Consequences of ozone layer depletion
Since the ozone layer absorbs UV-B ultraviolet light from the Sun, ozone layer depletion is expected to increase surface UV-B levels, which could lead to damage, including increases in skin cancer. This was the reason for the Montreal Protocol. Although decreases in stratospheric ozone are well-tied to CFCs and there are good theoretical reasons to believe that decreases in ozone will lead to increases in surface UV-B, there is no direct observational evidence linking ozone depletion to higher incidence of skin cancer in human beings. This is partly due to the fact that UV-A, which has also been implicated in some forms of skin cancer, is not absorbed by ozone, and it is nearly impossible to control statistics for lifestyle changes in the populace.
Increased UV
Ozone, while a minority constituent in the earth's atmosphere, is responsible for most of the absorption of UV-B radiation. The amount of UV-B radiation that penetrates through the ozone layer decreases exponentially with the slant-path thickness/ density of the layer. Correspondingly, a decrease in atmospheric ozone is expected to give rise to significantly increased levels of UV-B near the surface.
Biological effects of increased UV and microwave radiation from a depleted ozone layer
The main public concern regarding the ozone hole has been the effects of surface UV on human health. So far, ozone depletion in most locations has been typically a few percent and, as noted above, no direct evidence of health damage is available in most latitudes. Were the high levels of depletion seen in the ozone hole ever to be common across the globe, the effects could be substantially more dramatic. As the ozone hole over Antarctica has in some instances grown so large as to reach southern parts of Australia and New Zealand, environmentalists have been concerned that the increase in surface UV could be significant.
Effects of ozone layer depletion on Humans
UV-B (the higher energy UV radiation absorbed by ozone) is generally accepted to be a contributory factor to skin cancer. In addition, increased surface UV leads to increased tropospheric ozone, which is a health risk to humans. The increased surface UV also represents an increase in the vitamin D synthetic capacity of the sunlight.
The cancer preventive effects of vitamin D represent a possible beneficial effect of ozone depletion. In terms of health costs, the possible benefits of increased UV irradiance may outweigh the burden.
1. Basal and Squamous Cell Carcinomas -- The most common forms of skin cancer in humans, basal and squamous cell carcinomas, have been strongly linked to UV-B exposure. The mechanism by which UV-B induces these cancers is well understood — absorption of UV-B radiation causes the pyrimidine bases in the DNA molecule to form dimers, resulting in transcription errors when the DNA replicates. These cancers are relatively mild and rarely fatal, although the treatment of squamous cell carcinoma sometimes requires extensive reconstructive surgery. By combining epidemiological data with results of animal studies, scientists have estimated that a one percent decrease in stratospheric ozone would increase the incidence of these cancers by 2%.
2. Malignant Melanoma -- Another form of skin cancer, malignant melanoma, is much less common but far more dangerous, being lethal in about 15% - 20% of the cases diagnosed. The relationship between malignant melanoma and ultraviolet exposure is not yet well understood, but it appears that both UV-B and UV-A are involved. Experiments on fish suggest that 90 to 95% of malignant melanomas may be due to UV-A and visible radiation whereas experiments on opossums suggest a larger role for UV-B. Because of this uncertainty, it is difficult to estimate the impact of ozone depletion on melanoma incidence. One study showed that a 10% increase in UV-B radiation was associated with a 19% increase in melanomas for men and 16% for women. A study of people in Punta Arenas, at the southern tip of Chile, showed a 56% increase in melanoma and a 46% increase in nonmelanoma skin cancer over a period of seven years, along with decreased ozone and increased UV-B levels.
3. Cortical Cataracts -- Studies are suggestive of an association between ocular cortical cataracts and UV-B exposure, using crude approximations of exposure and various cataract assessment techniques. A detailed assessment of ocular exposure to UV-B was carried out in a study on Chesapeake Bay Watermen, where increases in average annual ocular exposure were associated with increasing risk of cortical opacity. In this highly exposed group of predominantly white males, the evidence linking cortical opacities to sunlight exposure was the strongest to date. However, subsequent data from a population-based study in Beaver Dam, WI suggested the risk may be confined to men. In the Beaver Dam study, the exposures among women were lower than exposures among men, and no association was seen. Moreover, there were no data linking sunlight exposure to risk of cataract in African Americans, although other eye diseases have different prevalences among the different racial groups, and cortical opacity appears to be higher in African Americans compared with whites.
4. Increased Tropospheric Ozone -- Increased surface UV leads to increased tropospheric ozone. Ground-level ozone is generally recognized to be a health risk, as ozone is toxic due to its strong oxidant properties. At this time, ozone at ground level is produced mainly by the action of UV radiation on combustion gases from vehicle exhausts.
Effects on Crops
An increase of UV radiation would be expected to affect crops. A number of economically important species of plants, such as rice, depend on cyanobacteria residing on their roots for the retention of nitrogen. Cyanobacteria are sensitive to UV light and they would be affected by its increase.
Effects on Plankton
Research has shown a widespread extinction of plankton two million years ago that coincided with a nearby supernova. There is a difference in the orientation and motility of planktons when excess of UV rays reach earth. Researchers speculate that the extinction was caused by a significant weakening of the ozone layer at that time when the radiation from the supernova produced nitrogen oxides that catalyzed the destruction of ozone (plankton are particularly susceptible to effects of UV light, and are vitally important to marine food webs).
Ozone depletion and global warming
Although they are often interlinked in the mass media, the connection between global warming and ozone depletion is not strong. There are four areas of linkage:
The same CO2 radiative forcing that produces near-surface global warming is expected to cool the stratosphere. This cooling, in turn, is expected to produce a relative increase in polar ozone (O3) depletion and the frequency of ozone holes.
Conversely, ozone depletion represents a radiative forcing of the climate system. There are two opposing effects: Reduced ozone causes the stratosphere to absorb less solar radiation, thus cooling the stratosphere while warming the troposphere; the resulting colder stratosphere emits less long-wave radiation downward, thus cooling the troposphere. Overall, the cooling dominates; the IPCC concludes that "observed stratospheric O3 losses over the past two decades have caused a negative forcing of the surface-troposphere system" of about −0.15 ± 0.10 watts per square meter (W/m²).
One of the strongest predictions of the greenhouse effect is that the stratosphere will cool. Although this cooling has been observed, it is not trivial to separate the effects of changes in the concentration of greenhouse gases and ozone depletion since both will lead to cooling. However, this can be done by numerical stratospheric modeling. Results from the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory show that above 20 km (12.4 miles), the greenhouse gases dominate the cooling.
Ozone depleting chemicals are also greenhouse gases. The increases in concentrations of these chemicals have produced 0.34 ± 0.03 W/m² of radiative forcing, corresponding to about 14% of the total radiative forcing from increases in the concentrations of well-mixed greenhouse gases.
World Ozone Day
In 1994, the United Nations General Assembly voted to designate September 16 as "World Ozone Day", to commemorate the signing of the Montreal Protocol on that date in 1987.
GLOBAL WARMING
Global warming is the increase in the average temperature of the Earth's near-surface air and oceans since the mid-twentieth century, and its projected continuation.
The average global air temperature near the Earth's surface increased 0.74 ± 0.18 °C (1.33 ± 0.32 °F) during the hundred years ending in 2005. The Intergovernmental Panel on Climate Change (IPCC) concludes "most of the observed increase in globally averaged temperatures since the mid-twentieth century is very likely due to the observed increase in anthropogenic (man-made) greenhouse gas concentrations" via an enhanced greenhouse effect. Natural phenomena such as solar variation combined with volcanoes probably had a small warming effect from pre-industrial times to 1950 and a small cooling effect from 1950 onward.
These basic conclusions have been endorsed by at least thirty scientific societies and academies of science, including all of the national academies of science of the major industrialized countries.
Climate model projections summarized by the IPCC indicate that average global surface temperature will likely rise a further 1.1 to 6.4 °C (2.0 to 11.5 °F) during the twenty-first century. This range of values results from the use of differing scenarios of future greenhouse gas emissions as well as models with differing climate sensitivity.
Increasing global temperature will cause sea level to rise, and is expected to increase the intensity of extreme weather events and to change the amount and pattern of precipitation. Other effects of global warming include changes in agricultural yields, trade routes, glacier retreat, species extinctions and increases in the ranges of disease vectors.
Remaining scientific uncertainties include the amount of warming expected in the future, and how warming and related changes will vary from region to region around the globe. Most national governments have signed and ratified the Kyoto Protocol aimed at reducing greenhouse gas emissions, but there is ongoing political and public debate worldwide regarding what, if any, action should be taken to reduce or reverse future warming or to adapt to its expected consequences.
The term "global warming" refers to the warming in recent decades and its projected continuation, and implies a human influence. The United Nations Framework Convention on Climate Change (UNFCCC) uses the term "climate change" for human-caused change, and "climate variability" for other changes. The term "anthropogenic global warming" (AGW) is sometimes used when focusing on human-induced changes.
Recent Temperature changes
Global temperatures on both land and sea have increased by 0.75 °C (1.35 °F) relative to the period 1860–1900, according to the instrumental temperature record. This measured temperature increase is not significantly affected by the urban heat island effect. Since 1979, land temperatures have increased about twice as fast as ocean temperatures (0.25 °C per decade against 0.13 °C per decade). Temperatures in the lower troposphere have increased between 0.12 and 0.22 °C (0.22 and 0.4 °F) per decade since 1979, according to satellite temperature measurements. Temperature is believed to have been relatively stable over the one or two thousand years before 1850, with possibly regional fluctuations such as the Medieval Warm Period or the Little Ice Age.
Sea temperatures increase more slowly than those on land both because of the larger effective heat capacity of the oceans and because the ocean can lose heat by evaporation more readily than the land. The Northern Hemisphere has more land than the Southern Hemisphere, so it warms faster. The Northern Hemisphere also has extensive areas of seasonal snow and sea-ice cover subject to the ice-albedo feedback. More greenhouse gases are emitted in the Northern than Southern Hemisphere, but this does not contribute to the difference in warming because the major greenhouse gases persist long enough to mix between hemispheres.
Causes of Global Warming
The Earth's climate changes in response to external forcing, including variations in its orbit around the Sun (orbital forcing), changes in solar luminosity, volcanic eruptions, and atmospheric greenhouse gas concentrations. The detailed causes of the recent warming remain an active field of research, but the scientific consensus is that the increase in atmospheric greenhouse gases due to human activity caused most of the warming observed since the start of the industrial era. This attribution is clearest for the most recent 50 years, for which the most detailed data are available. Some other hypotheses departing from the consensus view have been suggested to explain most of the temperature increase. One such hypothesis proposes that warming may be the result of variations in solar activity.
None of the effects of forcing are instantaneous. The thermal inertia of the Earth's oceans and slow responses of other indirect effects mean that the Earth's current climate is not in equilibrium with the forcing imposed. Climate commitment studies indicate that even if greenhouse gases were stabilized at 2000 levels, a further warming of about 0.5 °C (0.9 °F) would still occur.
The effects of forcing agents on the climate are complicated by various feedback processes.
One of the most pronounced feedback effects relates to the evaporation of water. Warming by the addition of long-lived greenhouse gases such as CO2 will cause more water to evaporate into the atmosphere. Since water vapor itself acts as a greenhouse gas, the atmosphere warms further; this warming causes more water vapor to evaporate (a positive feedback), and so on until other processes stop the feedback loop. The result is a much larger greenhouse effect than that due to CO2 alone. Although this feedback process causes an increase in the absolute moisture content of the air, the relative humidity stays nearly constant or even decreases slightly because the air is warmer. This feedback effect can only be reversed slowly as CO2 has a long average atmospheric lifetime.
Feedback effects due to clouds are an area of ongoing research. Seen from below, clouds emit infrared radiation back to the surface, and so exert a warming effect; seen from above, clouds reflect sunlight and emit infrared radiation to space, and so exert a cooling effect. Whether the net effect is warming or cooling depends on details such as the type and altitude of the cloud. These details are difficult to represent in climate models, in part because clouds are much smaller than the spacing between points on the computational grids of climate models. Nevertheless, cloud feedback is second only to water vapor feedback and is positive in all the models that were used in the IPCC Fourth Assessment Report.
A subtler feedback process relates to changes in the lapse rate as the atmosphere warms. The atmosphere's temperature decreases with height in the troposphere. Since emission of infrared radiation varies with the fourth power of temperature, longwave radiation emitted from the upper atmosphere is less than that emitted from the lower atmosphere. Most of the radiation emitted from the upper atmosphere escapes to space, while most of the radiation emitted from the lower atmosphere is re-absorbed by the surface or the atmosphere. Thus, the strength of the greenhouse effect depends on the atmosphere's rate of temperature decrease with height: if the rate of temperature decrease is greater the greenhouse effect will be stronger, and if the rate of temperature decrease is smaller then the greenhouse effect will be weaker. Both theory and climate models indicate that warming will reduce the decrease of temperature with height, producing a negative lapse rate feedback that weakens the greenhouse effect. Measurements of the rate of temperature change with height are very sensitive to small errors in observations, making it difficult to establish whether the models agree with observations.
Another important feedback process is ice-albedo feedback. When global temperatures increase, ice near the poles melts at an increasing rate. As the ice melts, land or open water takes its place. Both land and open water are on average less reflective than ice, and thus absorb more solar radiation. This causes more warming, which in turn causes more melting, and this cycle continues.
Positive feedback due to release of CO2 and CH4 from thawing permafrost, such as the frozen peat bogs in Siberia, is an additional mechanism that could contribute to warming. Similarly a massive release of CH4 from methane clathrates in the ocean could cause rapid warming, according to the clathrate gun hypothesis.
The ocean's ability to sequester carbon is expected to decline as it warms. This is because the resulting low nutrient levels of the mesopelagic zone (about 200 to 1000 m depth) limits the growth of diatoms in favor of smaller phytoplankton that are poorer biological pumps of carbon.
According to United Nations Environment Programme (UNEP), economic sectors likely to face difficulties related to climate change include banks, agriculture, transport and others. Developing countries dependent upon agriculture will be particularly harmed by global warming.
Although it is difficult to connect specific weather events to global warming, an increase in global temperatures may in turn cause broader changes, including glacial retreat, Arctic shrinkage, and worldwide sea level rise. Changes in the amount and pattern of precipitation may result in flooding and drought. There may also be changes in the frequency and intensity of extreme weather events. Other effects may include changes in agricultural yields, addition of new trade routes, reduced summer streamflows, species extinctions, and increases in the range of disease vectors.
Some effects on both the natural environment and human life are, at least in part, already being attributed to global warming. A 2001 report by the IPCC suggests that glacier retreat, ice shelf disruption such as that of the Larsen Ice Shelf, sea level rise, changes in rainfall patterns, and increased intensity and frequency of extreme weather events, are being attributed in part to global warming. While changes are expected for overall patterns, intensity, and frequencies, it is difficult to attribute specific events to global warming. Other expected effects include water scarcity in some regions and increased precipitation in others, changes in mountain snowpack, and adverse health effects from warmer temperatures.
Increasing deaths, displacements, and economic losses projected due to extreme weather attributed to global warming may be exacerbated by growing population densities in affected areas, although temperate regions are projected to experience some benefits, such as fewer deaths due to cold exposure. A summary of probable effects and recent understanding can be found in the report made for the IPCC Third Assessment Report by Working Group II. The newer IPCC Fourth Assessment Report summary reports that there is observational evidence for an increase in intense tropical cyclone activity in the North Atlantic Ocean since about 1970, in correlation with the increase in sea surface temperature, but that the detection of long-term trends is complicated by the quality of records prior to routine satellite observations. The summary also states that there is no clear trend in the annual worldwide number of tropical cyclones.
Additional anticipated effects include sea level rise of 110 to 770 millimeters (0.36 to 2.5 ft) between 1990 and 2100, repercussions to agriculture, possible slowing of the thermohaline circulation, reductions in the ozone layer, increased intensity (but less frequent) of hurricanes and extreme weather events, lowering of ocean pH, and the spread of diseases such as malaria and dengue fever. One study predicts 18% to 35% of a sample of 1,103 animal and plant species would be extinct by 2050, based on future climate projections. However, few mechanistic studies have documented extinctions due to recent climate change and one study suggests that projected rates of extinction are uncertain.
ULTRAVIOLET
Ultraviolet (UV) light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than soft X-rays. It is so named because the spectrum consists of electromagnetic waves with frequencies higher than those that humans identify as the colour violet.
UV light is typically found as part of the radiation received by the Earth from the Sun. Most humans are aware of the effects of UV through the painful condition of sunburn. The UV spectrum has many other effects, including both beneficial and damaging changes to human health.
The discovery of UV radiation was intimately associated with the observation that silver salts darken when exposed to sunlight. In 1801 the German physicist Johann Wilhelm Ritter made the hallmark observation that invisible rays just beyond the violet end of the visible spectrum were especially effective at darkening silver chloride-soaked paper. He called them "de-oxidizing rays" to emphasize their chemical reactivity and to distinguish them from "heat rays" at the other end of the visible spectrum. The simpler term "chemical rays" was adopted shortly thereafter, and it remained popular throughout the 19th century. The terms chemical and heat rays were eventually dropped in favor of ultraviolet and infrared radiation, respectively.
The name means "beyond violet" (from Latin ultra, "beyond"), violet being the colour of the shortest wavelengths of visible light. UV light has a shorter wavelength than that of violet light.
The electromagnetic spectrum of ultraviolet light can be subdivided in a number of ways. The draft ISO standard on determining solar irradiances (ISO-DIS-21348) describes the following ranges:
Name
|
Abbreviation
|
Wavelength range in nanometers
|
Energy per photon
|
|
Ultraviolet A
Long wave/ Black light
|
UVA
|
400 nm – 315 nm
|
3.10 – 3.94 eV
|
|
Near
|
NUV
|
400 nm – 300 nm
|
3.10 – 4.13 eV
|
|
Ultraviolet B or medium wave
|
UVB
|
315 nm – 280 nm
|
3.94 – 4.43 eV
|
|
Middle
|
MUV
|
300 nm – 200 nm
|
4.13 – 6.20 eV
|
|
Ultraviolet C, short wave, or germicidal
|
UVC
|
280 nm – 100 nm
|
4.43 – 12.4 eV
|
|
Far
|
FUV
|
200 nm – 122 nm
|
6.20 – 10.2 eV
|
|
Vacuum
|
VUV
|
200 nm – 10 nm
|
6.20 – 124 eV
|
|
Extreme
|
EUV
|
121 nm – 10 nm
|
10.2 – 124 eV
|
In photolithography, in laser technology, etc., the term deep ultraviolet or DUV refers to wavelengths below 300 nm. "Vacuum UV" is so named because it is absorbed strongly by air and is therefore used in a vacuum. In the long-wave limit of this region, roughly 150–200 nm, the principal absorber is the oxygen in air. Work in this region can be performed in an oxygen free atmosphere, pure nitrogen being commonly used, which avoids the need for a vacuum chamber.
A black light, or Wood's light, is a lamp that emits long wave UV radiation and very little visible light. Commonly these are referred to as simply a "UV light". Fluorescent black lights are typically made in the same fashion as normal fluorescent lights except that only one phosphor is used and the normally clear glass envelope of the bulb may be replaced by a deep-bluish-purple glass called Wood's glass, a nickel-oxide–doped glass, which blocks almost all visible light above 400 nanometers. The color of such lamps is often referred to in the trade as "blacklight blue" or "BLB." This is to distinguish these lamps from "bug zapper" blacklight ("BL") lamps that don't have the blue Wood's glass. The phosphor typically used for a near 368 to 371 nanometer emission peak is either europium-doped strontium fluoroborate (SrB4O7F:Eu2+) or europium-doped strontium borate (SrB4O7:Eu2+) while the phosphor used to produce a peak around 350 to 353 nanometers is lead-doped barium silicate (BaSi2O5:Pb+). "Blacklight Blue" lamps peak at 365 nm.
While "black lights" do produce light in the UV range, their spectrum is confined to the longwave UVA region. Unlike UVB and UVC, which are responsible for the direct DNA damage that leads to skin cancer, black light is limited to lower energy, longer waves and does not cause sunburn. However, UVA is capable of causing damage to collagen fibers and destroying vitamin A in skin.
A black light may also be formed by simply using Wood's glass instead of clear glass as the envelope for a common incandescent bulb. This was the method used to create the very first black light sources.
Some UV fluorescent bulbs specifically designed to attract insects for use in bug zappers use the same near-UV emitting phosphor as normal blacklights, but use plain glass instead of the more expensive Wood's glass. Plain glass blocks less of the visible mercury emission spectrum, making them appear light blue to the naked eye. These lamps are referred to as "blacklight" or "BL" in most lighting catalogs.
Ultraviolet light can also be generated by some light-emitting diodes.
The Sun emits ultraviolet radiation in the UVA, UVB, and UVC bands, but because of absorption in the atmosphere's ozone layer, 98.7% of the ultraviolet radiation that reaches the Earth's surface is UVA. (Some of the UVB and UVC radiation is responsible for the generation of the ozone layer.)
Ordinary glass is partially transparent to UVA but is opaque to shorter wavelengths while Silica or quartz glass, depending on quality, can be transparent even to vacuum UV wavelengths. Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm.
Human health-related effects of UV radiation
Beneficial effects
The Earth's atmosphere blocks UV radiation from penetrating through the atmosphere by 98.7%. A positive effect of UVB exposure is that it induces the production of vitamin D in the skin. It has been estimated that tens of thousands of premature deaths occur in the United States annually from a range of cancers due to vitamin D deficiency. Another effect of vitamin D deficiency is osteomalacia (the adult equivalent of rickets), which can result in bone pain, difficulty in weight bearing and sometimes fractures. Other studies show most people get adequate Vitamin D through food and incidental exposure.
Many countries have fortified certain foods with Vitamin D to prevent deficiency. Eating fortified foods or taking a dietary supplement pill is usually preferred to UVB exposure, due to the increased risk of skin cancer from UV radiation.
Too little UVB radiation leads to a lack of Vitamin D. Too much UVB radiation leads to direct DNA damages and sunburn. An appropriate amount of UVB (What is appropriate depends on your skin colour) leads to a limited amount of direct DNA damage. This is recognized and repaired by the body. Then the melanin production is increased which leads to a long lasting tan. This tan occurs with a 2 day lag phase after irradiation, but it is much less harmful and long lasting than the one obtained from UVA.
Harmful effects
An overexposure to UVB radiation can cause sunburn and some forms of skin cancer. In humans, prolonged exposure to solar UV radiation may result in acute and chronic health effects on the skin, eye, and immune system. However the most deadly form - malignant melanoma - is mostly caused by the indirect DNA damage (free radicals and oxidative stress). This can be seen from the absence of a UV-signature mutation in 92% of all melanoma.
UVC rays are the highest energy, most dangerous type of ultraviolet light. Little attention has been given to UVC rays in the past since they are filtered out by the atmosphere. However, their use in equipment such as pond sterilization units may pose an exposure risk, if the lamp is switched on outside of its enclosed pond sterilization unit.
Skin : UVA, UVB and UVC can all damage collagen fibers and thereby accelerate aging of the skin. Both UVA and UVB destroy vitamin A in skin which may cause further damage. In the past UVA was considered less harmful, but today it is known, that it can contribute to skin cancer via the indirect DNA damage (free radicals and reactive oxygen species). It penetrates deeply but it does not cause sunburn. UVA does not damage DNA directly like UVB and UVC, but it can generate highly reactive chemical intermediates, such as hydroxyl and oxygen radicals, which in turn can damage DNA. Because it does not cause reddening of the skin (erythema) it cannot be measured in the SPF testing. There is no good clinical measurement of the blocking of UVA radiation, but it is important that sunscreen block both UVA and UVB.
UVB light can cause direct DNA damage. The radiation excites DNA molecules in skin cells, causing aberrant covalent bonds to form between adjacent cytosine bases, producing a dimer. When DNA polymerase comes along to replicate this strand of DNA, it reads the dimer as "AA" and not the original "CC". This causes the DNA replication mechanism to add a "TT" on the growing strand. This is a mutation, which can result in cancerous growths and is known as a "classical C-T mutation". The mutations that are caused by the direct DNA damage carry a UV signature mutation that is commonly seen in skin cancers. The mutagenicity of UV radiation can be easily observed in bacteria cultures. This cancer connection is one reason for concern about ozone depletion and the ozone hole. UVB causes some damage to collagen but at a very much slower rate than UVA.
Eye :High intensities of UVB light are hazardous to the eyes, and exposure can cause welder's flash (photokeratitis or arc eye) and may lead to cataracts, pterygium, and pinguecula formation.
Protective eyewear is beneficial to those who are working with or those who might be exposed to ultraviolet radiation, particularly short wave UV.
Many polymers used in consumer products are degraded by UV light, and need addition of UV absorbers to inhibit attack, especially if the products are used externally and so exposed to sunlight. The problem appears as discoloration or fading, cracking and sometimes, total product disintegration if cracking has proceeded far enough. The rate of attack increases with exposure time and sunlight intensity.
It is known as UV degradation, and is one form of polymer degradation. Sensitive polymers include thermoplastics, such as polypropylene and polyethylene as well as speciality fibres like aramids. UV absorption leads to chain degradation and loss of strength at sensitive points in the chain structure. They include tertiary carbon atoms, which in polypropylene occur in every repeat unit.
In addition, many pigments and dyes absorb UV and change colour, so paintings and textiles may need extra protection both from sunlight and fluorescent lamps, two common sources of UV radiation. Old and antique paintings such as watercolour paintings for example, usually need to be placed away from direct sunlight. Common window glass provides some protection by absorbing some of the harmful UV, but valuable artifacts need shielding.
Blockers and absorbers
Ultraviolet Light Absorbers (UVAs) are molecules used in organic materials (polymers, paints, etc.) to absorb UV light in order to reduce the UV degradation (photo-oxidation) of a material. A number of different UVAs exist with different absorption properties. UVAs can disappear over time, so monitoring of UVA levels in weathered materials is necessary.
In sunscreen, ingredients which absorb UVA/UVB rays, such as avobenzone and octyl methoxycinnamate, are known as absorbers. They are contrasted with physical "blockers" of UV radiation such as titanium dioxide and zinc oxide.
Applications of UV
Security
To help thwart counterfeiters, sensitive documents (e.g. credit cards, driver's licenses, passports) may also include a UV watermark that can only be seen when viewed under a UV-emitting light. Passports issued by most countries usually contain UV sensitive inks and security threads. Visa stamps and stickers on passports of visitors contain large and detailed seals invisible to the naked eye under normal lights, but strongly visible under UV illumination. Passports issued by many nations have UV sensitive watermarks on all pages of the passport. Currencies of various countries' banknotes have an image, as well as many multicolored fibers, that are visible only under ultraviolet light.
Fluorescent lamps
Fluorescent lamps produce UV radiation by ionising low-pressure mercury vapour. A phosphorescent coating on the inside of the tubes absorbs the UV and converts it to visible light.
The main mercury emission wavelength is in the UVC range. Unshielded exposure of the skin or eyes to mercury arc lamps that do not have a conversion phosphor is quite dangerous.
The light from a mercury lamp is predominantly at discrete wavelengths. Other practical UV sources with more continuous emission spectra include xenon arc lamps (commonly used as sunlight simulators), deuterium arc lamps, mercury-xenon arc lamps, metal-halide arc lamps, and tungsten-halogen incandescent lamps.
Astronomy
In astronomy, very hot objects preferentially emit UV radiation. Because the ozone layer blocks many UV frequencies from reaching telescopes on the surface of the Earth, most UV observations are made from space.
Biological surveys and pest control
Some animals, including birds, reptiles, and insects such as bees, can see into the near ultraviolet. Many fruits, flowers, and seeds stand out more strongly from the background in ultraviolet wavelengths as compared to human color vision. Scorpions glow or take on a yellow to green color under UV illumination. Many birds have patterns in their plumage that are invisible at usual wavelengths but observable in ultraviolet, and the urine and other secretions of some animals, including dogs, cats, and human beings, is much easier to spot with ultraviolet.
Many insects use the ultraviolet wavelength emissions from celestial objects as references for flight navigation. A local ultraviolet emissor will normally disrupt the navigation process and would eventually attract to itself the flying insect.
Ultraviolet traps are used to eliminate various small flying insects. They are attracted to the UV light, and are killed using an electric shock, or trapped once they come into contact with the device. Different designs of ultraviolet light traps are also used by entomologists for collecting nocturnal insects during faunistic survey studies.
Spectrophotometry
UV/ VIS spectroscopy is widely used as a technique in chemistry, to analyze chemical structure, most notably conjugated systems. UV radiation is often used in visible spectrophotometry to determine the existence of fluorescence in a given sample.
Analyzing minerals
Ultraviolet lamps are also used in analyzing minerals, gems, and in other detective work including authentication of various collectibles. Materials may look the same under visible light, but fluoresce to different degrees under ultraviolet light; or may fluoresce differently under short wave ultraviolet versus long wave ultraviolet.
Chemical markers
UV fluorescent dyes are used in many applications (for example, biochemistry and forensics). The Green Fluorescent Protein (GFP) is often used in genetics as a marker. Many substances, such as proteins, have significant light absorption bands in the ultraviolet that are of use and interest in biochemistry and related fields. UV-capable spectrophotometers are common in such laboratories.
Photochemotherapy
Exposure to UVA light while the skin is hyper-photosensitive by taking psoralens is an effective treatment for psoriasis called PUVA. Due to psoralens potentially causing damage to the liver, PUVA may only be used a limited number of times over a patient's lifetime
Phototherapy
Ultraviolet radiation has other medical applications, in the treatment of skin conditions such as psoriasis and vitiligo. UVA radiation can be used in conjunction with psoralens (PUVA treatment). UVB radiation is rarely used in conjunction with psoralens. In cases of psoriasis and vitiligo, UV light with wavelength of 311 nm is most effective.
Exposure to UVB light, particularly the 310 nm narrowband UVB range, is an effective long-term treatment for many skin conditions like psoriasis, vitiligo, eczema, and many others. UVB phototherapy does not require additional medications or topical preparations for the therapeutic benefit; only the light exposure is needed. However, phototherapy can be effective when used in conjunction with certain topical treatments such as anthralin, coal tar, and Vitamin A and D derivatives, or systemic treatments such as methotrexate and soriatane.
Typical treatment regimes involve short exposure to UVB rays 3 to 5 times a week at a hospital or clinic, and for the best results, up to 30 or more sessions may be required.
Photolithography
Ultraviolet radiation is used for very fine resolution photolithography, a procedure where a chemical known as a photoresist is exposed to UV radiation which has passed through a mask. The light allows chemical reactions to take place in the photoresist, and after development (a step that either removes the exposed or unexposed photoresist), a geometric pattern which is determined by the mask remains on the sample. Further steps may then be taken to "etch" away parts of the sample with no photoresist remaining.
UV radiation is used extensively in the electronics industry because photolithography is used in the manufacture of semiconductors, integrated circuit components and printed circuit boards.
Checking electrical insulation
A new application of UV is to detect corona discharge (often simply called "corona") on electrical apparatus. Degradation of insulation of electrical apparatus or pollution causes corona, wherein a strong electric field ionizes the air and excites nitrogen molecules, causing the emission of ultraviolet radiation. The corona degrades the insulation level of the apparatus. Corona produces ozone and to a lesser extent nitrogen oxide which may subsequently react with water in the air to form nitrous acid and nitric acid vapour in the surrounding air.
Sterilization
Ultraviolet lamps are used to sterilize workspaces and tools used in biology laboratories and medical facilities. A low pressure mercury vapor discharge tube floods the inside of a hood with shortwave UV light when not in use, sterilizing microbiological contaminants from irradiated surfaces.
Disinfecting drinking water
UV radiation can be an effective viricide and bactericide. Disinfection using UV radiation is commonly used in wastewater treatment applications and is finding an increased usage in drinking water treatment. Many bottlers of spring water use UV disinfection equipment to sterilize their water.
Food processing
As consumer demand for fresh and "fresh-like" food products increases, the demand for nonthermal methods of food processing is likewise on the rise. In addition, public awareness regarding the dangers of food poisoning is also raising demand for improved food processing methods. Ultraviolet radiation is used in several food processes to kill unwanted microorganisms.
Fire detection
UV detectors are sensitive to most fires, including hydrocarbons, metals, sulfur, hydrogen, hydrazine, and ammonia. Arc welding, electrical arcs, lightning, X-rays used in nondestructive metal testing equipment (though this is highly unlikely), and radioactive materials can produce levels that will activate a UV detection system. The presence of UV-absorbing gases and vapors will attenuate the UV radiation from a fire, adversely affecting the ability of the detector to detect flames.
Ultraviolet detectors generally use either a solid-state device, such as one based on silicon carbide or aluminium nitride, or a gas-filled tube as the sensing element. UV detectors which are sensitive to UV light in any part of the spectrum respond to irradiation by sunlight and artificial light. Virtually all fires emit some radiation in the UVB band, while the Sun's radiation at this band is absorbed by the Earth's atmosphere. The result is that the UV detector is "solar blind", meaning it will not cause an alarm in response to radiation from the Sun, so it can easily be used both indoors and outdoors.
Curing of inks, adhesives, varnishes and coatings
Certain inks, coatings and adhesives are formulated with photoinitiators and resins. When exposed to the correct energy and irradiance in the required band of UV light, polymerization occurs, and so the adhesives harden or cure. Usually, this reaction is very quick, a matter of a few seconds. Applications include glass and plastic bonding, optical fiber coatings, the coating of flooring, UV Coating and paper finishes in offset printing, and dental fillings.
Deterring substance abuse in public places
UV lights have been installed in some parts of the world in public restrooms, and on public transport, for the purpose of deterring substance abuse. The blue color of these lights, combined with the fluorescence of the skin, make it harder for intravenous drug users to find a vein. The efficacy of these lights for that purpose has been questioned, with some suggesting that drug users simply find a vein outside the public restroom and mark the spot with a marker for accessibility when inside the restroom. There is currently no published evidence supporting the idea of a deterrent effect.
Erasing EPROM modules
Some EPROM (electronically programmable read-only memory) modules are erased by exposure to UV radiation. These modules often have a transparent glass (quartz) window on the top of the chip that allows the UV radiation in. These have been largely superseded by EEPROM and flash memory chips in most devices.
Preparing low surface energy polymers
UV radiation is useful in preparing low surface energy polymers for adhesives. Polymers exposed to UV light will oxidize thus raising the surface energy of the polymer. Once the surface energy of the polymer has been raised, the bond between the adhesive and the polymer will not be smaller.
Reading otherwise illegible papyruses
Using multi-spectral imaging it is possible to read illegible papyruses, such as the burned papyruses of the Villa of the Papyri or of Oxyrhynchus. The technique involves taking pictures of the illegible papyruses using different filters in the infrared or ultraviolet range, finely tuned to capture certain wavelengths of light. Thus, the optimum spectral portion can be found for distinguishing ink from paper on the papyrus surface.
Lasers
Ultraviolet lasers have applications in industry (laser engraving), medicine (dermatology and keratectomy), free air secure communications and computing (optical storage). They can be made by applying frequency conversion to lower-frequency lasers.
SEA LEVEL RISE
Sea-level rise is an increase in sea level. Multiple complex factors may influence this change.
Sea-level has risen about 130 meters (400 ft) since the peak of the last ice age about 18,000 years ago. Most of the rise occurred before 6,000 years ago. From 3,000 years ago to the start of the 19th century sea level was almost constant, rising at 0.1 to 0.2 mm/yr. Since 1900 the level has risen at 1 to 2 mm/yr; since 1993 satellite altimetry from TOPEX/ Poseidon indicates a rate of rise of 3.1 ± 0.7 mm per yr. Church and White in 2006 found a sea-level rise from January 1870 to December 2004 of 195 mm, a 20th century rate of sea-level rise of 1.7 ± 0.3 mm per yr and a significant acceleration of sea-level rise of 0.013 ± 0.006 mm per year. If this acceleration remains constant, then the 1990 to 2100 rise would range from 280 to 340 mm.
Sea-level rise can be a product of global warming through two main processes: thermal expansion of sea water and widespread melting of land ice. Global warming is predicted to cause significant rises in sea level over the course of the twenty-first century.
Local and eustatic sea level change
Local mean sea level (LMSL) is defined as the height of the sea with respect to a land benchmark, averaged over a period of time (such as a month or a year) long enough that fluctuations caused by waves and tides are smoothed out. One must adjust perceived changes in LMSL to account for vertical movements of the land, which can be of the same order (mm/yr) as sea level changes. Some land movements occur because of isostatic adjustment of the mantle to the melting of ice sheets at the end of the last ice age. The weight of the ice sheet depresses the underlying land, and when the ice melts away the land slowly rebounds. Atmospheric pressure, ocean currents and local ocean temperature changes also can affect LMSL.
“Eustatic” change (as opposed to local change) results in an alteration to the global sea levels, such as changes in the volume of water in the world oceans or changes in the volume of an ocean basin.
Short term and periodic changes
There are many following factors which can produce short-term (a few minutes to 14 months) changes in sea level.
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I Short-term (periodic) causes
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Time scale
(P = period) |
Vertical effect
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Periodic sea level changes
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Diurnal and semidiurnal astronomical tides
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12–24 h P
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0.2–10+ m
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Long-period tides
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Rotational variations (Chandler wobble)
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14 month P
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Meteorological and oceanographic fluctuations
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Atmospheric pressure
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Hours to months
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–0.7 to 1.3 m
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Winds (storm surges)
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1–5 days
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Up to 5 m
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Evaporation and precipitation (may also follow long-term pattern)
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Days to weeks
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Ocean surface topography (changes in water density and currents)
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Days to weeks
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Up to 1 m
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El Niño/southern oscillation
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6 mo every 5–10 yr
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Up to 0.6 m
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Seasonal variations
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Seasonal water balance among oceans (Atlantic, Pacific, Indian)
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Seasonal variations in slope of water surface
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River runoff/floods
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2 months
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1 m
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Seasonal water density changes (temperature and salinity)
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6 months
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0.2 m
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Seiches
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Seiches (standing waves)
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Minutes to hours
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Up to 2 m
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Earthquakes
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Tsunamis (generate catastrophic long-period waves)
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Hours
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Up to 10 m
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Abrupt change in land level
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Minutes
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Up to 10 m
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Longer term changes
Various factors affect the volume or mass of the ocean, leading to long-term changes in eustatic sea level. The two primary influences are temperature (because the volume of water depends on temperature), and the mass of water locked up on land and sea as fresh water in rivers, lakes, glaciers, polar ice caps, and sea ice. Over much longer geological timescales, changes in the shape of the oceanic basins and in land/sea distribution will affect sea level.
Observational and modelling studies of mass loss from glaciers and ice caps indicate a contribution to sea-level rise of 0.2 to 0.4 mm/yr averaged over the 20th century.
Glaciers and ice caps
Each year about 8 mm (0.3 inch) of water from the entire surface of the oceans falls into the Antarctica and Greenland ice sheets as snowfall. If no ice returned to the oceans, sea level would drop 8 mm every year. Although approximately the same amount of water returns to the ocean in icebergs and from ice melting at the edges, scientists do not know which is greater — the ice going in or the ice coming out. The difference between the ice input and output is called the mass balance and is important because it causes changes in global sea level.
Geological influences
At times during Earth's long history, continental drift has arranged the land masses into very different configurations from those of today. When there were large amounts of continental crust near the poles, the rock record shows unusually low sea levels during ice ages, because there was lots of polar land mass upon which snow and ice could accumulate. During times when the land masses clustered around the equator, ice ages had much less effect on sea level. However, over most of geologic time, long-term sea level has been higher than today. Only at the Permian-Triassic boundary ~250 million years ago was long-term sea level lower than today. Long term changes in sea level are the result of changes in the oceanic crust, with a downward trend expected to continue in the very long term.
During the glacial/interglacial cycles over the past few million years, sea level has varied by somewhat more than a hundred metres. This is primarily due to the growth and decay of ice sheets (mostly in the northern hemisphere) with water evaporated from the sea.
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Long-term causes
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Range of effect
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Vertical effect
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Change in volume of ocean basins
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Plate tectonics and seafloor spreading (plate divergence/convergence) and change in seafloor elevation (mid-ocean volcanism)
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Eustatic
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0.01 mm/yr
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Marine sedimentation
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Eustatic
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< 0.01 mm/yr
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Change in mass of ocean water
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Melting or accumulation of continental ice
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Eustatic
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10 mm/yr
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• Climate changes during the 20th century
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•• Antarctica (the results of increasing precipitation)
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Eustatic
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-0.2 to 0.0 mm/yr
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•• Greenland (from changes in both precipitation and runoff)
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Eustatic
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0.0 to 0.1 mm/yr
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• Long-term adjustment to the end of the last ice age
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•• Greenland and Antarctica contribution over 20th century
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Eustatic
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0.0 to 0.5 mm/yr
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Release of water from earth's interior
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Eustatic
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Release or accumulation of continental hydrologic reservoirs
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Eustatic
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Uplift or subsidence of Earth's surface (Isostasy)
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Thermal-isostasy (temperature/density changes in earth's interior)
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Local effect
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Glacio-isostasy (loading or unloading of ice)
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Local effect
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10 mm/yr
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Hydro-isostasy (loading or unloading of water)
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Local effect
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Volcano-isostasy (magmatic extrusions)
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Local effect
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Sediment-isostasy (deposition and erosion of sediments)
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Local effect
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< 4 mm/yr
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Tectonic uplift/subsidence
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Vertical and horizontal motions of crust (in response to fault motions)
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Local effect
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1-3 mm/yr
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Sediment compaction
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Sediment compression into denser matrix (particularly significant in and near river deltas)
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Local effect
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Loss of interstitial fluids (withdrawal of groundwater or oil)
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Local effect
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≤ 55 mm/yr
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Earthquake-induced vibration
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Local effect
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Departure from geoid
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Shifts in hydrosphere, aesthenosphere, core-mantle interface
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Local effect
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Shifts in earth's rotation, axis of spin, and precession of equinox
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Eustatic
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External gravitational changes
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Eustatic
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Evaporation and precipitation (if due to a long-term pattern)
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Local effect
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Based on the projected increases stated above, the The Intergovernmental Panel on Climate Change (IPCC) report notes that current and future climate change would be expected to have a number of impacts, particularly on coastal systems. Such impacts may include increased coastal erosion, higher storm-surge flooding, inhibition of primary production processes, more extensive coastal inundation, changes in surface water quality and groundwater characteristics, increased loss of property and coastal habitats, increased flood risk and potential loss of life, loss of nonmonetary cultural resources and values, impacts on agriculture and aquaculture through decline in soil and water quality, and loss of tourism, recreation, and transportation functions.
There is an implication that many of these impacts will be detrimental. The report does, however, note that owing to the great diversity of coastal environments; regional and local differences in projected relative sea level and climate changes; and differences in the resilience and adaptive capacity of ecosystems, sectors, and countries, the impacts will be highly variable in time and space and will not necessarily be negative in all situations.
Statistical data on the human impact of sea level rise is scarce. A study in the April, 2007 issue of Environment and Urbanization reports that 634 million people live in coastal areas within 30 feet (9.1 m) of sea level. The study also reported that about two thirds of the world's cities with over five million people are located in these low-lying coastal areas.
Drowning of Islands
IPCC assessments suggest that deltas and small island states are particularly vulnerable to sea level rise caused by both thermal expansion and ocean volume. Relative sea level rise (mostly caused by subsidence) is currently causing substantial loss of lands in some deltas. Sea level changes have not yet been conclusively proven to have directly resulted in environmental, humanitarian, or economic losses to small island states, but the IPCC and other bodies have found this a serious risk scenario in coming decades.
Many reports have focused the island nations of the Pacific, notably the Polynesian island of Tuvalu, which based on more severe flooding events in recent years, was thought to be "sinking" due to sea level rise. A scientific review in 2000 reported that based on University of Hawaii gauge data, Tuvalu had experienced a negligible increase in sea-level of 0.07 mm a year over the past two decades, and that ENSO had been a larger factor in Tuvalu's higher tides in recent years. A subsequent study by John Hunter from the University of Tasmania, however, adjusted for ENSO effects and the movement of the gauge (which was thought to be sinking). Hunter concluded that Tuvalu had been experiencing sea-level rise of about 1.2 mm per year. The recent more frequent flooding in Tuvalu may also be due to an erosional loss of land during and following the actions of 1997 cyclones Gavin, Hina, and Keli.
Sea level rise estimates from satellite altimetry are 3.1 +/- 0.4 mm/ yr for 1993-2003 (Leuliette et al. (2004)). This exceeds those from tide gauges. It is unclear whether this represents an increase over the last decades; variability; true differences between satellites and tide gauges; or problems with satellite calibration.
Since 1992 the NASA/ CNES TOPEX/ Poseidon (T/P) and Jason-1 satellite programs have provided measurements of sea level change. The data show a mean sea level increase of 2.8 ± 0.4 mm/ yr. This includes an apparent increase to 3.7 ± 0.2 mm/ yr during the period 1999 through 2004. Satellites ERS-1 (July 17, 1991-March 10, 2000), ERS-2 (April 21, 1995-), and Envisat (March 1, 2002-) also have sea surface altimeter components but are of limited use for measuring global mean sea level due to less detailed coverage. TOPEX/ Poseidon began their series of measurements in 1992, and the scientific mission was ended in October 2005. Jason-1, launched December 7, 2001, has now taken over the mission, and is flying the same ground track.
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