Atmospheric Chemistry and Climate in the Anthropocene
Atmospheric Chemistry
Introduction
Atmospheric circulation includes the movement of air on a global scale. It is the manner in which that heat is distributed throughout the atmosphere, from equatorial regions that are warmer to polar regions that are colder. The circulation of air in the atmosphere varies somewhat from year to year, but overall, the basic mechanism of circulation remains the same. This helps produce a stable global climate.
Atmospheric circulation is linked to ocean temperature and winds. An example of this relationship are two naturally occurring variations in the temperature of the tropical Pacific Ocean known as El Niño and La Niño.
The warming sea temperature in the case of El Niño and cooling temperature in the case of La Niña can persist for several years. These alterations affect wind patterns, which in turn alter weather. For example, the greater frequency and severity of tropical storms that occurred in the equatorial Atlantic Ocean in 2005 and 2006 (the best known example from the U.S. perspective was Hurricane Katrina) has been linked to the increased upper atmosphere easterly blowing winds that were stimulated by a La Niña. These winds reduce vertical wind shear-wind changes with altitude-which increases the likelihood of the formation of thunderstorms and tropical storms.
Whether global warming is altering atmospheric circulation is still debatable, as of 2007. However, measurements of circulation of air over the tropical Pacific have provided evidence of human-induced changes.
Historical Background and Scientific Foundations
Circulation of air in the atmosphere has likely occurred ever since the formation of the modern-day atmosphere following the appearance of life on Earth. The basis for atmospheric circulation is the differential heating of Earth. Tropical regions are heated more so than the polar regions because the thickness of the atmosphere that the sun’s rays penetrate through is greater at the poles. As a result, the tropical atmosphere is warmer than the atmosphere over the poles, which causes the movement of warm air northward or southward from the equator.
Winds are essential to atmospheric circulation. Aside from winds that occur temporarily, the general circulation of the atmosphere involves surface winds that blow regularly. There are three so-called belts of wind in each hemisphere. Polar easterlies are found from 60 to 90 degrees latitude. In the Northern Hemisphere, these blow from the northeast to the southwest, and from the southeast to the northwest in the Southern Hemisphere.
The prevailing winds from 30 to 60 degrees latitude are called prevailing westerlies. In the Northern Hemisphere, the prevailing westerlies blow from the southwest to the northeast, and from the northwest to the southeast in the Southern Hemisphere. Finally, the region from the equator to 30 degrees latitude north and south is the area where tropical easterlies are found. In the Northern Hemisphere, they blow from the northeast to the southwest, and from the southeast to the northwest in the Southern Hemisphere. Tropical easterlies are also called trade winds. The northern and southern trade winds converge near the equator in a zone; this intertropical convergence zone is an area of cloud and thunderstorms that encircles Earth. It is also an area where winds can be light and variable. Centuries ago, mariners could be calmed for days in the doldrums.
The directions of the winds in these belts are influenced by what are known as cells. The Northern and Southern Hemisphere polar winds are guided by northern and southern polar cells, which are created because of Earth’s rotation. The hemispheric trade winds are guided by a cell called the Ferrel cell, while the tropical easterlies are guided by the Hadley cell.
The polar and Hadley cells are closed loops; warm, more southerly air rises and moves northward. As the air cools, it sinks and moves southward where it heats and rises again, completing the loop. The reason that the winds in the belts do not move north-south and south north is because of the Coriolis effect-the influence that Earth’s rotation exerts.
In addition to these three air circulation cells, other cells operate horizontally, from west-to-east to east-to west.
The horizontal circulation occurs because Earth’s surface is composed of land and water. Water absorbs heat more slowly than does the land, and loses heat to the atmosphere more slowly than land. On a small scale, the result is clear along the ocean coast, where the winds blow onto shore during the day, as cooler air over the sea migrates toward land, and blows out to sea at night, when the air over the ground is cooler than the sea air.
On a much larger scale, this back and forth flow of air occurs over a period of months or even years. Atmospheric circulation is crucial for the global climate and the global pattern of precipitation. The movement of air from regions of low pressure, which tends to encourage precipitation, to regions of high pressure, which do not favor precipitation, helps to distribute moisture through the atmosphere.
Impacts and Issues
Climate changes due to altered atmospheric circulation are not new. Scientists have evidence that altered air circulation in the tropical Pacific Ocean, similar to that which occurs during El Niño, triggered a global climate change about one million years ago. Then, the changed circulation of air caused the polar ice sheets to grow in area, which lengthened the periods of glaciations (the ice ages).
This research has relevance in modern times, for it indicates that tropical regions are very influential to global climate. Thus, conditions that alter atmospheric circulation can change the global climate. It is known that Earth’s atmosphere is warming, due to the increased retention of heat. One reason for this has been suggested to be the gradual accumulation of greenhouse gases-gases produced by human activities.
The link between human activity and atmospheric change used to be very contentious. In 2007, however, only a small minority of scientists still argued that atmospheric warming is free from human influence. The question of whether human activities are influencing atmospheric circulation, however, remains contentious. As one example, a paper published in Nature in 2006 reported on data gathering from 1861 to the early years of the 21st century, which revealed that the difference in pressure between the higher pressure of the western Pacific to the lower pressure of the eastern Pacific has declined over the past 150 years. The data were used in several computer models of climate; some models factored in the influence to pressure change of only natural conditions, and others had the added influence of human activities. The model that incorporated human-influenced atmospheric change most closely matched the actual data.
Other scientists are skeptical of the concluded link between human activities and atmospheric change, because data collected on sea surface temperatures for a much longer time do not support the air pressure data. Whether or not human activities have influenced the changed environment over the Pacific Ocean, however, it is clear that change has occurred, and that such changes in atmospheric circulation do affect global climate.
Words to Know
Coriolis Effect: A pseudo force describing the deflection of winds due to the rotation of Earth, which produce a clockwise or counterclockwise rotation of storm systems in the Southern and Northern Hemispheres, respectively.
Greenhouse Gases: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth’s surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth’s atmosphere, causing global warming and global climate change.
Ice Age: Period of glacial advance.
Intertropical: Literally, between the tropics: usually refers to a narrow belt along the equator where convergence of air masses of the Northern and Southern Hemispheres produces a low-pressure atmospheric condition.
Polar Cells: Air circulation patterns near the poles: relatively warm, moist air approaches the pole at a high altitude, cools, sinks at the pole, and flows southward at a lower altitude. Because of Earth’s rotation, air approaching or receding from the poles flows eastward, producing the polar easterlies.
Trade Winds: Surface air from the horse latitudes (subtropical regions) that moves back toward the equator and is deflected by the Coriolis Force, causing the winds to blow from the Northeast in the Northern Hemisphere and from the Southeast in the Southern Hemisphere. These steady winds are called trade winds because they provided trade ships with an ocean route to the New World.
Wind Cells: More commonly called convective or convection cells; vertical structures of moving air formed by warm (less-dense) air welling up in the center and cooler (more-dense) air sinking around the perimeter. Thunderstorms are shaped by convective cells.
See also: Abrupt Climate Change; Atmospheric Chemistry; Atmospheric Pollution; Atmospheric Structure; El Niño and La Niña; Global Warming; Greenhouse Effect.
Bibliography
Books
Barry, Roger G. Atmosphere, Weather and Climate. Oxford, United Kingdom: Routledge, 2003. Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa. The Atmosphere: An Introduction to Meteorology. New York: Prentice Hall, 2006.
Trefil, Calvo. Earth’s Atmosphere. Geneva, IL: McDougal Littell, 2005.
Periodical
Vecchi, Gabriel A., Brian J. Soden, Andrew T. Wittenberg, et al. ‘‘Weakening of Tropical Pacific Atmospheric Circulation Due to Anthropogenic Forcing.’’ Nature 441, no. 7089 (May 4, 2006): 73-76
