainfall.
3. Water Vapor
Today, water vapor produces two-thirds of the world’s greenhouse effect. All of the other gases – carbon dioxide, methane, nitrous oxide, halocarbons, etc. – contribute the other third. The effect of water vapor is so significant that the global average temperature would be below freezing without it.
Warm air can contain more moisture than cold air. This is the basis of the water vapor feedback. As the atmospheric temperature rises and the amount of water vapor increases, the greenhouse effect is enhanced, further increasing temperature.
The water vapor feedback is critical for producing the glacial/interglacial cycles. Uncertainty in the magnitude of the water vapor feedback is an important source of uncertainty in projecting future climate warming.
A Common Source of Confusion
Does the water vapor added to the atmosphere by cooling towers and smokestacks contribute to global warming?
These sources are tiny compared to natural evaporation from the land and ocean. However, the water vapor feedback is important in increasing water vapor concentration and the greenhouse effect.
4. Ice-Reflectivity
Ice-free surfaces tend to absorb more solar energy than ice-covered surfaces. Therefore, snow and ice cover have a cooling effect on the Earth. If global warming reduces the global snow and ice cover, the warming will be enhanced because more solar energy will be absorbed. This ice-reflectivity feedback does not operate in polar regions during the winter, when it is always dark or the Sun is very low in the sky.
© National Academy of Sciences
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