An Introduction

According to the National Academy of Sciences, the Earth's surface temperature has risen by about 1 degree Fahrenheit in the past century, with accelerated warming during the past two decades. There is new and stronger evidence that most of the warming over the last 50 years is attributable to human activities. Human activities have altered the chemical composition of the atmosphere through the buildup of greenhouse gases – primarily carbon dioxide, methane, and nitrous oxide. The heat-trapping property of these gases is undisputed although uncertainties exist about exactly how earth’s climate responds to them. Go to the Emissions section for much more on greenhouse gases.

Our Changing Atmosphere

Energy from the sun drives the earth’s weather and climate, and heats the earth’s surface; in turn, the earth radiates energy back into space. Atmospheric greenhouse gases (water vapor, carbon dioxide, and other gases) trap some of the outgoing energy, retaining heat somewhat like the glass panels of a greenhouse.
Without this natural “greenhouse effect,” temperatures would be much lower than they are now, and life as known today would not be possible. Instead, thanks to greenhouse gases, the earth’s average temperature is a more hospitable 60°F. However, problems may arise when the atmospheric concentration of greenhouse gases increases.

Since the beginning of the industrial revolution, atmospheric concentrations of carbon dioxide have increased nearly 30%, methane concentrations have more than doubled, and nitrous oxide concentrations have risen by about 15%. These increases have enhanced the heat-trapping capability of the earth’s atmosphere. Sulfate aerosols, a common air pollutant, cool the atmosphere by reflecting light back into space; however, sulfates are short-lived in the atmosphere and vary regionally.

Why are greenhouse gas concentrations increasing? Scientists generally believe that the combustion of fossil fuels and other human activities are the primary reason for the increased concentration of carbon dioxide. Plant respiration and the decomposition of organic matter release more than 10 times the CO2 released by human activities; but these releases have generally been in balance during the centuries leading up to the industrial revolution with carbon dioxide absorbed by terrestrial vegetation and the oceans.

What has changed in the last few hundred years is the additional release of carbon dioxide by human activities. Fossil fuels burned to run cars and trucks, heat homes and businesses, and power factories are responsible for about 98% of U.S. carbon dioxide emissions, 24% of methane emissions, and 18% of nitrous oxide emissions. Increased agriculture, deforestation, landfills, industrial production, and mining also contribute a significant share of emissions. In 1997, the United States emitted about one-fifth of total global greenhouse gases.

Estimating future emissions is difficult, because it depends on demographic, economic, technological, policy, and institutional developments. Several emissions scenarios have been developed based on differing projections of these underlying factors. For example, by 2100, in the absence of emissions control policies, carbon dioxide concentrations are projected to be 30-150% higher than today’s levels.
 

Changing Climate

Global mean surface temperatures have increased 0.5-1.0°F since the late 19th century. The 20th century's 10 warmest years all occurred in the last 15 years of the century. Of these, 1998 was the warmest year on record. The snow cover in the Northern Hemisphere and floating ice in the Arctic Ocean have decreased. Globally, sea level has risen 4-8 inches over the past century. Worldwide precipitation over land has increased by about one percent. The frequency of extreme rainfall events has increased throughout much of the United States.
 

Increasing concentrations of greenhouse gases are likely to accelerate the rate of climate change. Scientists expect that the average global surface temperature could rise 1-4.5°F (0.6-2.5°C) in the next fifty years, and 2.2-10°F (1.4-5.8°C) in the next century, with significant regional variation. Evaporation will increase as the climate warms, which will increase average global precipitation. Soil moisture is likely to decline in many regions, and intense rainstorms are likely to become more frequent. Sea level is likely to rise two feet along most of the U.S. coast.

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Uncertainties

What's Known  |  What's Likely  |  What's Unknown  | 

Like many fields of scientific study, there are uncertainties associated with the science of global warming. This does not imply that all things are equally uncertain. Some aspects of the science are based on well-known physical laws and documented trends, while other aspects range from 'near certainty' to 'big unknowns.'

What's Known for Certain?

Scientists know for certain that human activities are changing the composition of Earth's atmosphere. Increasing levels of greenhouse gases, like carbon dioxide (CO2 ), in the atmosphere since pre-industrial times have been well documented. There is no doubt this atmospheric buildup of carbon dioxide and other greenhouse gases is largely the result of human activities.

It's well accepted by scientists that greenhouse gases trap heat in the Earth's atmosphere and tend to warm the planet. By increasing the levels of greenhouse gases in the atmosphere, human activities are strengthening Earth's natural greenhouse effect. The key greenhouse gases emitted by human activities remain in the atmosphere for periods ranging from decades to centuries.

A warming trend of about 1°F has been recorded since the late 19th century. Warming has occurred in both the northern and southern hemispheres, and over the oceans. Confirmation of 20th-century global warming is further substantiated by melting glaciers, decreased snow cover in the northern hemisphere and even warming below ground.

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What's Likely but not Certain?

Figuring out to what extent the human-induced accumulation of greenhouse gases since pre-industrial times is responsible for the global warming trend is not easy. This is because other factors, both natural and human, affect our planet's temperature. Scientific understanding of these other factors – most notably natural climatic variations, changes in the sun's energy, and the cooling effects of pollutant aerosols – remains incomplete.

Nevertheless, the Intergovernmental Panel on Climate Change (IPCC) stated there was a "discernible" human influence on climate; and that the observed warming trend is "unlikely to be entirely natural in origin." In the most recent Third Assessment Report (2001), IPCC wrote "There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities."

In short, scientists think rising levels of greenhouse gases in the atmosphere are contributing to global warming, as would be expected; but to what extent is difficult to determine at the present time.

As atmospheric levels of greenhouse gases continue to rise, scientists estimate average global temperatures will continue to rise as a result. By how much and how fast remain uncertain. IPCC projects further global warming of 2.2-10°F (1.4-5.8°C) by the year 2100. This range results from uncertainties in greenhouse gas emissions, the possible cooling effects of atmospheric particles such as sulfates, and the climate's response to changes in the atmosphere.

The IPCC states that even the low end of this warming projection "would probably be greater than any seen in the last 10,000 years, but the actual annual to decadal changes would include considerable natural variability."
 

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What are the Big Unknowns?

Scientists have identified that our health, agriculture, water resources, forests, wildlife and coastal areas are vulnerable to the changes that global warming may bring. But projecting what the exact impacts will be over the 21st century remains very difficult. This is especially true when one asks how a local region will be affected.

Scientists are more confident about their projections for large-scale areas (e.g., global temperature and precipitation change, average sea level rise) and less confident about the ones for small-scale areas (e.g., local temperature and precipitation changes, altered weather patterns, soil moisture changes). This is largely because the computer models used to forecast global climate change are still ill-equipped to simulate how things may change at smaller scales. [See the U.S. Climate section for more detail on climate models.]

Some of the largest uncertainties are associated with events that pose the greatest risk to human societies. IPCC cautions, "Complex systems, such as the climate system, can respond in non-linear ways and produce surprises." There is the possibility that a warmer world could lead to more frequent and intense storms, including hurricanes. Preliminary evidence suggests that, once hurricanes do form, they will be stronger if the oceans are warmer due to global warming. However, the jury is still out whether or not hurricanes and other storms will become more frequent.

More and more attention is being aimed at the possible link between El Niño events – the periodic warming of the equatorial Pacific Ocean – and global warming. Scientists are concerned that the accumulation of greenhouse gases could inject enough heat into Pacific waters such that El Niño events become more frequent and fierce. Here too, research has not advanced far enough to provide conclusive statements about how global warming will affect El Niño.

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Climate Models

Scientists generally agree on the likely rise in the average global temperatures over the next century. Unfortunately, projecting the change in particular regions is more difficult. Nevertheless, there is a general consensus that temperatures will warm throughout the United States. However, scientists are unable to say whether particular regions will receive more or less rainfall; and for many regions they are unable to even state whether a wetter or a drier climate is more
likely.

Virtually all published estimates of how the climate could change in the United States are the results of computer models of the atmosphere known as "general circulation models." These complicated models are able to simulate many features of the climate, but they are still not accurate enough to provide reliable forecasts of how the climate may change; and the several models often yield contradictory results. For the time being, however, these models are about all we have to say how the climate may change in particular areas.

Given the unreliability of these models, researchers trying to understand the future impacts of climate change generally analyze different scenarios from several different climate models. The hope is that, by using a wide variety of different climate models, one’s analysis can include the entire range of scientific uncertainty. For all of these reasons, EPA reiterates the warning provided by all climate modelers to people considering the impacts of future climate change: the projections of climate change in specific areas are not forecasts but are reasonable examples of how the climate might change.

Climate model projections fall broadly into two categories which are known as "CO2 doubling" and "transient" scenarios. The "CO2 doubling" scenarios represent the climate model’s estimate of how the climate would change if the level of CO2 in the atmosphere was doubled and the climate had several decades to reach a new equilibrium. These scenarios were particularly common with older versions of the climate models, which generally analyzed how climate might change without attempting to calculate how the ocean currents might change. They generally do not consider the cooling effect of sulfates or other aerosols.

More recently, elaborate models of the ocean currents have been added to the climate models. The transient scenarios mostly use these more elaborate "coupled ocean-atmosphere" models. Instead of simply calculating how the climate and oceans would respond to a doubling of CO2, these models use the historic and projected changes in concentrations of greenhouse gases and calculate how the climate might change each year until some date in the remote future. Many of these model calculations include the cooling effects of sulfate aerosols.

 

Regional Temperatures

The historical temperature record shows that a rise in the global average temperature does not automatically imply that every part of the world warms. The cooling from sulfates may offset the warming in some areas. Moreover, natural fluctuations in the jet stream and other factors often can cause the Eastern United States to be unusually cool when the West is unusually warm (as well as the reverse). During the summer of 1988, when the East suffered severely hot and dry weather, cold relatively deep ocean water began to flow to the sea surface off the mid-Atlantic Coast, keeping the coastal zone unusually cool. Scientists have not ruled out the possibility that global warming could induce such shifts, which could lead to little or no warming in some areas while other areas warm by much more than the 1.0-3.5°C (3-8°F) expected for the world as a whole.

The region of the United States that has been the most thoroughly examined is the area from 35-50°N and 85-105°W. The transient climate model results suggest that if global temperatures warm 2.6°C, the combined impact of aerosols and greenhouse gases is likely to warm this region approximately 1.5-3.5°C (3-6.5°F) during winter. The same models suggest, however, that summer temperatures will warm between 0 and 0.5°C (less than 1°F). Our actual uncertainty for future temperature change is probably at least twice as great as these ranges suggest, because global warming is also uncertain.

Moreover, the cooling effect of aerosols may prove to be less than assumed by the climate models. When the effect of aerosols is eliminated, however, the same models estimate that summers could warm by 3.5-5°C (6-9°F), and winters by 4-5°C (7.5-9°F). Other climate models, which have estimated the impact of greenhouse gases but not aerosols, suggest that summers in that region could warm 1.2-4.4°C and winters by 1.2-5.8°C.

Temperature projections vary for other regions as well. For example, the Max Plank Institute’s model suggests that California will warm approximately 1°C (2°F) in summer and 3°C (5°F) in winter, while the United Kingdom’s Hadley Centre estimates that both winter and summer could warm by about 3°C (5°F).
 

Regional Precipitation

The nation's water resources are sensitive both to rising temperatures and changes in precipitation. Although scientists expect global temperatures to rise approximately 0.5 to 1.5°C (1-3°F) by the year 2050, most climate models suggest that warming over land--including the continental United States--will be greater than the warming over the sea. Because higher temperatures increase evaporation and plant transpiration, rainfall would generally have to increase just to maintain current levels of water availability. Holding other factors constant, the potential for evaporation and transpiration increases about 5-10% per degree (C) throughout most of the United States (Waggoner and Revelle 1990).

There is a general consensus that annual worldwide precipitation and evaporation will increase a few percent for every degree of warming. But there is considerably less certainty about rainfall in particular locations, and whether the rainfall will increase enough to offset the increased evaporation. Many scientists, however, believe that middle latitudes such as that of the United States will see drier summers: Assuming that the land warms more than the sea, evaporation over the land will increase by more than the evaporation over the sea that produces rainfall. Thus, summer rainfall may not increase by as much as evaporation.

For specific locations, however, it is currently impossible to confidently project even the direction, let along the magnitude or timing, of the seasonal or even annual changes in precipitation. In the Central North American region, the two models that include the effect of sulfates estimate that rainfall may increase slightly more than evaporation, leading to modest increases in soil moisture during both winter and summer.

A more pessimistic picture emerges, however, from the nine climate models that have considered the implications of greenhouse gases without the cooling effect of aerosols. During winter, precipitation changes range from a decline of 15 percent to an increase of 18 percent; during summer, the changes range from a decline of 8 percent to an increase of 6 percent. The scenarios that show an increase in precipitation also project warming of 4-5°C (7-9°F), which would generally cause evaporation to increase by 20 to 50 percent. Thus, all of the scenarios suggest that summers will be drier if the cooling effect of sulfates does not occur. Some of these models also suggest that winters could be drier, while others project wetter winters.

Whether or not annual or seasonal rainfall increases, many climate models project that rainfall will occur in a smaller number of heavier storms, and that the number of dry days is likely to increase. An Australian climate model, for example, projects that total rainfall in the Midwest will decline by about 5 percent, but that heavy rainstorms would occur 2 to 5 times as often. The National Center for Atmospheric Research also expects fewer but heavier rainstorms. The Max Plank Institute estimates that in central North America, 3-month-long dry spells could become about 50 percent more frequent with a 2°C warming.

 

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Ozone: What is it and why we care about it?

Ozone (O₃) is a relatively unstable molecule made up of three atoms of oxygen (O). Although it represents only a tiny fraction of the atmosphere, ozone is crucial for life on Earth.

Depending on where ozone resides, it can protect or harm life on Earth. Most ozone resides in the stratosphere (a layer of the atmosphere between 10 and 40 km above us), where it acts as a shield to protect Earth's surface from the sun's harmful ultraviolet radiation. With a weakening of this shield, we would be more susceptible to skin cancer, cataracts, and impaired immune systems. Closer to Earth in the troposphere (the atmospheric layer from the surface up to about 10 km), ozone is a harmful pollutant that causes damage to lung tissue and plants. The amounts of "good" stratospheric and "bad" tropospheric ozone in the atmosphere depend on a balance between processes that create ozone and those that destroy it. An upset in the ozone balance can have serious consequences for life on Earth, as scientists are finding evidence that changes are occurring in ozone levels—the "bad" tropospheric ozone is increasing in the air we breathe, and the "good" stratospheric ozone is decreasing in our protective ozone layer. This article describes processes that regulate "good" ozone levels.

Relative ozone levels, October 1998. From the Total Ozone Mapping Spectrometer Earth Probe (TOMS EP). Low ozone levels are transparent (blue) while high levels are opaque (white)

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Ozone Balance in Stratosphere :

Ozone :

In the stratosphere, ozone is created primarily by ultraviolet radiation. When high-energy ultraviolet rays strike ordinary oxygen molecules (O₂), they split the molecule into two single oxygen atoms, known as atomic oxygen. A freed oxygen atom then combines with another oxygen molecule to form a molecule of ozone. There is so much oxygen in our atmosphere, that these high-energy ultraviolet rays are completely absorbed in the stratosphere

The relative heights of atmospheric layers. Also view an animation (1.2MB) which shows the outer atmosphere — the thermosphere and exosphere.

The formation of ozone, also animated. (950k)

Ozone is extremely valuable since it absorbs a range of ultraviolet energy. When an ozone molecule absorbs even low-energy ultraviolet radiation, it splits into an ordinary oxygen molecule and a free oxygen atom. Usually this free oxygen atom quickly re-joins with an oxygen molecule to form another ozone molecule. Because of this "ozone-oxygen cycle," harmful ultraviolet radiation is continuously converted into heat.

Natural reactions other than the "ozone-oxygen cycle" described above also affect the concentration of ozone in the stratosphere. Because ozone and free oxygen atoms are highly unstable, they react very easily with nitrogen, hydrogen, chlorine, and bromine compounds that are found naturally in Earth's atmosphere (released from both land and ocean sources). For example, single chlorine atoms can convert ozone into oxygen molecules and this ozone loss balances the production of ozone by high-energy ultraviolet rays striking oxygen molecules.

In addition to the natural ozone balance, scientists have found that ozone levels change periodically as part of regular natural cycles such as the changing seasons, winds, and long time scale sun variations. Moreover, volcanic eruptions may inject materials into the stratosphere that can lead to increased destruction of ozone.

Over the Earth's lifetime, natural processes have regulated the balance of ozone in the stratosphere. A simple way to understand the ozone balance is to think of a leaky bucket. As long as water is poured into the bucket at the same rate that water is leaking out, the amount or level of water in the bucket will remain the same. Likewise, as long as ozone is being created at the same rate that it is being destroyed, the total amount of ozone will remain the same.

Starting in the early 1970's, however, scientists found evidence that human activities were disrupting the ozone balance. Human production of chlorine-containing chemicals such as chlorofluorocarbons (CFCs) has added an additional factor that destroys ozone. CFCs are compounds made up of chlorine, fluorine and carbon bound together. Because they are extremely stable molecules, CFCs do not react easily with other chemicals in the lower atmosphere. One of the few forces that can break up CFC molecules is ultraviolet radiation. In the lower atmosphere, CFCs are protected from ultraviolet radiation by the ozone layer itself. CFC molecules thus are able to migrate intact up into the stratosphere. Although the CFC molecules are heavier than air, the air currents and mixing processes of the atmosphere carry them into the stratosphere.

Once in the stratosphere, the CFC molecules are no longer shielded from ultraviolet radiation by the ozone layer. Bombarded by the sunÕs ultraviolet energy, CFC molecules break up and release chlorine atoms. Free chlorine atoms then react with ozone molecules, taking one oxygen atom to form chlorine monoxide and leaving an ordinary oxygen molecule.

The destruction of ozone, also animated. (1MB)

If each chlorine atom released from a CFC molecule destroyed only one ozone molecule, CFCs would pose very little threat to the ozone layer. However, when a chlorine monoxide molecule encounters a free atom of oxygen, the oxygen atom breaks up the chlorine monoxide, stealing the oxygen atom and releasing the chlorine atom back into the stratosphere to destroy more ozone. This reaction happens over and over again, allowing a single atom of chlorine to act as a catalyst, destroying many molecules of ozone.

Fortunately, chlorine atoms do not remain in the stratosphere forever. When a free chlorine atom reacts with gases such as methane (CH₄), it is bound up into a molecule of hydrogen chloride (HCl), which can be carried downward from the stratosphere into the troposphere, where it can be washed away by rain. Therefore, if humans stop putting CFCs and other ozone-destroying chemicals into the stratosphere, the ozone layer eventually may repair itself.

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What are good and bad ozone ?

Ozone either protects or harms organisms on Earth depending on the height at which it is found. In the high stratosphere "good" ozone acts like a shield protecting Earth's flora and fauna against the Sun's ultraviolet (UV) radiation. An excess of UV radiation increases the risk of getting skin cancer or ocular diseases. It may also weaken the natural immunity of human beings and animals.

On the other hand, near the surface, in the air we breathe, ozone is a pollutant, which in large amounts causes damage to man, animals and plants. It makes up about 10 % of all ozone, and is produced, for example, as a consequence of the exhaust fumes from cars. After a thunderstorm one can smell a sharp scent in the air. The same kind of "electric" smell may also occasionally be sensed while traveling on the metro. This is "bad" ozone, which in large amounts irritates the eyes and lungs. In summertime ozone may cause smog in large cities.

During the Earth's past history, natural conditions have varied the amount of ozone in the atmosphere. The amount of ozone depends on the balance between its production and depletion. The ozone balance can be understood by thinking of a leaky bucket. As long as one pours water into the bucket at the same rate as water is dripping out of it, the amount of water in the bucket remains constant. In a similar way, as long as atmospheric ozone is produced as fast as it is depleted, the total amount stays in balance.

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How is ozone produced and destroyed ?

 

The stratosphere lies in a region about 10-50 km above the surface of the Earth. In the stratosphere, UV radiation from the Sun both creates and destroys ozone. When the UV rays encounter an oxygen molecule (O2), the molecule splits into two oxygen atoms (O). After that, the released oxygen atom can link up with an oxygen molecule, and produce an ozone molecule (O3).

Ozone has the ability to absorb UV radiation emitted by the Sun. This feature makes the ozone layer a molecular shield which protects the Earth against UV radiation. On absorbing UV radiation, an ozone molecule divides into an oxygen molecule and an oxygen atom. The free oxygen atom can consequently join up with an oxygen molecule to create a new ozone molecule, or alternatively grab an oxygen atom from an ozone molecule, leading to the formation of two oxygen molecules. These ozone formation and depletion processes caused by UV radiation are known as Chapman reactions.

Consisting of three oxygen atoms, ozone is an extremely unstable molecule. It reacts readily, releasing its extra oxygen atom, which bonds itself to other elements occurring in nature. These elements are naturally released for example from the soil and oceans, and they have also always been present in the stratosphere. Such natural phenomena as seasonality, winds and volcanic eruptions also affect the distribution of the ozone in the atmosphere.

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How does the action of man affect the ozone layer of the atmosphere?

During the last few decades it has been noticed that man's actions have begun to disturb the ozone balance. The ozone concentration in the upper atmosphere has been declining since the 1970's. The cause of the chemical ozone depletion is the presence of chlorine and bromine originating in man-made freons and halogen compounds.

The main reason for the ozone depletion is the stratosphere becoming polluted by compounds containing chlorine (freons) or bromine (halogens). In addition, ozone is dissipated by nitrous oxide. Due to the fact that the compounds destroying ozone are chemically stable while in the lower atmosphere, they do not break up until they have been carried with the airstreams into the upper atmosphere. Dissipation is caused by powerful UV radiation, as a consequence of which highly reactive chlorine and bromine atoms and their compounds are born. During its lifetime, a single atom or compound of this kind breaks down numerous ozone molecules into ordinary two-atom oxygen molecules, thus preventing the natural reformation of ozone molecules.

CFC compounds include chlorine, fluorine and carbon. For half a century they have been used among other things in producing refrigerators, air conditioning devices and insulating materials.

Fortunately, CFC compounds, chlorine and bromine atoms do not remain in the stratosphere for ever. When, say, a chlorine atom reacts with certain gases (with methane, for example), it forms a hydrochloride molecule. Under the influence of gravity, hydrochloride molecules drift from the stratosphere into the lower atmosphere, i.e. the troposphere (extending from a height of 0 to 10 km above the ground), and finally down to the ground.

If man through his own actions can succeed in preventing freons and other ozone-destroying compounds from drifting into the stratosphere, the damage to the ozone layer will gradually be repaired by itself. The ozone layer is estimated to be at its thinnest at the beginning of the next century, after which it is believed the upper atmosphere will slowly recover within 50 years. A prerequisite for this is that the international agreements restricting the use of compounds destructive to ozone are observed. Measurements carried out in the last few years indicate that concentrations of the most serious ozone-destructive compounds in the atmosphere have been declining since the beginning of the 1990's.

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Ozone Depletion

Especially clearly ozone depletion has been observed in the middle atmosphere above the Antarctic. Since the 1980's, observations have indicated that ozone depletion has been getting ever stronger and wider above the Antarctic. During the last few years, the ozone layer has been thinned at its greatest by over 60 %. The ozone layer gets depleted in September - October, after which the loss is again made up in November-December. The reason for this is the extreme coldness. In temperatures sinking to as low as -80 degrees Celsius, small cloud crystals are formed in the stratosphere, which release chlorine in a reactive form, binding the nitrogen molecules required to take part in the restoring reactions. If the Sun has already began to radiate the area after the Arctic night, the catalytic ozone loss process will get started, leading to a wide ozone depletion over the course of several weeks.

In 1992-1996, unusually strong periodical ozone depletions were observed in the northern hemisphere in the winter and spring time. In 1993, in the ozone layer above Finland, for example, areas were detected with an ozone depletion as large as 40 %. It is believed that these depletions may have been a consequence of, among other things, the eruption of Mt. Pinatubo in 1991. This eruption spewed a huge number of particles into the air, which then found their way into the stratosphere. Pollutants in the air may have enhanced the ozone loss caused by those particles.

The extent of the observed ozone losses have to be compared with the significant variation naturally occurring in the ozone amount. In Finland, the thickness of the ozone layer is, in a normal year, about 30 % smaller in the autumn than that in the spring. At the equator, there is on average 35 % less ozone than there is at the latitude of Finland. Due to the circulation of the atmosphere, even variations of over 20 % of the average during one day are not unusual.

In the northern hemisphere, the ozone layer has so far escaped Antarctic-like depletion. The reason for this is that in the Arctic the upper atmosphere is in winter warmer than it is in the Antarctic, which prevents the formation of stratospheric clouds. This, in turn, is caused by differences in the land-ocean distribution, and atmospheric flow differences between the hemispheres. Mid-atmospheric cooling and low-atmospheric warming caused by the global climate change can, however, make it possible for an Antarctic-like ozone loss to also form in the northern hemisphere in the near future

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Has UV radiation increased

As the ozone layer gets thinner, UV radiation at the surface of the Earth increases. If the ozone amount decreases by 10 % during the spring and summer, the annual UV dose increases by about 12 %. Some indications of increasing UV doses have been noticed during short incidental depletion episodes in Finland during the last years. This had nevertheless only a minor effect on the annual dose, because, according to measurements, the burning UV radiation had increased by over 20 % for only a few days in April and May. People's behavior in the Sun is still dominating on the total UV exposure they receive.

Early in 1996, the climatological conditions in the mid-atmosphere of the northern polar region were exceptionally favourable for the occurrence of ozone depletion. According to stratospheric temperature observations made in Finland, temperatures lower than -78 degrees Celsius, which are conducive to ozone loss, have so far been occurring more frequently than at any time since the beginning of the 1960's, when measurements were started. Ozone depletion has been at its strongest in early 1996 above the Nordic countries and Greenland, and the loss of ozone has been estimated to stem mainly from chemical factors. At its largest, the ozone depletion has been 40 %. Consequently, the UV radiation has occasionally been stronger than the average for the season, but it has still remained below 1/3 of the summertime radiation intensity in Finland.

The Finnish Centre for Radiation and Nuclear Safety (STU) and the Finnish Meteorological Institute (FMI) are developing accurate methods for the measurement of solar UV radiation. UV radiation has been regularly measured by the Finnish Meteorological Institute since 1991, and at the moment there are UV measurement devices at six different sites ranging from Utö up to Sodankylä. The Finnish Meteorological Institute commenced measurements of total column ozone at Sodankylä in 1987 and at Jokioinen in 1991. Measurements of ozone concentrations at different heights in the atmosphere were started in 1988.

The Finnish Meteorological Institute has developed facilities to monitor ozone and the UV situation in real time, so that the observations may be forwarded daily for public use. Additionally, the behavior of the ozone layer is being monitored globally with the aid of satellite observations. Announcements regarding ozone and UV observations are made as the need arises.

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How does UV radiation affect man?

The most well-known effect of UV radiation is the slight reddening or burning of the skin in sunshine. Tanning will occur when the UV radiation causes a pigment called melanin to form in the pigment cells of the skin. This only demonstrates the skin's attempt to protect itself against further damage.

Over a period of years, exposure to radiation originating either from the Sun or solariums causes damages in the skin's connective tissues, so-called photo ageing. This shows itself as a thickening of the skin, as wrinkles and decreasing elasticity. Elastine and collagen fibres determining the firmness and elasticity of the skin are damaged. UV radiation increases the risk of getting skin cancer.

UV radiation enhances the dimming of the eye's lens, which means that potential cataracts begin to evolve at earlier ages. Part of the UV radiation reaches the back of the eye, causing cells in the retina to slowly begin to deteriorate. Damage will in time particularly occur to near vision. Radiation is partly absorbed in the lens of an adult eye, but will go right through the lens of a child, reaching the back of the eye. For this reason, children's eyes in particular should be protected against strong sunlight.

Strong UV radiation can also cause inflammation of the cornea, snow blindness. Symptoms of this kind of an infection include the eyes becoming reddish, a sensitivity to light, enhanced excretion of tears, the feeling of having some dirt in one's eye, and pain. The trauma appears 3-12 hours after exposure. Thanks to the quick regeneration of the eye cells, symptoms will normally disappear within a few days. A long-term exposure to UV radiation may cause permanent damage to the cornea.

UV radiation may weaken the immune system taking care of the body's defence against e.g. infection. These effects are not restricted to the part of skin actually subject to exposure, but may also occur on shielded parts of skin and in the whole immune system.

At the present time, the significance of the immune system weakening caused by UV radiation is not properly understood.

UV radiation also benefits health, generating vitamin D production on the skin. The required amount of radiation is, however, quite small: in summer, an exposure of 15 minutes to the hands and face is adequate. There is no need to get a tan for this purpose. Vitamin D is also found in food. People living in Finland and following a normal diet get enough vitamin D in their food, even in winter. In the treatment of some skin diseases such as psoriasis, UV radiation is being effectively exploited. Under a doctor's control, the benefit from the treatment is much greater than any consequential increase in skin cancer risk.

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What Kinds of effects does UV radiation have on nature?

The greatest risks connected with the depletion of ozone in the stratosphere are ecological. Exposure tests made in USA and Australia have showed that over one hundred species are sensitive to changes in UV radiation, most important of these being the soy bean and a particular pine (loblolly pine) growing in USA. The University of Oulu has investigated the impact of UV radiation on coniferous trees. It has been noted that differences between trees growing in different areas are extremely large. For example, a pine brought from the Kola Peninsula began to grow better than before after receiving boosted UV radiation, whereas the needles of a pine from Kittilä began to shrink under the same treatment. Old needles are able to protect themselves by strengthening the wax coating the outermost layer of their needles and by increasing the amount of protective pigment. In contrast to this, young growing needles suffer easily.

On a global scale, half of the carbon annually bound up in biological assimilation is produced by plankton in oceanic ecosystems. In this way, plankton maintains the basic production of the oceanic food chain. The tolerance of plankton to UV radiation has been found to be very variable, depending on species, which may lead to a disturbance of the balance between separate species. At present, research concerning the impact of UV radiation on oceanic ecosystems is still in its infancy.

International research has revealed that some species of rice suffer from even minor increases in UV radiation, while other species capable of tolerating even intense radiation have also been found. With the help of research, as well as the efficient breeding and cultivation of strong species it will be possible to be prepared for years with a considerably decreased prevailing level of ozone level

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Monitoring Ozone from Space

Since the 1920's, ozone has been measured from the ground. Scientists place instruments at locations around the globe to measure the amount of ultraviolet radiation getting through the atmosphere at each site. From these measurements, they calculate the concentration of ozone in the atmosphere above that location. These data, although useful in learning about ozone, are not able to provide an adequate picture of global ozone concentrations.

Contrary to the image created by the term "ozone layer," the amount and distribution of ozone molecules in the stratosphere vary greatly over the globe. Ozone molecules drift and swirl around the stratosphere in changing concentrations--much as clouds do in the satellite weather pictures you see on television news. Therefore, scientists observing ozone fluctuations over just one spot could not be confident that a change in local ozone levels meant an alteration in global ozone levels, or simply a fluctuation in the concentration over that particular spot. Satellites have given scientists the ability to overcome this problem because they provide a picture of what is happening simultaneously over the entire Earth.

Scientists now are confident that ozone is being depleted worldwide--partly due to human activities. However, scientists still need to determine how much of the loss is the result of human activity, and how much is the result of fluctuations in natural cycles.

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Predicting Ozone Levels

If scientists can separate the human and natural causes of ozone depletion, they can formulate improved models for predicting ozone levels. The predictions of early models already have been used by policy makers to determine what can be done to reduce the ozone depletion caused by humans. For example, faced with the strong possibility that CFCs could cause serious damage to the ozone layer, policy makers from around the world in 1987 signed a treaty known as the Montreal Protocol. This treaty set strict limits on the production and use of CFCs. By 1990, the growing amount of scientific evidence against CFCs prompted diplomats to strengthen the requirements of the Montreal Protocol. The revised treaty called for a complete phase out of CFCs by the year 2000.

However, scientists agree that much remains to be learned about the interactions that affect ozone. To create accurate models, scientists must study simultaneously all of the factors affecting ozone creation and destruction. Moreover, they must study these factors from space continuously, over many years, and over the entire globe. NASA's Earth Observing System (EOS) will allow scientists to study ozone in just this way. The EOS series of satellites will carry a sophisticated group of instruments that will measure the interactions of the atmosphere that affect ozone. Building on more than 20 years of data gathered by previous NASA missions, these measurements will increase dramatically our knowledge of the chemistry and dynamics of the upper atmosphere and our understanding of how human activities are affecting Earth's protective ozone layer.

The initial monitoring of ozone was driven by curiosity about the circulation in the upper levels of the atmosphere. Because measurements of total ozone were observed to be related to the passage of weather systems, it was used for many years as an aid to weather forecasting. Now, of course, the focus is very much on the depletion of the ozone layer due to anthropogenic pollutants and the ensuing negative biological impacts. The Bureau monitors ozone so that it can have data for the intitialisation and verification of global modeling and analysis products, so that we can detect long term trends and in order to resolve questions about the dynamics of the stratosphere and the ozone layer.

The Bureau’s network is a part of the WMO’s Global Atmosphere Watch. It is in a relatively data sparse part of the globe and the global modeling effort is greatly enhanced by the availability of the Australian data, especially the profile data which are becoming increasingly important as reductions in mid-latitude lower stratospheric ozone are observed. It is clear that the Bureau’s surface based network is a vital ingredient in this ‘public good’ research. The fact that atmospheric ozone was monitored for many years before it became an issue highlights the benefit of long term monitoring. Without the measurements that Dobson began in the 1920s, we would be far less capable of reaching conclusions about the status of, and trends in, the ozone layer today.

Ozone is also an important issue in the Climate Change debate. Ozone is a greenhouse gas and stratospheric ozone is a radiatively important constituent responsible for heating of the upper troposphere. Most crucial for climate simulations is the distribution of ozone near the tropopause, as the minimum temperatures here contrast most with the Earth’s surface and allows for the maximum forcing of the earth-troposphere system.

Stratospheric ozone observations- a brief history

Schönbein discovered ozone in 1839 and in 1850 it was determined that it was a naturally occurring atmospheric constituent. By 1860 surface ozone was being measured at hundreds of locations in Europe. The measurements at Paris from 1873 show levels there were half of what they are today. In 1879 Cornu suggested that the rather sharp limitation of the end of the solar spectrum as received at the ground was due to absorption in the atmosphere and, in 1880, Hartley postulated the existence of a layer above the troposphere, the stratosphere, where ozone was responsible for the absorption of solar ultra-violet radiation at wavelengths between 200 and 300nm. In 1921 Dobson and Lindeman, both working on meteors at the Clarendon Laboratory at Oxford University discovered that the temperature in the stratosphere increased with height in contrast to the troposphere. They concluded that radiative processes must dominate in the stratosphere and, as had been predicted by Hartley, that the source of the energy must be from the absorption of solar UV radiation by ozone.

The first measurements of total ozone, namely the total amount of atmospheric ozone between the surface of the Earth and the outermost reaches of the atmosphere, were driven by curiosity about the upper atmosphere. Since it was known that most of the atmospheric ozone should reside in the stratosphere, it was apparent that aspects of the stratosphere could be studied by measuring total ozone. In 1919 and 1920 Fabry and Buisson designed a special spectrograph and began making measurements at Marseilles. They found, quite rightly, that the amount of ozone in an atmospheric column was about 3mm (STP). In 1924 the first Fry spectrograph was built at Oxford. Seven of these instruments were eventually deployed in a network that included the Southern Hemisphere (Montezuma, Chile). In 1927 the Chilean instrument was placed in New Zealand. The Fry spectrographs were very successful, provided data which allowed the general relationship between ozone and the pressure distribution to be found and gave the first picture of the seasonal variation of ozone over much of the globe. Their chief drawbacks were that they required direct sunlight, when the sun was not too low and the measurements took a very long time to obtain because of the photographic developing required. The first photoelectric spectrophotometer that could reliably measure total ozone was produced by Dobson in 1926 (Dobson #1) and about 14 were made before the Second World War. During the 1930s-40s, further research was undertaken on how total ozone amounts correlated with weather patterns, and increased interest was directed on monitoring ozone in support of weather forecasting. This was of particular interest during the War when ground-based total ozone measurements could supplement the loss of weather information caused by the sinking of weather monitoring ships in the Atlantic. Also, during the War, stratospheric humidity was of importance for long range, high flying reconnaissance planes for which a vapour trail would be a dead give away.

Internationally, concerted measurements of ozone commenced in the International Geophysical Year (1957) and following this, during the 1960s, it became evident that the measurement of ozone could provide information concerning large-scale planetary atmospheric circulation. This is possible since in the lower middle atmosphere ozone is a tracer whose lifetime is large compared to the time involved in atmospheric circulation processes. Models of this circulation were initially derived based on the information provided by ozone measurements.

During the 1960s concerns were raised about the effect of atmospheric nuclear tests on the ozone layer but the first concrete indication that human activities could damage the ozone layer came in 1971 when Johnston pointed out that the large fleet of supersonic aircraft proposed by the US would feed considerable amounts of nitric oxide into or just below the ozone layer. Research had shown that oxides of nitrogen were very efficient destroyers of ozone. In 1974 a landmark paper by Molina and raised the possibility of ozone loss in the stratosphere due to presence of halogen-containing man-made chemicals, including the CFCs; first produced commercially in the early 1930s by DuPont, in the atmosphere. Since ozone absorbs almost all solar radiation below about 300nm and prevents this harmful radiation from arriving at the Earth’s surface, intense international debates followed. Also as a result was the upgrading and expansion of ozone measurements worldwide and the development of more sophisticated satellite-borne instruments for measuring ozone and ozone-related trace gases. The first comprehensive satellite observations were started in 1978 with the Nimbus-7 satellite which carried a Total Ozone Mapping Spectrometer (TOMS) instrument. International debate led to international treaty (Vienna Convention and its Montreal Protocol on Substances that Deplete the Ozone Layer). However it was not until concern following the discovery of the Antarctic ozone hole in 1985, followed by the air-borne measurements over Antarctica by NASA in the Spring of 1987 that a causal link between ozone loss and man-made chemicals was established, that the international treaties became strongly influential.

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Last update on 10-03--06