The atmosphere of the Earth is divided into 5 layers. From closest and thickest to farthest and thinnest the layers are: troposphere, stratosphere, mesosphere, thermosphere and exosphere. The majority of the atmosphere’s ozone resides in the stratosphere, which extends from 6 miles above the Earth’s surface to 31 miles. Humans rely heavily on the absorption of ultraviolet B rays by the ozone layer because UV-B radiation causes skin cancer and can lead to genetic damage. The ozone layer has historically protected the Earth from the harmful UV rays, although in recent decades this protection has diminished due to stratospheric ozone depletion.
History of Ozone Depletion
Ozone depletion is largely a result of man-made substances. Humans have introduced gases and chemicals into the atmosphere that have rapidly depleted the ozone layer in the last century. This depletion makes humans more vulnerable to the UV-B rays which are known to cause skin cancer as well as other genetic deformities. The possibility of ozone depletion was first introduced by scientists in the late 1960′s as dreams of super sonic transport began to become a reality. Scientists had long been aware that nitric oxide (NO) can catalytically react with ozone (O3) to produce O2 molecules; however, NO molecules produced at ground level have a half life far too short to make it into the stratosphere. It was not until the advent of commercial super sonic jets (which fly in the stratosphere and at an altitude much higher then conventional jets) that the potential for NO to react with stratospheric ozone became a possibility. The threat of ozone depletion from commercial super sonic transport was so great that it is often cited as the main reason why the US federal government pulled support for its development in 1971. Fear of ozone depletion was abated until 1974 when Sherwood Rowland and Mario Molina discovered that chlorofluorocarbons could be photolyzed by high energy photons in the stratosphere. They discovered that this process could releasing chlorine radicals that would catalytically react with O3 and destroy the molecule. This process is called the Rowland-Molina theory of O3 depletion.
The Ozone Hole
From 1985 to 1988, researchers studying atmospheric properties over the south pole kept noticing significantly reduced concentrations of ozone directly over the continent of Antarctica. For three years it was assumed that the ozone data was incorrect and was due to some type of instrument malfunction. In 1988, researchers finally realized their error and concluded that an enormous hole in the ozone layer had indeed developed over Antarctica. Examination of NASA satellite data later showed that the hole had begun to develop in the mid 1970′s.
The ozone hole over Antarctica is formed by a slew of unique atmospheric conditions over the continent that combine to create an ideal environment for ozone destruction.
- Because Antarctica is surrounded by water, winds over the continent blow in a unique clockwise direction creating a so called “polar vortex” that effectively contains a single static air mass over the continent. As a result, air over Antarctica does not mix with air in the rest of the earth’s atmosphere.
- Antarctica has the coldest winter temperatures on earth, often reaching -110 F. These chilling temperatures result in the formation of polar stratospheric clouds (PSC’s) which are a conglomeration of frozen H2O and HNO3. Due to their extremely cold temperatures, PSC’s form an electrostatic attraction with CFC molecules as well as other halogenated compounds
As spring comes to Antarctica, the PSC’s melt in the stratosphere and release all of the halogenated compounds that were previously absorbed to the cloud. In the antarctic summer, high energy photons are able to photolyze the halogenated compounds, freeing halogen radicals that then catalytically destroy O3. Because Antarctica is constantly surrounded by a polar vortex, radical halogens are not able to be diluted over the entire globe. The ozone hole develops as result of this process.
Resent research suggests that the strength of the polar vortex from any given year is directly correlated to the size of the ozone hole. In years with a strong polar vortex, the ozone hole is seen to expand in diameter, where as in years with a weaker polar vortex, the ozone hole is noted to shrink
Q: What is the causes for ozone depletion?
A: There are many causes for ozone depletion. The main reason for this is because of these things called CFC’s. CFC’s are found in everyday products such as refrig…
Q: What is the cause of ozone depletion?
A: People. your car. aircraft flights. rocket launches. using to much energy. wasting water. cfc chemicals from aerosal cans. people.
Q: What are the causes of ozone depletion?
A: this link helped me to study the causes. According to which different gases and human activities are responsible for it. Source(s)
Q: What Are The Causes Of Ozone Depletion?
A: According to m the causes of ozone depletion is the excessive use of CF Cs in our refrigerants Anonymous
Q: What Are the Causes of Ozone Depletion?
A: The ozone layer is a portion of the atmosphere that absorbs ultraviolet radiation. Ozone is the molecule responsible for this. Its structure consists of three
Harm to human health:
- More skin cancers, sunburns and premature aging of the skin.
- More cataracts, blindness and other eye diseases: UV radiation can damage several parts of the eye, including the lens, cornea, retina and conjunctiva.
- Cataracts (a clouding of the lens) are the major cause of blindness in the world. A sustained 10% thinning of the ozone layer is expected to result in almost two million new cases of cataracts per year, globally (Environment Canada, 1993).
- Weakening of the human immune system (immunosuppression). Early findings suggest that too much UV radiation can suppress the human immune system, which may play a role in the development of skin cancer.
Adverse impacts on agriculture, forestry and natural ecosystems:
- Several of the world’s major crop species are particularly vulnerable to increased UV, resulting in reduced growth, photosynthesis and flowering. These species include wheat, rice, barley, oats, corn, soybeans, peas, tomatoes, cucumbers, cauliflower, broccoli and carrots.
- The effect of ozone depletion on the Canadian agricultural sector could be significant.
- Only a few commercially important trees have been tested for UV (UV-B) sensitivity, but early results suggest that plant growth, especially in seedlings, is harmed by more intense UV radiation.
Damage to marine life:
- In particular, plankton (tiny organisms in the surface layer of oceans) are threatened by increased UV radiation. Plankton are the first vital step in aquatic food chains.
- Decreases in plankton could disrupt the fresh and saltwater food chains, and lead to a species shift in Canadian waters.
- Loss of biodiversity in our oceans, rivers and lakes could reduce fish yields for commercial and sport fisheries.
- In domestic animals, UV overexposure may cause eye and skin cancers. Species of marine animals in their developmental stage (e.g. young fish, shrimp larvae and crab larvae) have been threatened in recent years by the increased UV radiation under the Antarctic ozone hole.
- Wood, plastic, rubber, fabrics and many construction materials are degraded by UV radiation.
- The economic impact of replacing and/or protecting materials could be significant.
B. Impact on the Biosphere
1. Marine Ecosystems
The effects on aquatic ecosystems, especially on phytoplankton and larvae of higher organisms, are of particular concern. Marine phytoplankton play a fundamental role both in the food chain as well as the oceanic carbon cycle by which atmospheric carbon dioxide is converted into oxygen. See Figure 13. Approximately 30 percent of the world’s animal protein for human consumption comes from the sea (Tevini, 1983). The base of the marine food chain are the phytoplankton organisms which are concentrated in high latitudes where reductions in stratospheric ozone are predicted to cause the greatest increase in the amount of UV-B radiation reaching the Earth’s surface. Equatorial regions contain densities of phytoplankton approximately 10 to 100 times smaller than the circumpolar regions (UNEP, 1994). Additional concentrations of phytoplankton occur in upwelling areas along the continental shelves. Investigations in Antarctica indicate that current UV-B radiation levels already affect phytoplankton productivity (UNEP, 1994; Tevini, 1993). Current UV-B radiation levels are also limiting factors for early developmental stages of fish, shrimp, crab, amphibians, and other animals (UNEP, 1994).
Quantitative estimates of the potential effects of increased UV-B radiation on the marine ecosystem are questionable given the current state of knowledge. A complicating factor is that small changes could cause nonlinear (multiplicative) reactions. One study estimated that a 16 percent reduction in stratospheric ozone levels would produce a five percent loss of phytoplankton productivity, leading to a loss of approximately seven million tons of fish from the annual fisheries harvest (UNEP, 1994).
Terrestrial plants vary considerably in their response to UV-B radiation between species and even between cultivars of the same species. Plants have several mechanisms to ameliorate or repair adverse effects from UV-B radiation, and may acclimate to a certain extent to increased UV-B radiation levels. In agriculture, reduction in stratospheric ozone will require the use of UV-B tolerant cultivars and the development of new ones. Scientific evidence indicates that there will be an adverse effect on crops, but the magnitude of these effects cannot be estimated given the current state of knowledge (UNEP, 1994; Tevini, 1993).
The risks of increased UVB due to stratospheric ozone depletion includes damage to crops and aquatic organisms, increased formation of ground-level smog, and accelerated weathering of outdoor plastics.
3. Global Warming
Another concern relates to the global warming potential associated with decreases in stratospheric ozone. However, recent modeling studies conclude that decreases in stratospheric ozone serve to cool the global climate (WMO, 1994).
C. Impact on Humans
In addition to the above effects on the biosphere, increased UVB can have direct effects on humans including increased skin cancer, cataracts, and suppression of the human immune response system.
1. Skin Cancer
Laboratory and epidemiological studies demonstrate that UVB causes nonmelanoma skin cancer and plays a major role in malignant melanoma development. A major effort over the last several decades has been to understand the results of human epidemiological studies that have investigated the relationship between various forms of skin cancer and increased UV-B radiation. The USEPA has used the results of these studies to support its rulemaking on the protection of stratospheric ozone, concluding that it may be reasonably anticipated that an increase in UV-B radiation caused by a decrease in the ozone column would result in increased incidences of cutaneous malignant melanomas (potentially mortal skin cancers). In addition to the conclusions reached by USEPA, other analyses have been published which acknowledge the adverse relationship between reduced stratospheric ozone and increased cancer incidences (Shea, 1988; Van Der Leun, 1986).
Non-melanoma skin cancers mainly include basal cell carcinoma and squamous cell carcinoma. The mortality rate from non-melanoma skin cancer is less than or equal to one percent in areas with good medical care (UNEP, 1994; Tevini, 1993). An estimated 1.2 million cases occur worldwide annually (UNEP, 1994). The development of non-melanoma skin cancer is correlated strongly to exposure to sunlight and sufficient scientific information is available to roughly forecast the effects of increase UV-B radiation. A one percent decrease in stratospheric ozone is estimated to cause an increase of approximately 2.3 percent in non-melanoma skin cancer (UNEP, 1994; Tevini, 1993).
For melanoma skin cancer, sufficient scientific information is not available to project increased incidences. The incidence of melanoma skin cancer is lower than non-melanoma skin cancer by a factor of ten (Tevini, 1993) for an estimated 120,000 cases worldwide annually. However, the mortality is much higher, approximately 25 percent in areas with good medical care (Tevini, 1993). Rather than cumulative exposure to UV-B radiation, studies suggest that melanoma may be produced by severe episodic exposures (sunburn). These results are inconclusive. Earlier estimates by USEPA (1987) were that each one percent decrease in stratospheric ozone would increase the incidence of melanomas by one to two percent and mortality by 0.8 to 1.5 percent. Because of the many uncertainties involved, these estimates are considered questionable (Tevini, 1993).
Potential human health effects on the eyes include increased incidence of “snowblindness” and cataracts. The medical term for snowblindness is photokeratitis, an acute inflammation of the superficial layers of the eyes. The effect is dose related and can cause lasting damage in severe cases. Increased UV radiation will likely increase incidences. However, eye protection is available and a single incident is usually sufficient to encourage use of protective sunglasses (UNEP, 1994; Tevini, 1993). Sufficient information is available with regard to cataracts to roughly forecast increases. An approximate 0.5 percent increase in cataracts would occur for each one percent drop in stratospheric ozone (UNEP, 1994; Tevini, 1993). An estimated 17 million people in the world are blind due to cataracts (Tevini, 1993). Based on calculations presented in the cited reference (Tevini, 1993), the number of additional blindness annually due to cataracts is estimated as 680,000. A one percent drop in stratospheric ozone therefore could cause an additional 3,400 cases of blindness due to cataracts each year.
3. Immune System
Laboratory studies have also shown that increased exposure UVB weakens the immune response system. A reduction in the efficiency of the immune system could lead to increases in cancers and infectious disease. The complexity of the immune system, which is comprised of several subsystems that help and suppress each other, and the complex reactions of different types of diseases to UV-radiation prevent any quantitative predictions of the effects of increased UV-B radiation given the current state of scientific knowledge (UNEP, 1994; Tevini, 1993).
All sunlight contains some UVB, even with normal ozone levels. It is always important to limit exposure to the sun. However, ozone depletion will increase the amount of UVB, which will then increase the risk of health effects.