Earth Science: Atmospheric Science

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Earth Science: Atmospheric Science


Atmospheric sciences are a group of disciplines that comprise the formal study of the atmosphere that envelopes Earth, from just above the ground all the way to near space, where a vacuum exists. Traditionally it is split into three fields of study that developed in approximately the following historical sequence: meteorology, which studies the motion and phenomena of the troposphere, also seeks to predict the weather and explain the various processes involved in weather and atmospheric phenomena; climatology, the study of the atmosphere at a given place over multiple years or greater periods of time; and aerology (or aeronomy), sometimes viewed as a subfield of meteorology, which focuses mainly on chemical and physical reactions that occur within the five major atmospheric layers.

Historical Background and Scientific Foundations

From ancient times, man has been keenly aware of the power of nature and the need to better understand the atmosphere and mitigate the impact of hurricanes, tornadoes, drought, torrential downpours, blizzards, and other types of severe weather. The ancient Greeks were the first to write about weather and climate, producing three key texts known today: On Airs, Waters, and Places by Hippocrates (460–375 BC) which among other things discusses the impact of climate on human health; De Ventis by Theophrastus (372–287 BC), an extensive discussion of the origin and function of winds; and Aristotle's (384–322 BC) Meteorologica, the first textbook on meteorology (which included everything between Earth and the moon). Aristotle discussed a wide range of weather phenomena including winds, clouds, rain, snow, hail, thunder, lightning, and rainbows, but time quickly showed that virtually all of his explanations were incorrect, based as they were on reason with no basis in experimentation. Arabic scholars over the next several centuries added to the knowledge of the Greek philosophers and began to keep records of weather phenomena in their areas.

European interest in finding new lands, people, and riches in the sixteenth century led to the discovery of Arabic books and translations of ancient Greek texts on meteorology. Unfortunately, these voyages of discovery also generated fanciful and wildly exaggerated accounts of weather conditions, climate, and natural phenomena that would negatively influence scientific understanding of Earth's atmosphere for several centuries.

Galileo Galilei (1564–1642) invented a simple thermometer called a thermoscope in 1592 that estimated temperature change by measuring the expansion of air. Despite its pleasing aesthetics, it had no scale and was unreliable, although replicas of this device are still available today. Galileo's device was further improved into a clinical thermometer by fellow Italian Santorio Santorii (1561–1636) in 1612.

Evangelista Torricelli of Italy (1608–1647), an assistant to Galileo, invented a mercury barometer in 1643. The device consisted of a glass tube 3.9 feet (1.2 m) long, filled with mercury and inverted into a dish. Atmospheric pressure exerted on the mercury controlled the height of the mercury and created a vacuum at the top of the column. Torricelli's barometer was also used as a model for the creation of a liquid-in-glass thermometer that was invented in Florence, Italy, sometime in the mid-seventeenth century.

In his book Les Météores, René Descartes (1596–1650) reworked many of Aristotle's ideas from the Meteorologica; and while it perpetuated many false notions, it moved meteorology onto a somewhat firmer footing with an emphasis on the need for hypotheses and experiments. Perhaps most importantly, in 1662 Irish chemist and physicist Robert Boyle (1627–1691) formulated “Boyle's Law,” the basic mathematical relationship between pressure and volume of a gas. In Paris, meanwhile, the government started to collect daily weather data. While the records from these early years are not standardized or complete, it set a global precedent regarding their importance.

Eighteenth-century technological advances in instrumentation allowed much more precise measurements of atmospheric data and encouraged systematic data collection on weather, climate, and the atmosphere, including the use of kites to measure temperatures above Earth's surface, building on the pioneering work of American scientist Benjamin Franklin's (1706–1790)

study of lightning in 1752. English scientist Sir Edmond Halley (1656–1742), for whom Halley's comet is named, used wind data from ships' logs to construct the first meteorological map in 1668. He focused on the tropical surface winds, called the trade winds. The map was a significant aid to ocean navigation and signaled the beginning of a much more systematic understanding of the dynamic nature of the atmosphere.

Halley knew that warm air rises near the equator and that cooler air moves downward over Earth's surface from its northernmost regions, but he thought that air simply “followed the sun” as it moved from east to west across the sky. Some 50 years later amateur meteorologist and member of the Royal Society George Hadley (1685–1768) was mystified as to why winds in the Northern Hemisphere, which he thought should flow in a straightforward northerly direction, had a pronounced westerly flow. He published information about the trade winds from various sources in his 1735 book, Concerning the Cause of the General Trade-Winds.

He argued that sunlight in the equatorial zone strikes Earth at almost a right angle, and therefore each unit area of the surface receives more sunlight than areas farther north. Since warm air is less dense than cold air at the same pressure, this warmer air must rise and be replaced by cooler air flowing toward the equator from higher latitudes. Applying Newton's first law of motion, Hadley attributed the result to the rotation of Earth. Later study would show that this was incorrect, since in a rotating system it is angular momentum that is conserved, not linear momentum, but his general description moved the study of the atmosphere forward significantly. Temperature readings became more standardized with the introduction of the centigrade or Celsius scale in 1736, named after its inventor, astronomer Anders Celsius (1701–1744) of Sweden.

English chemist Henry Cavendish (1731–1810) discovered hydrogen as a component of the atmosphere and settled a running scientific dispute as to whether the composition of the air varied from place to place. Conducting over 60 experiments throughout what is today central London, Cavendish established that there was virtually no difference in the composition of the air in various locations. He calculated a concentration for oxygen in the atmosphere at 20.83%, a number that is only 0.12% removed from today's accepted value of 20.95%. He went on to show that there was no variation between air on the ground and that a few thousand feet up. Fellow Englishman John Dalton (1766–1844), the founder of modern chemistry, made pioneering studies that discerned correctly that no chemical reaction was involved in the evaporation of water into the atmosphere—a subject quite controversial at the time.

The atmospheric sciences began to develop more rapidly in the early nineteenth century, as did the physical and biological sciences. Weather data such as cloud types, temperature, atmospheric pressure, wind velocity, precipitation and other basic measures were now collected systematically in many places around the world, including New Haven, Connecticut, which has the longest continuous sequence of records in the United States, dating back to 1779. Some weather-observation stations in England have records that go back over 325 years, but most weather stations have continuous records that span only the last century or so. The United States War Department established a national weather service in 1870, at first using the army signal office to collect daily weather reports via telegraph from about 500 observers across the nation. The weather service was moved to the Department of Agriculture in 1891 and renamed the National Weather Bureau.

Alexander von Humboldt (1769–1859), a leading scientist of the day, used data from various places to construct the first world map of mean annual temperatures in 1816, using isotherms to connect identical temperature readings. Just the year before, much of North America and Europe experienced unusually cold weather as a result of the explosion of Mount Tambora in Indonesia in April 1815. The eruption sent 3.5 million cubic feet (100,000 cubic meters) of volcanic debris into the upper atmosphere, where it shrouded the entire globe.

By 1827 Prussian physicist and meteorologist Heinrich Wilhelm Dove (1803–1879) used weather maps and other information to develop his laws of storms that laid the foundation for weather prediction. Among his other important observations were that tropical cyclones rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. He investigated the effects of climate on the growth of plants. He was also fascinated by the manner in which heat is distributed over Earth's surface. Dove believed that low-pressure storm systems formed as polar and equatorial air masses met. Other refinements in this period included the invention of the psychrometer in 1825, the invention of the pyrheliometer to measure insolation in 1837, and the systematic use of balloons to collect measurements from varied levels of the lower atmosphere.

Map making that employed atmospheric data developed rapidly during this century. William Redfield (1789–1857) created the first weather map in the United States in 1831. He was drawn into the study of meteorology when he observed firsthand the damage of the “Great September Gale” of 1821 in southern New England. He determined from observations he made at the time and studying the aftermath in Connecticut that it was a “progressive whirlwind” (what we now call a hurricane). He noted that trees in Connecticut had fallen toward the northwest in the eastern part of the state and toward the southeast in the western part. From this data and other information, Redfield correctly deduced that the wind patterns were in a counterclockwise direction. He also concluded that low-pressure systems were caused by a spinning air mass that moved air from the center to the periphery of the storm, creating a calm spot in the middle. Since small mountains seemed to stop a hurricane's advance, he and other amateur meteorologists of the time wrongly concluded that hurricanes extended only a mile or so into the atmosphere, a belief that would only disappear after the laws of physics derived in the 1920s and 1930s laid them to rest. Redfield's many scientific interests led to his election in 1848 as the first president of the American Association for the Advancement of Science.

Fellow American meteorologist James Pollard Espy (1785–1860) proposed an alternate theory to Redfield's, arguing that air just above Earth's surface rises due to heat it receives from the sun; as it does so, it takes water vapor with it. (Today we realize that the air is not heated directly by the sun but rather that Earth's atmosphere acts as a reflector, trapping heat from the sun near Earth's surface, the albedo effect.) As this column of heated air rises and barometric pressure decreases, the air expands and becomes less dense. This causes the water vapor in the air to condense, which leads to the formation of a cloud. The condensation process releases heat that provides a mechanism by which more moist air can be drawn up into the storm. Espy's ideas linked barometric pressure and temperature and showed how the two concepts are related to low-pressure system formation.

German geographer Heinrich Berghaus (1797–1884) and his nephew Hermann Berghaus (1828–1890) developed a world map of precipitation in 1845 with their widely read Physikalischer Atlas. Heinrich Wilhelm Dove published maps of mean monthly temperatures in 1848 and Austrian geographer Alexander Supan (1847–1920) created a map of world temperature regions in 1879. Supan divided the world into five major climatic zones and created 34 climatic provinces within them. This was the first major taxonomy of climate zones and greatly advanced the field by bringing some order to the accumulating data. The Frenchman E. Renou drew the first map of mean pressure for Western Europe in 1862.

With the discovery of the stratosphere in 1902 and the ozone layer in 1913, the twentieth century witnessed a rapid acceleration in scientific understanding of the atmosphere as its different layers were clearly delineated.

Norwegian meteorologist Vilhelm Bjerknes (1862–1951), who was trained as a mathematical physicist, advanced a theory of atmospheric circulation that included both hydrodynamics and thermodynamics. His ideas were later influential in oceanography as well. Bjerknes concentrated on seven variables—air pressure, temperature, density, water vapor content, and three factors related to wind. Drawing on his extensive knowledge of physics, he knew that all seven had accompanying physical laws that could account for changes within them. He coupled this knowledge with a detailed and systematic approach to drawing weather maps.

With regard to wind for example, he created streamlines (lines of constant direction) and isotachs (lines of constant wind speed) from which vertical wind speeds could be determined using graphical calculus. By 1918 Bjerknes and his colleagues began to articulate a polar front theory regarding the weather in the Scandanavian countries, adapting the word “front” from its military use in World War I (1914–1918), because he viewed weather as a struggle between warm and cold currents in a global conflict. Bjerknes's influential approach to meteorology, which treated it as a product of the movement of distinct air masses and fronts that form on the boundaries between them, has become the basis for modern weather prediction. Bjerknes and his colleagues founded the Bergen School of Meteorology in 1917.

In addition to contributing terminology, World War I led to several technological innovations that benefited the study of the atmosphere, most notably aircraft. Predicting weather patterns and climate was essential to battlefield strategy not only due to the combatants' active air corps, but also to ensure that large-scale gas attacks did not go awry and lead to gassing one's own troops.


Prior to Hurricane Katrina's devastation of New Orleans and a significant stretch of the Gulf Coast in 2005, the worst natural disaster in the history of the United States was the Galveston, Texas, hurricane of 1900. The city was nearly destroyed; the death toll ranged between 6,000 and 12,000 people.

Galveston, at the turn of the century, was on its way to becoming a significant American metropolis. But the developers who built on Galveston Island, a low sandbar in the Gulf of Mexico, had ignored reports of an 1841 storm that had flooded the island.

Around August 27, 1900, a storm began forming in the Cape Verde Islands in the North Atlantic. Heading westward, the storm struck Cuba the first week of September, dumping 24.34 inches (87 cm) of rain. A Havana observatory issued a statement predicting that the storm would likely intensify once it headed toward Florida. Meteorologists at the U.S. Weather Bureau in Washington, D.C., however, previously frustrated by “alarmist” reports they received from Cuba about the severity of West Indian storms, discounted the alarm.

By September 5 the storm was gradually heading northward. Weather Bureau forecasters predicted that it would move northward and threaten both Florida and the East Coast, ignoring Cuban forecasters' speculation that the storm would move northwestward into the Gulf of Mexico instead. On September 6 the Weather Bureau erroneously reported that the storm was 150 miles northwest of Key West and as late as that afternoon warned fishermen in New Jersey to remain in port. Meanwhile the storm was getting larger, faster, and heading toward Texas.

A ship, the Louisiana, ran into the storm in the Gulf early in the afternoon of the sixth and clocked wind speeds of 100 miles per hour (161 km/h). The very next day, the steamship Pensacola was nearly destroyed by the storm, but managed to remain afloat. Unfortunately, neither vessel was equipped with a wireless transmitter, so no warnings could be sent.

By the morning of the sixth it was clear that the storm was not going to hit Florida, and all U.S. Weather Bureau offices along the Gulf of Mexico were ordered to hoist storm warnings. There was no effort to indicate the possible severity of the storm, a standard practice at the time to avoid alarming the public or crippling business interests.

On Friday, September 7, Isaac M. Cline, chief of the local Weather Bureau office in Galveston, awoke to the sound of heavy breakers on the beach and confusing information from headquarters. Monitoring the breakers throughout the day and into the next, he found the interval between them lengthening, a sign that a large storm was likely brewing out in the Gulf. On the other hand, the sky was clear; neither high cirrus clouds nor the brick-dust sky that Cline believed were always associated with hurricanes were visible. The barometer was falling, however, and by Saturday morning the sea had risen high enough that the seaward end of a number of city streets were flooded. Cline telegraphed Washington, D.C., of this news, noting that opposing winds were now in play that he had not seen before. Still, no hurricane warning was issued from his office to the citizens of Galveston—such warnings had to be issued from Washington.

Cline did not realize the severity of the situation in part because he accepted that a storm surge like the one that swept the Ganges River basin in 1876, killing over 100,000 people, was impossible in Galveston because of the way that waters shoaled as they approached its coastline. He also thought that Galveston Bay would absorb any serious flooding, an argument he had earlier used to rebut the city's need for an expensive sea wall. At the same time, Cline seemed to ignore the 1841 inundation of Galveston Island and the complete havoc wrought by two separate hurricanes that struck the port of Indianola, 150 miles (240 km) southwest of the city, in 1875 and 1886; the last one devastated the city so severely it was never rebuilt.

Whether Cline ultimately issued a warning sometime in late morning or early afternoon Saturday is still a matter of dispute. No survivors remembered any warning being given. The storm struck the city full force on Saturday afternoon; Cline himself recorded that by 7:30 PM the water rose as much as 4 feet (1.2 m) within a few seconds, quickly reaching a level of about 10 feet (3 m) above the ground where his own home stood. At least another 5-foot (1.5-m) rise occurred within the next hour, coupled with winds as high as 140 mph (225 km/h). Cline's wife Cora, with 31 others who had sought refuge at his home, perished. Cline and his three children clung to debris for over three hours before the waters subsided enough to find high ground.

No one knew of the disaster until well into the following day. Even then, early reports of the dead, missing, and injured were viewed in Houston as impossibly high. Subsequentely, Clara Barton, founder of the American Red Cross and a veteran of Civil War battlefield carnage, reported she had never seen anything like it. Corpses were burned in funeral pyres.

The storm's devastation left profound changes in its wake. The city began construction of a 17-foot- (5-m-) high, 6-mile (10-km-) long seawall and started the painstaking process of raising the city to as much as 17 feet (5 m) above preexisting levels. From the 1920s to 1961, the seawall was extended still farther to its current length of 10 miles (16 km). These precautions proved their worth, substantially reducing damage from violent storms.

Meteorologists quickly figured out that the Germans had a major advantage because winds on the Western front made it easier for them to glide safely back to the German lines in the event of engine failure, while Allied planes were afforded no such wind assistance, and always had to be sure that they knew how far they were from troop lines so that if their engine failed they would be safely behind their own lines. Bjerknes seized on the public's fascination with aircraft to advocate for a global network of weather observatories and trans-Atlantic travel, a dream fulfilled in the 1930s.

By 1925 several nations were engaged in systematic collection of atmospheric data from aircraft. Radiosondes were employed for the first time in 1928 to capitalize on advances in balloons, radiometry, and knowledge of wind currents. This practice continues as the United Nations' World Meteorological Organization (WMO) coordinates the daily launch of hundreds of meteorological balloons to study the upper atmosphere and improve local weather forecasting.

The use of rockets to study the atmosphere followed the pioneering work of American inventor Robert H. Goddard (1882–1945), who launched a scientific rocket in the early 1930s with an attached camera and a barometer to collect data high up in the atmosphere. The Meteorological Rocket Network (MRN) was launched at North American missile ranges in 1959 under supervision of the military and later expanded to a global, nonmilitary system that embraces some 60 sites worldwide today. The MRN has roughly doubled the amount of the atmosphere that can be subjected to relatively thorough study. The United States Weather Bureau, as it was called in the 1930s, conducted large-scale analyses of air masses and adopted much of the work of the Bergen School of Meteorology. The bureau also funded the creation of more elaborate communication systems to facilitate weather forecasting.

World War II (1939–1945) spurred further developments in the study of the atmosphere, as the movement of troops, supplies, and tactics were strongly dependent on accurate weather forecasting and prediction of major climatic changes. The U.S. armed forces, for example, employed some 8,000 weather officers. During the battle for the Marshall Islands in 1942, when American naval task forces realized their vulnerability to Japanese counterattacks, they used weather fronts as natural barriers to detection, following the front when possible toward safe harbor. The U.S. Navy flew an aircraft directly into the eye of a hurricane for the first time in 1943 to collect scientific data and better understand how to protect troops and ships from these large natural phenomena. The chief weather forecaster for the Royal Air Force in Britain, Norwegian-born Sverre Petterssen (1898–1974) built on emerging knowledge of the jet stream and put it to use in nightly bombing runs over Germany, thereby conserving bomber fuel and extending the planes' range. The use of long-range weather forecasting (up to five days with reasonable accuracy) led to successful Allied forecasts for amphibious landings in both North Africa and on the beaches of Normandy.

Radar was another technological advance in World War II that turned out to have profound implications for atmospheric sciences. Radar today is used for the short-term prediction of rain or severe weather at ground locations and to study severe storms, precipitation development, and the spatial structure of precipitation patterns. Television weather reports routinely show live Doppler radar pictures of precipitation. High atmospheric radar is used to investigate winds and the thermal structure of the upper atmosphere.

A third boon from World War II was the extensive number of ex-military aircraft that were purchased by or donated to university researchers, increasing their ability to track storms and take measurements in the lower atmosphere. This included the American Thunderstorm Project, where, by flying through developing thunderstorms, scientists developed a much more accurate understanding of their formation, dynamics, and life cycle.

Monitoring atmospheric data and phenomena from space began in 1960 when the United States launched the first meteorological satellite, the Television and Infrared Observation Satellite (TIROS) I, which transmitted images of large-scale weather systems. The TIROS series of satellites and their successors continue to provide improved detection of developing major weather patterns, including the successful warning of Hurricane Carla in 1963 that permitted an early evacuation and saved many lives. Data from these types of satellites was supplemented in the United States by the deployment of a national Doppler radar network in 1990 that provides even more fine-grained tracking of storm systems. Efforts similar to those in the United States have spread to many countries including Russia, Japan, China, India, and the countries of Europe.

The National Science Foundation (NSF) established the National Center for Atmospheric Research at the University of Colorado at Boulder in 1960. It became a focal point for the entire community of researchers across the United States and for visiting scholars from many other nations. NSF provided substantial research funding and support for graduate education in atmospheric studies from the late 1950s onwards, and also gave the field its present name of atmospheric sciences. The American Meteorological Society picked up this designation in 1962 when it retitled its lead journal the Journal of Atmospheric Sciences and launched a new Journal of Applied Meteorology to affirm that the study of atmosphere is about much more than just weather prediction.

Military developments also helped advance atmo-spheric sciences during the 20 years a number of nations practiced open-air nuclear weapons testing. The release of radioactive particles high into the atmosphere required atmospheric scientists to track them; this generated new understanding of upper atmospheric wind patterns. This work contributed to the construction of the 1963 Nuclear Test Ban Treaty between the United States and Soviet Union, ending above-ground nuclear tests by these two nations. It also established the clear role of the upper atmosphere's influence on weather in the troposphere, an insight derived in large part from one of Bjerknes's former students, Swedish-American

meteorologist Carl-Gustaf Rossby (1898–1957) at the Massachusetts Institute of Technology.

The U.S. National Climate Program Act of 1978 was a significant development for atmospheric sciences globally. It included a number of elements that greatly advanced scientific understanding of the atmosphere, including provisions for developing new methods to improve climate forecasts, gathering global data on a continuing basis, systems for disseminating climatological data and information, mechanisms for climate-related studies, experimental climate forecast centers, increasing international cooperation in climatology, and a robust program of research and development in atmospheric studies. Increased cooperation among nations led to the Global Atmospheric Research Programme (GARP) in the 1960s and 1970s.

John von Neumann (1903–1957), the Hungarian-born American mathematician and contributor to ENIAC, the first computer, was granted government funding to attempt to model the weather beginning in 1946. He drew upon earlier work by British physicist Lewis Fry Richardson (1881–1953) who developed a way to evaluate analytical equations describing the weather. Although the first computer weather predictions from this work in 1949 and 1951 were crude by today's standards, von Neuman's pioneering efforts attracted technically oriented people into meteorology to create more sophisticated models that progressively improved weather prediction and better understanding of the possible impact of human activities on the planet's atmosphere.

American mathematician Edward N. Lorenz (1917–) showed mathematically that a very small change in initial conditions within the atmosphere could have dramatic effects on later events. This led to the rigorous application of chaos theory to meteorology. Today's models for weather and global climate change employ the power of supercomputers and retrieve and utilize millions of data points and various simulations. Despite the sophistication of these models, Earth's atmosphere and the billions of interactions within it that occur on a daily basis are still vastly more complicated than the most advanced computer model of this dynamic system.

Coordinating global atmospheric information among nations and organizations is the responsibility of the World Meteorological Organization, an official agency of the United Nations. Since 1966 this global network has successfully identified and tracked every tropical storm, as well as all other major weather events.

Twentieth-century atmospheric sciences developed in close concert with the disciplines associated with oceanography. Wallace S. Broecker (1931–) studied the circulating currents of the northern Atlantic and discovered that they convey heat from tropical waters to the shores of northwest Europe. Broecker's “conveyor belt” dramatically influences the weather patterns of northern Europe and results in a much warmer climate that would be possible without this warm oceanic circulation system.

In the 1920s British physicist and statistican Sir Gilbert Walker (1868–1958), head of the Indian Meteorological Survey, discovered an inverse relationship between the air pressure over the Indian Ocean and over the Pacific Ocean. He called this the southern oscillation. Scientists linked this phenomenon to one well known to fisherman off the coast of Peru, who for centuries had noticed the ocean generally warmed around Christmas, a phenomenon they called El Niño (Spanish for “little boy”) in reference to the celebration of the birth of Jesus. This rise in water temperature along the equator in the Pacific Ocean was linked to heavier seasonal rains in South America. At UCLA, Norwegian-born American meteorologist Jacob Bjerknes (son of Vilhelm) tied the two together and christened them El Niño southern oscillation (ENSO). La Niña (little girl) or El Viejo (old man) is the opposite end of the oscillation when the Pacific Ocean water at the equator turns cooler than normal. Today scientists know that ENSO and La Niña affect both climate and fish populations in each hemisphere. Each oscillation lasts about five years, with the most intense temperature difference amounting to about 50°F (10°C). Climatic data gathered from the study of tree rings documents the existence of this oscillation over the past 750 years. Steeper changes in temperature observed in the past decade have led some scientists to speculate that this is further evidence of global warming.

Aerology, the third branch of the atmospheric sciences, has broadened and deepened considerably in the twentieth century with advances in chemistry and physics and aided by increased use of sophisticated monitoring instruments on the ground and in near space. The Swedish chemist Svante Arrhenius (1859–1927) presented a paper to the Stockholm Physical Society in 1895 in which he argued that widespread combustion of fossil fuels (e.g., coal and petroleum) would lead to global warming. Smog in cities within Great Britain was identified as a major health concern in 1905 and coal and chemical combustion were linked to the deaths of thousands of people between 1850–1960, including a particularly bad episode of London-type smog that led to over 4,000 deaths in London in 1952. The burgeoning city of Los Angeles had to contend with photo-chemical smog as early as the late 1940s although it was not described scientifically until 1951 by Dutch chemist Arie Haagen-Smit (1900–1977), who produced ozone in a laboratory, simulating a daily occurrence in the Los Angeles basin.

Modern Cultural Connections

In 1970 Paul Crutzen (1933–) reported that nitrogen oxides released as waste products from soil bacteria rise all the way into the stratosphere. There they are broken apart by sunlight, a chemical reaction that depletes the amount of protective ozone (a special form of isotopic

oxygen that helps shield Earth's surface from ultraviolet solar radiation) in the atmosphere. His report was not widely accepted within the scientific community until Mario Molina (1943–) and Sherwood Rowland (1927–), working at the University of California at Irvine reported in 1974 that chlorofluorocarbon (CFC) gases accelerate the decay of the ozone layer. Invented in 1928, CFCs were in wide use by the early 1970s as refrigerant gases in air conditioners and in aerosol cans. The 1995 Nobel Prizewinning research of these three chemists from Holland, Mexico, and the United States respectively, coupled with observations from James Anderson (1944–) that the concentration of chlorine oxide in the upper atmosphere above Texas was much higher than predicted, led to a flurry of research.

The ozone layer, discovered in 1913, was first measured over the Antarctic by a British survey team in

1957. By the 1970s it was clear that it was diminishing; by 1985 measurements indicated a layer 100 times thinner than in the 1970s. In 1982, Anderson and a team from Harvard University used a U2 aircraft to measure the ozone layer and reported the presence of a hole over Antarctica, with a similar hole discovered over the Arctic. Informed speculation suggested that both were caused by the widespread use of CFCs. In light of the growing scientific concern and the work of many environmental groups, governments around the globe agreed on a series of international protocols named after the city in which they were negotiated. The Montreal Protocols of 1987, 1990, and 1992 phased out the use of CFCs in most nations of the world, and mandated the substitution of new substances; China and India have yet to sign the agreement. The other major discovery in the 1980s and 1990s was that Earth appeared to be warming; this effect was also suspected to be caused by human activity.

The study of the atmosphere over the centuries demonstrates a number of important themes. Foremost is the role that technology has played. Second, military needs and funding have given scientists new tools and support for much basic and applied work. Third, controversies among scientists and a multidisciplinary approach have been critical to acquiring better knowledge. Finally, advances in atmospheric sciences have come from scientists and nonscientists around the world, not just a single nation or region. It is truly an international science and important to the future of the planet we call home.

Atmospheric science today is a highly complex set of disciplines working together to elucidate the highly convoluted manner in which gas, radiative, aerosol, meteorological, cloud, transport, and surface processes combine their effects moment by moment in Earth's atmosphere. Comparative study of nearby planets with atmospheres has sharpened our understanding of our own. Land masses, oceans and seas, the sun, and human activity all contribute to the dynamic system of Earth's atmosphere and it is clear that the composition of the atmosphere has changed dramatically over time. While many major advances in our understanding of this gaseous envelope that surrounds Earth have occurred, there are still vastly more mysteries to be solved.

Primary Source Connection

Ozone is generally found both in the troposphere, which extends 5 to 9 miles (8 to 14.5 km) above sea level, and in the stratosphere, which extends between 10 and 31 miles (17 and 50 km) in altitude. In the troposphere, ozone is considered a pollutant because it contributes to the formation of smog. In the stratosphere, ozone occurs naturally and acts as an important protective shield against harmful radiation from the sun. Stratospheric ozone absorbs much of the ultraviolet energy at

wavelengths between 240 and 320 nm that impinges on Earth. This ultraviolet radiation is responsible for sunburn, skin cancer, damage to vegetation, and higher rates of genetic mutation in many invertebrate animals.

Ozone is formed in the stratosphere by sunlight. Oxygen molecules are generally found as two oxygen atoms joined together, symbolized as O2. When ultra-violet energy from the sun strikes an oxygen molecule, it can dissociate the two atoms of oxygen, O. Oxygen atoms are extremely reactive and quickly join with a molecule of oxygen to form the ozone molecule (O3).

Ozone can be destroyed naturally when an atom of oxygen joins with ozone to form O4. The O4 molecule then splits to form two molecules of oxygen, O2. However, this reaction occurs too slowly to account for the concentrations of ozone that are actually found in the stratosphere. Research has shown that other gases have the ability to destroy ozone as well. In particular, manmade chemicals that contain chlorine, known as chlorofluorocarbons or CFCs, are particularly effective at destroying ozone. CFCs are used in refrigeration and cooling systems, aerosols and solvents. Also, gases containing bromine and nitrogen oxides are involved in the breakdown of ozone. Nitrogen oxides result from the burning of fossil fuels.

In the following article, the authors discuss holes in the protective ozone layer, their potential causes, and the possibility of closing them.


May 26, 2006: Think of the ozone layer as Earth's sunglasses, protecting life on the surface from the harmful glare of the sun's strongest ultraviolet rays, which can cause skin cancer and other maladies.

People were understandably alarmed, then, in the 1980s when scientists noticed that manmade chemicals in the atmosphere were destroying this layer. Governments quickly enacted an international treaty, called the Montreal Protocol, to ban ozone-destroying gases such as CFCs then found in aerosol cans and air conditioners.

Today, almost 20 years later, reports continue of large ozone holes opening over Antarctica, allowing dangerous UV rays through to Earth's surface. Indeed, the 2005 ozone hole was one of the biggest ever, spanning 24 million sq km in area, nearly the size of North America.

Listening to this news, you might suppose that little progress has been made. You'd be wrong.

While the ozone hole over Antarctica continues to open wide, the ozone layer around the rest of the planet seems to be on the mend. For the last 9 years, worldwide ozone has remained roughly constant, halting the decline first noticed in the 1980s.

The question is why ? Is the Montreal Protocol responsible? Or is some other process at work?

It's a complicated question. CFCs are not the only things that can influence the ozone layer; sunspots, volcanoes and weather also play a role. Ultraviolet rays from sun-spots boost the ozone layer, while sulfurous gases emitted by some volcanoes can weaken it. Cold air in the stratosphere can either weaken or boost the ozone layer, depending on altitude and latitude. These processes and others are laid out in a review just published in the May 4th issue of Nature: “The search for signs of recovery of the ozone layer” by Elizabeth Weatherhead and Signe Andersen.

Sorting out cause and effect is difficult, but a group of NASA and university researchers may have made some headway. Their new study, entitled “Attribution of recovery in lower-stratospheric ozone,” was just accepted for publication in the Journal of Geophysical Research. It concludes that about half of the recent trend is due to CFC reductions.

Lead author Eun-Su Yang of the Georgia Institute of Technology explains: “We measured ozone concentrations at different altitudes using satellites, balloons and instruments on the ground. Then we compared our measurements with computer predictions of ozone recovery, [calculated from real, measured reductions in CFCs].” Their calculations took into account the known behavior of the sunspot cycle (which peaked in 2001), seasonal changes in the ozone layer, and Quasi-Biennial Oscillations, a type of stratospheric wind pattern known to affect ozone.

What they found is both good news and a puzzle.

The good news: In the upper stratosphere (above roughly 18 km), ozone recovery can be explained almost entirely by CFC reductions. “Up there, the Montreal Protocol seems to be working,” says co-author Mike Newchurch of the Global Hydrology and Climate Center in Huntsville, Alabama.

The puzzle: In the lower stratosphere (between 10 and 18 km) ozone has recovered even better than changes in CFCs alone would predict. Something else must be affecting the trend at these lower altitudes.

The “something else” could be atmospheric wind patterns. “Winds carry ozone from the equator where it is made to higher latitudes where it is destroyed. Changing wind patterns affect the balance of ozone and could be boosting the recovery below 18 km,” says Newchurch. This explanation seems to offer the best fit to the computer model of Yang et al. The jury is still out, however; other sources of natural or manmade variability may yet prove to be the cause of the lower-stratosphere's bonus ozone. Whatever the explanation, if the trend continues, the global ozone layer should be restored to 1980 levels sometime between 2030 and 2070. By then even the Antarctic ozone hole might close—for good.

Patrick L. Barry

Tony Phillips


See Also Chemistry: Chaos Theory; Earth Science: Climate Change; Earth Science: Oceanography and Water Science.



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Dennis Cheek

Kim Cheek

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