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In March 2007,a former White House official accused
of improperly editing reports on global warming defended his editing changes
Monday, saying they reflected views in a 2001 report by the National
Academy of Sciences. House Democrats said the 181 changes made in three
climate reports reflected a consistent attempt to emphasize the uncertainties
surrounding the science of climate change and undercut the broad conclusions
that man-made emissions are warming the earth.
Philip Cooney, former chief of staff at the White
House Council on Environmental Quality, acknowledged at a House hearing
that some of the changes he made were "to align these communications with
the administration's stated policy" on climate change. It seems the Bush
administrative knows no limits when it comes to policy. That policy is
lying to US citizens and the global community. "My concern is that there
was a concerted White House effort to inject uncertainty into the climate
debate," said Rep. Henry Waxman (news, bio, voting record), D-Calif., chairman
of the House Government Reform Committee.
Cooney said that many of the changes he made
to the reports, such as uncertainty about the regional impact of climate
change and limits on climate modeling, reflected findings of a 2001 National
Academy of Sciences report on climate. James Hansen, director of
NASA's Goddard Institute for Space Studies and one of the country's leading
climate scientists, who said the White House repeatedly tried to control
what government scientists say to the public and media about climate change.
Sir John Houghton,of the Intergovernmental Panel
predicts the sea level is likely to rise by about half a meter by
the year 2100 and that will lead to a more intense
hydrological cycle.In laymen terms this means areas with
heavy rain fall will intensify while global areas
with little rain fall will receive even less.The ecology summit held
in 1997 at Kyoto,Japan where the world's industrial
nations agreed to try and lessen the green house effect will
need to step up their plans on the lessening
of global emissions. The amount of carbon dioxide has already
increased by 30% since 1750 and if no action
is taken soon,it will increase 60% by the middle of this century
making ecology one of the world's top priority
concerns of the next century. Many scientists have already
stated that the world's fresh water supply will
be in serious shortage by the year 2025 and will not be able to
supply the basic needs of 18 nations in the MidEast.
This surely will cause conflict between nations including
Iran, Iraq, Israel and the Palestine area (as
if they needed more issues to deal with as it is.) and cause tension
world wide if drastic measures are not taken
immediately by everyone.If anything would cause a world panic
it would be the shortage of water.Look what happened
when oil was in short supply.
The invasion of Kuwait in 1990 by Iraq leader
Suddam Hussein which brought the armies of the world to drive him out
had little to do with the liberation of Kuwait-
but more like the stopping of a ruthless leader controlling the Persian
Gulf
in which most of the world's oil supply comes
from. Oil is the second largest financial business in the world and to
let a
dictator try and control it would be disastrous.
With Military weaponry as the world's largest economic business and
drugs follow up in third I foresee global
water shortages as a possibility to future global conflict within the next
20 years
at which time a estimated 8 to 12 more nations
in the world will have nuclear war head capabilities.
"These disturbing results demonstrate that
global warming is coming home to roost," said Adam Markham,
director of the wildlife fund's climate program.
"The story will only get worse unlessgovernments and
business take the steps to stop it."Ocean temperatures
have risen three degrees Fahrenheit in some places
over the past 60 years and will rise another
5.5 degrees over the next century if greenhouse gas emissions
continue to grow at current rates, the
report said.
Global warming has coincided with an increased
incidence of the El Niño phenomenon, in which warm
water concentrated in the eastern Pacific creates
volatile weather patterns, it said. Centuries ago El Niño
occurred every two to 15 years,but the pattern
was repeated five times between 1990 and 1997 and record
high global average temperatures were recorded
in 1997 and 1998,the report said.The oceanic heat has
devastated coral reefs and ice shelves that house
species including algae, plankton and crustaceans,
cutting the food supply to larger animals including
whales, penguins and sea lions, it said. Rising sea levels
also threaten to ruin coastal wetlands and other
habitats that support marine animals and commercial fisheries,
the report concluded.
The shortest, or interannual, time scale relates to natural variations that are perceived as years of unusual weather--e.g., excessive heat, drought, or storminess. Such changes are so common in many regions that any given year is about as likely to be considered as exceptional as typical. The best example of the influence of the oceans on interannual climate anomalies is the occurrence of El Niño conditions in the eastern Pacific Ocean at irregular intervals of about 3-10 years. The stronger El Niño episodes of enhanced ocean temperatures (2-8 C above normal) are typically accompanied by altered weather patterns around the globe, such as droughts in Australia, northeastern Brazil, and the highlands of southern Peru, excessive summer rainfall along the coast of Ecuador and northern Peru, severe winter storminess along the coast of central Chile, and unusual winter weather along the west coast of North America.The effects of El Niño have been documented in Peru since the Spanish conquest in 1525. The Spanish term "la corriente de El Niño" was introduced by fishermen of the Peruvian port of Paita in the 19th century; it refers to a warm, southward ocean current that temporarily displaces the normally cool, northward-flowing Humboldt, or Peru, Current. (The name is a pious reference to the Christ child, chosen because of the typical appearance of the countercurrent during the Christmas season.) By the end of the 19th century Peruvian geographers recognized that every few years this countercurrent is more intense than normal, extends farther south, and is associated with torrential rainfall over the otherwise dry northern desert. The abnormal countercurrent also was observed to bring tropical debris, as well as such flora and fauna as bananas and aquatic reptiles, from the coastal region of Ecuador farther north. Increasingly during the 20th century, El Niño has come to connote an exceptional year rather than the original annual event.
As Peruvians began to exploit the guano of marine birds for fertilizer in the early 20th century, they noticed El Niño-related deteriorations in the normally high marine productivity of the coast of Peru as manifested by large reductions in the bird populations that depend on anchovies and sardines for sustenance. The preoccupation with El Niño increased after mid-century, as the Peruvian fishing industry rapidly expanded to exploit the anchovies directly. (Fish meal produced from the anchovies was exported to industrialized nations as a feed supplement for livestock.) By 1971 the Peruvian fishing fleet had become the largest in its history; it had extracted very nearly 13 million metric tons of anchovies in that year alone. Peru was catapulted into first place among fishing nations, and scientists expressed serious concern that fish stocks were being depleted beyond self-sustaining levels, even for the extremely productive marine ecosystem of Peru. The strong El Niño of 1972-73 captured world attention because of the drastic reduction in anchovy catches to a small fraction of prior levels. The anchovy catch did not return to previous levels, and the effects of plummeting fish meal exports reverberated throughout the world commodity markets.
El Niño was only a curiosity to the scientific community in the first half of the 20th century, thought to be geographically limited to the west coast of South America. There was little data, mainly gathered coincidentally from foreign oceanographic cruises, and it was generally believed that El Niño occurred when the normally northward coastal winds off Peru, which cause the upwelling of cool, nutrient-rich water along the coast, decreased, ceased, or reversed in direction. When systematic and extensive oceanographic measurements were made in the Pacific in 1957-58 as part of the International Geophysical Year, it was found that El Niño had occurred during the same period and was also associated with extensive warming over most of the Pacific equatorial zone. Eventually tide-gauge and other measurements made throughout the tropical Pacific showed that the coastal El Niño was but one manifestation of basinwide ocean circulation changes that occur in response to a massive weakening of the westward-blowing trade winds in the western and central equatorial Pacific and not to localized wind anomalies along the Peru coast.
Studies suggest that the ENSO can affect mid-latitude climates, modulating the position and intensity of the polar-front jet stream (see above). An El Niño event that began in early 1982 lasted well into 1983. It was accompanied by unusual weather events outside of the equatorial Pacific region. Western Europe suffered from record summer heat. Late-fall temperatures in the United States were very cold; winter was mild; spring rains were far above normal and spring temperatures in the central part of the country were extremely cold; and summer conditions in the Midwest and Southeast were extremely hot and dry. These abnormal weather conditions were consistent with a shift in the jet-stream pattern away from its normal one and were with little doubt caused in part by the intense El Niño.
The ENSO appears to be a truly disruptive force that wreaks havoc on life. In 1982-83 it not only caused the drought in Australia and the flooding along the western coast of South America and the loss of the anchovy catch there, but it also damaged corn, soybean, and other summer crops in the United States, which resulted in losses amounting to billions of dollars. Clearly a better understanding of the El Niño and its associated atmospheric effects is needed, leading perhaps to predictive skill.
As was explained earlier, the oceans can moderate the climate of certain
regions. Not only do they affect such geographic variations, but they also
influence temporal changes in climate. The time scales of climate variability
range from a few years to millions of years and include the so-called ice
age cycles that repeat every 20,000 to 40,000 years, interrupted by interglacial
periods of "optimum" climate, such as the present. The climatic modulations
that occur at shorter scales include such periods as the Little Ice Age
from the early 16th to the mid-19th centuries, when the global average
temperature was approximately 1 C lower than it is today. Several climate
fluctuations on the scale of decades have occurred in the 20th century,
such as warming from 1910 to 1940, cooling from 1940 to 1970, and the warming
trend since 1970.
Although many of the mechanisms of climate change are understood, it
is usually difficult to pinpoint the specific causes. Scientists acknowledge
that climate can be affected by factors external to the land-ocean-atmosphere
climate system, such as variations in solar brightness, the shading effect
of aerosols injected into the atmosphere by volcanic activity, or the increased
atmospheric concentration of "greenhouse" gases (e.g., carbon dioxide,
nitrous oxide, methane, and chlorofluorocarbons) produced by human activities.
However, none of these factors explain the periodic variations observed
during the 20th century, which may simply be manifestations of the natural
variability of climate. The existence of natural variability at many time
scales makes the identification of causative factors such as human-induced
warming more difficult. Whether change is natural or caused, the oceans
play a key role and have a moderating effect on influencing factors.
Studying the Causes of Droughts and other Climatic PatternsAnother subject still poorly understood is the occurrence of droughts in areas of highly variable rainfall. In the early 1970s and again in the early 1980s the Sahel region of Africa suffered periods of severe drought, resulting in widespread famine and death. There have been many Sahelian droughts before, but the consequences of the recent droughts have been exacerbated by increased populations of people and grazing animals. The combination of drought and population growth results in desertification. It remains an unanswered scientific question as to whether the deterioration of the Sahel and other marginal lands is part of a long-term natural change or whether it is a result of human activities. Some evidence for long-range interactions in the occurrence of droughts and other climatic regimes comes from studies of the ocean currents. It is known that the oceans are a major controlling influence on climate, but the processes involved remain the subject of active research. Some clues have been revealed by studies of El Niño, a minor branch of the Pacific Equatorial Countercurrent that flows south along the coasts of Colombia and Ecuador where it meets the northward-flowing Peru Current. The cold Peru Current keeps rainfall along the coastal area of Peru very low but maintains a rich marine life, which in turn supports major bird populations and a fishing industry. In certain years El Niño becomes much stronger, forcing the Peru Current to the south. Storms rake the coast, causing flooding and erosion. The sudden change in sea temperatures causes dramatic decreases in plankton production and, consequently, in fish and bird populations. Catastrophic El Niño events occurred in 1925, 1933, 1939, 1944, 1958, and 1983. It is thought that the global changes associated with this last event included severe droughts in Australia and Central America and floods in the southwestern United States and Ecuador. Explanations of the El Niño events have invoked both local and long-range interactions in the circulation of the Pacific winds and currents. The study of such dramatic events, enhanced by remote sensing and computer modeling, is a major stimulus to understanding the general circulation of the Earth's atmosphere and oceans.
The ocean, particularly in areas where the surface is warm, also supplies moisture to the atmosphere. This in turn contributes to the heat budget of those areas in which the water vapour is condensed into clouds, liberating latent heat in the process, frequently in high latitudes and in locations remote from the ocean where the moisture was taken up.
The great ocean currents are themselves wind-driven--set in motion by the drag of the winds over vast areas of the sea surface, especially where waves increase the friction. At the limits of the warm currents, particularly where they abut directly upon a cold current, as at the left flank of the Gulf Stream in the neighbourhood of the Grand Banks off Newfoundland and at the subtropical and Antarctic convergences in the oceans of the Southern Hemisphere, the strong thermal gradients in the sea surface result in marked differences in the heating of the atmosphere on either side of the boundary. These temperature gradients tend to position and guide the strongest flow of the jet stream (see below Jet streams) in the atmosphere above and thereby influence the development and steering of weather systems.
Interactions between the ocean and the atmosphere proceed in both directions. They also operate at different rates. Some interesting lag effects, which are of value in long-range weather forecasting, arise through the considerably slower circulation of the ocean. Thus, enhanced strength of the easterly trade winds over low latitudes of the Atlantic, north and south of the Equator, impels more water toward the Caribbean and Gulf of Mexico, producing a stronger flow and greater warmth in the Gulf Stream approximately six months later. Anomalies in the position of the Gulf Stream-Labrador Current boundary, which produce a greater or lesser extent of warm water near the Grand Banks, so affect the energy supply to the atmosphere and the development and steering of weather systems from that region that they are associated with rather persistent anomalies of weather pattern over the British Isles and northern Europe. Anomalies in the equatorial Pacific and in the northern limit of the Kuroshio (also called the Japan Current) seem to have effects on a similar scale. Indeed, through their influence on the latitude of the jet stream and the wavelength (that is, the spacing of cold trough and warm ridge regions) in the upper westerlies, these ocean anomalies exercise an influence over the atmospheric circulation that spreads to all parts of the hemisphere.
Sea-surface temperature anomalies that recur in the equatorial Pacific at variable intervals of two to seven years can sometimes produce major climatic perturbations. Such an anomaly is known as El Niño (Spanish for "The Child"; it was so named by Peruvian fishermen who noticed its onset during the Christmas season).
During an El Niño event, warm surface water flows eastward from
the equatorial Pacific, in at least partial response to weakening of the
equatorial easterly winds, and replaces the normally cold upwelling surface
water off the coast of Peru and Ecuador that is associated with the northward
propagation of the cold Peru (or Humboldt) Current. The change in sea-surface
temperature transforms the coastal climate from arid to wet. The event
also affects atmospheric circulation in both hemispheres and is associated
with changes in precipitation in regions of North America, Africa, and
the western Pacific.
The atmosphere extends from the surface of the Earth to heights of thousands of kilometres, where it gradually merges with the solar wind--a stream of charged atomic particles that flows outward from the outermost regions of the Sun. The composition of the atmosphere is more or less constant with height to an altitude of about 100 kilometres.
The atmosphere is commonly described in terms of distinct layers, or regions. Most of the atmosphere is concentrated in the troposphere, which extends from the surface to an altitude of about 15 kilometres. The behaviour of the gases in this layer is controlled by convection. This process involves the turbulent, overturning motions resulting from buoyancy of near-surface air that is warmed by the Sun. Convection maintains a vertical temperature gradient (i.e., temperatures decline with altitude) of roughly 6 C per kilometre (10.8 F per kilometre) through the troposphere. At the top of the troposphere, which is called the tropopause, temperatures fall to about -60 C (-76 F). The troposphere is the region where virtually all water vapour exists and where all weather occurs.
The dry, tenuous stratosphere lies above the troposphere and extends to an altitude of about 50 kilometres. Convective motions are weak or absent in the stratosphere; motions instead tend to be horizontally oriented. The temperature in this layer increases with altitude.
In the upper stratospheric regions, absorption of ultraviolet light from the Sun breaks down oxygen molecules; recombination of oxygen atoms with O2 molecules into ozone (O3) creates the ozone layer, which shields the lower ecosphere from harmful short-wavelength radiation.
Above the relatively warm stratopause is the even more tenuous mesosphere, in which temperatures again decline with altitude, reaching roughly -85 C at the mesopause. Temperatures then rise with increasing height through the overlying layer known as the thermosphere. Above about 100 kilometres, in the ionosphere, there is an increasing fraction of charged, or ionized, particles. Spectacular visible auroras are generated in this region, particularly along circular zones around the poles, by episodic precipitation of energetic particles.
The general circulation of the Earth's atmosphere is driven by solar energy, which falls preferentially in equatorial latitudes. Atmospheric redistribution of heat poleward is strongly affected by the Earth's rapid rotation and the associated Coriolis force at nonequatorial latitudes (which adds an east-west component to the direction of the winds), resulting in about three latitudinal cells of circulation in each hemisphere. Instabilities produce the characteristic high-pressure areas and low-pressure storms of the mid-latitudes as well as the fast, eastward-moving jet streams of the upper troposphere that guide the paths of storms. The oceans are massive reservoirs of heat, and their slowly changing currents and temperatures also influence weather and climate, as in the so-called El Niño episodes (see OCEANS: Impact of ocean-atmosphere interactions on weather and climate: The El Niño phenomenon). (see also Index: atmospheric circulation, ocean-atmosphere interaction)
The Earth's atmosphere is not a static feature of the environment. Rather its composition has evolved over time in concert with life and continues to change as human activities alter the ecosphere. Roughly halfway through the history of the Earth, the atmosphere's unusual complement of free oxygen began to develop owing to photosynthesis by blue-green algae and subsequently evolving plant life. Accumulation of oxygen eventually made it possible for respirating animals to move out onto the land.
The Earth's climate at any location varies with the seasons, but there are also longer-term variations in global climate. Volcanic explosions, such as the 1991 eruption of Mount Pinatubo in the Philippines, can inject great quantities of particulates into the stratosphere, which remain suspended for years, decreasing atmospheric transparency and resulting in measurable cooling worldwide. Rare, giant impacts of asteroids and comets can have even more profound effects. The dominant climate variations observed in the recent geologic record are the ice ages, which are linked to small variations in the Earth's geometry with respect to the Sun. (see also Index: volcanic eruption)
The Sun is believed to have been less luminous during the early history of the Earth, so if other planetary conditions were identical with those of today, the oceans would have been frozen. But it is expected that there was much more carbon dioxide in the Earth's atmosphere during earlier periods, which would have enhanced greenhouse warming. In this phenomenon, heat radiated by the surface is trapped by gases such as carbon dioxide in the atmosphere and reradiated back to the surface, thereby warming it. There is presently 105 times more carbon dioxide buried in carbonate rocks in the Earth's crust than in the atmosphere, in sharp contrast with Venus, whose atmospheric evolution followed a different course. (see also Index: greenhouse effect)
The amount of carbon dioxide in the atmosphere is rising steadily, however, and has increased by more than 10 percent in the last 30 years owing to the burning of fossil fuels (e.g., coal, oil, and natural gas) and the destruction of tropical rain forests, such as that of the Amazon River basin. A further doubling by the middle of the 21st century could lead to a global warming of a few degrees, which would have profound effects on the sea level and on agriculture.
Of more immediate concern is the impact of human activities on the stratospheric ozone layer. Complex chemical reactions involving traces of man-made chlorofluorocarbons have recently created temporary holes in the ozone layer, particularly over Antarctica, during polar spring. More disturbing, however, is the discovery of a growing depletion of ozone over temperate latitudes, where a large percentage of the world's population resides, since the ozone layer serves as a shield against ultraviolet radiation, which has been found to cause skin cancer.
GREENHOUSE EFFECT INDUCED BY CARBON DIOXIDE AND OTHER TRACE GASES
In spite of these long-term possibilities, the greenhouse problem has received the least policy-oriented attention of any of the three major issues at hand. There are various reasons for this: (1) The problem is fraught with technical uncertainties. (2) It has perceived "winners" and "losers"--economic and otherwise. (3) No one nation acting alone can do much to counteract the CO2 buildup in the atmosphere. (4) Dealing with the problem substantively could be expensive and even alter life-styles. (5) There is no way of proving the validity of the greenhouse theory to everyone's satisfaction except by "performing the experiment" on the real climatic system, which would necessarily involve all living things on Earth. (6) The principal greenhouse gas, CO2, is an inherent by-product of the utilization of a commodity that is most fundamental to the economic viability of the world--fossil-fuel energy. (This fact more than any other explains why the greenhouse problem is so difficult to solve.)
It seems appropriate to break down the issue of greenhouse warming into a series of stages and then consider how policy questions might be addressed against the background of these more technical stages. The present discussion will deal with the problem specifically as it relates to increasing atmospheric CO2 for the sake of simplicity, though other related questions certainly can be dealt with in the same manner.
(UNCED), byname EARTH SUMMIT, conference held at Rio de Janeiro, Brazil (June 3-14, 1992), to reconcile worldwide economic development with protection of the environment. The Earth Summit was the largest gathering of world leaders in history, with 117 heads of state and representatives of 178 nations in all attending. By means of treaties and other documents signed at the conference, most of the world's nations nominally committed themselves to the pursuit of economic development in ways that would protect the Earth's environment and nonrenewable resources.The main documents agreed upon at the Earth Summit are as follows. The Convention on Biological Diversity is a binding treaty requiring nations to take inventories of their plants and wild animals and protect their endangered species. The Framework Convention on Climate Change, or Global Warming Convention, is a binding treaty that requires nations to reduce their emission of carbon dioxide, methane, and other "greenhouse" gases thought to be responsible for global warming; the treaty stopped short of setting binding targets for emission reductions, however. The Declaration on Environment and Development, or Rio Declaration, laid down 27 broad, nonbinding principles for environmentally sound development. Agenda 21 outlined global strategies for cleaning up the environment and encouraging environmentally sound development. The Statement of Principles on Forests, aimed at preserving the world's rapidly vanishing tropical rainforests, is a nonbinding statement recommending that nations monitor and assess the impact of development on their forest resources and take steps to limit the damage done to them.
The Earth Summit was hampered by disputes between the wealthy industrialized nations of the North (i.e., western Europe and North America) and the poorer developing countries of the South (i.e., Africa, Latin America, the Middle East, and parts of Asia). In general, the countries of the South were reluctant to hamper their economic growth with the environmental restrictions urged upon them by the North unless they received increased Northern financial aid, which they claimed would help make environmentally sound growth possible.
Ozone is 1.5 times as dense as oxygen; at -112 C (-170 F) it condenses
to a dark blue liquid, which freezes at -251.4 C (-420 F). The gas decomposes
rapidly at temperatures above 100 C (212 F) or, in the presence of certain
catalysts, at room temperatures. Although it resembles oxygen in many respects,
ozone is much more reactive; hence, it is an extremely powerful oxidizing
agent, particularly useful in converting olefins into aldehydes, ketones,
or carboxylic acids. Because it can decolorize many substances, it is used
commercially as a bleaching agent for organic compounds; as a strong germicide
it is used to sterilize drinking water as well as to remove objectionable
odours and flavours
The debate concerning the environmental impact of the SST has had a lasting effect on the development of atmospheric science, spawning a new interdisciplinary program of research linking chemists, physicists, and biologists in a common effort to understand the stratosphere. The program, with international participation, has been remarkably successful and has led to a new view of the interdependence of the atmosphere, hydrosphere, and biosphere.
The concern over the effects of exhaust gases from supersonic aircraft was soon followed by a new issue: the possibility that chlorine atoms released by decomposition of chlorofluorocarbons could have a larger and more persistent effect on stratospheric ozone. CFC's were developed first in the 1930s but found widespread use only in the years following World War II. They were employed with great success by U.S. troops in the Pacific to dispense insecticides from aerosol spray cans. This led to many commercial uses, from propellants and refrigerants to foaming agents and degreasers and a host of other applications. Moreover, the use of CFC's rapidly spread from the United States to Europe and the Far East. All this changed in 1975, when it was recognized that the release of CFC's to the atmosphere could pose a serious problem for stratospheric ozone. Production of F-12 declined from a peak of about 4.5 105 metric tons in 1975 to about 3.4 105 metric tons in 1982. A similar drop was registered for F-11.
Much of the work undertaken since the mid-1970s has focused on the effects of CFC's on the assumption that the composition of the atmosphere was otherwise constant. It has become clear, however, that the response of ozone depends not simply on the abundance of CFC's but also on the abundances of methane, nitrous oxide, and carbon monoxide. These species, too, are changing. Current models suggest that a continuing release of CFC's at the rate registered in 1980, other gases remaining constant, would lead to a reduction in stratospheric ozone by about 5 percent. Maximum impact is predicted to occur at altitudes above 25 kilometres. An increase in nitrous oxide of 20 percent is expected to cause a reduction in ozone of about 2 percent. An increase in carbon dioxide should lead to a reduction in stratospheric temperatures with a consequent reduction in the anticipated impact of CFC's and nitrous oxide on ozone. The effect of an increasing burden of methane is more complex. Oxidation of methane provides a source of ozone at low altitude, while the reaction of chlorine with methane converts chlorine radicals to hydrogen chloride, resulting in a reduction in the impact of CFC's between 30 and 40 kilometres.
Models suggest that the change in the column density of stratospheric ozone to date should be relatively small. Reductions in ozone at high altitude, near 40 kilometres, ought to be balanced by excess production at low altitudes due in part to the higher level of methane and in part to NOx released by high-altitude aircraft. Observational evidence is consistent with this view. A statistical analysis concluded that the change in the ozone column from 1970 to 1983 averaged -0.003 percent per decade. It showed, however, that a small though statistically significant drop in ozone--a decline of about 2 percent--occurred at altitudes above 30 kilometres between 1970 and 1980.
There has been a new development since 1985. That year, Joseph C. Farman
and his associates at the British Antarctic Survey reported that the level
of ozone over Antarctica had dropped precipitously every October since
1982, with the first such change apparent as early as 1978. A number of
theories, or more properly hypotheses, have been advanced to account for
this phenomenon. Several implicate effects of anthropogenic chlorine, enhanced
by small quantities of bromine. Others suggest that the reduction may be
due to a diminished supply of ozone from low latitudes, reflective of a
change in stratospheric dynamics. In any case, the phenomenon was quite
unexpected and serves as a powerful warning that current scientific understanding
of the stratosphere is still rudimentary.
By the mid-1970s, various U.S. investigators had determined that chlorofluorocarbons
(CFCs), widely employed as propellants in aerosol spray cans, could reduce
the amount of stratospheric ozone significantly. A temporary ban was imposed
on the use of certain CFCs in the United States, but only after much emotional
debate among environmental and industrial scientists, reports by the National
Academy of Sciences, and the development by industry of economically viable
substitutes for spray-can propellants.
Sulfur dioxide results primarily from the burning of large amounts of soft coal and high-sulfur oil. It is toxic to a wide range of plants at concentrations as low as 0.25 part per million (ppm) of air (i.e., on a volume basis, one part per million represents one volume of pure gaseous toxic substance mixed in one million volumes of air) for 8 to 24 hours. Gaseous and particulate fluorides are more toxic to sensitive plants than is sulfur dioxide because they are accumulated by leaves. They are also toxic to animals that feed on such foliage. Fluorine injury is common near metal-ore smelters, refineries, and industries making fertilizers, ceramics, aluminum, glass, and bricks.
Ozone and peroxyacetyl nitrate injury (also called oxidant injury) are more prevalent in and near cities with heavy traffic problems. Exhaust gases from internal combustion engines contain large amounts of hydrocarbons (substances that principally contain carbon and hydrogen molecules--gasoline, for example). Smaller amounts of unconsumed hydrocarbons are formed by combustion of fossil fuels (e.g., coal, oil, natural gas) and refuse burning. Ozone, peroxyacetyl nitrate, and other oxidizing chemicals (smog) are formed when sunlight reacts with nitrogen oxides and hydrocarbons. This pollutant complex is damaging to susceptible plants many kilometres from its source. Ozone and peroxyacetyl nitrate are capable of causing injury if present at levels of 0.01 to 0.05 part per million for several hours.
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