|By Jeannie Allen · September 6, 2001|
The sun radiates energy in a wide range of wavelengths, most of which are invisible to human eyes. The shorter the wavelength, the more energetic the radiation, and the greater the potential for harm. Ultraviolet (UV) radiation that reaches the Earths surface is in wavelengths between 290 and 400 nm (nanometers, or billionths of a meter). This is shorter than wavelengths of visible light, which are 400 to 700 nm.
UV radiation from the sun has always played important roles in our environment, and affects nearly all living organisms. Biological actions of many kinds have evolved to deal with it. Yet UV radiation at different wavelengths differs in its effects, and we have to live with the harmful effects as well as the helpful ones. Radiation at the longer UV wavelengths of 320-400 nm, called UV-A, plays a helpful and essential role in formation of Vitamin D by the skin, and plays a harmful role in that it causes sunburn on human skin and cataracts in our eyes. The incoming radiation at shorter wavelengths, 290-320 nm, falls within the UV-B part of the electromagnetic spectrum. (UV-B includes light with wavelengths down to 280 nm, but little to no radiation below 290 nm reaches the Earth’s surface). UV-B causes damage at the molecular level to the fundamental building block of life deoxyribonucleic acid (DNA).
DNA readily absorbs UV-B radiation, which commonly changes the shape of the molecule in one of several ways. The illustration below illustrates one such change in shape due to exposure to UV-B radiation. Changes in the DNA molecule often mean that protein-building enzymes cannot read the DNA code at that point on the molecule. As a result, distorted proteins can be made, or cells can die.
But living cells are smart. Over millions of years of evolving in the presence of UV-B radiation, cells have developed the ability to repair DNA. A special enzyme arrives at the damage site, removes the damaged section of DNA, and replaces it with the proper components (based on information elsewhere on the DNA molecule). This makes DNA somewhat resilient to damage by UV-B.
In addition to their own resiliency, living things and the cells they are made of are protected from excessive amounts of UV radiation by a chemical called ozone. A layer of ozone in the upper atmosphere absorbs UV radiation and prevents most of it from reaching the Earth. Yet since the mid-1970s, human activities have been changing the chemistry of the atmosphere in a way that reduces the amount of ozone in the stratosphere (the layer of atmosphere ranging from about 11 to 50 km in altitude). This means that more ultraviolet radiation can pass through the atmosphere to the Earths surface, particularly at the poles and nearby regions during certain times of the year.
Without the layer of ozone in the stratosphere to protect us from excessive amounts of UV-B radiation, life as we know it would not exist. Scientific concern over ozone depletion in the upper atmosphere has prompted extensive efforts to assess the potential damage to life on Earth due to increased levels of UV-B radiation. Some effects have been studied, but much remains to be learned.
next: Effects on the Biosphere
Some Effects of Ultraviolet-B (UV-B) Radiation on the
We know that increased exposure to UV-B radiation has specific effects on human health, crops, terrestrial ecosystems, aquatic ecosystems, and biogeochemical cycles. (Biogeochemical cycles refers to the cycling of chemicals such as carbon and energy throughout the Earth system.) This article will touch briefly on these effects, then will explain what determines how much UV we are getting and how we know.
The effects of UV-B radiation on human skin are varied and widespread. UV-B induces skin cancer by causing mutation in DNA and suppressing certain activities of the immune system. The United Nations Environment Program estimates that a sustained 1 percent depletion of ozone will ultimately lead to a 2-3 percent increase in the incidence of non-melanoma skin cancer. UV-B may also suppress the bodys immune response to Herpes simplex virus and to skin lesion development, and may similarly harm the spleen.
Our hair and clothing protect us from UV-B, but our eyes are vulnerable. Common eye problems resulting from over-exposure to UV-B include cataracts, snow blindness, and other ailments, both in humans and animals. While many modern sunglasses offer some UV protection, a significant amount of UV can still reach our eyes in a high exposure situation.
With regard to plants, UV-B impairs photosynthesis in many species. Overexposure to UV-B reduces size, productivity, and quality in many of the crop plant species that have been studied (among them, many varieties of rice, soybeans, winter wheat, cotton, and corn). Similarly, overexposure to UV-B impairs the productivity of phytoplankton in aquatic ecosystems. UV-B increases plants susceptibility to disease. Scientists have found it affects enzyme reactions that conduct fundamental biological functions, it impairs cellular division in developing sea urchin eggs, and it changes the movements and orientation of tiny organisms as they move through ocean waters. Since some species are more vulnerable to UV-B than others, an increase in UV-B exposure has the potential to cause a shift in species composition and diversity in various ecosystems. Because UV-B affects organisms that move nutrients and energy through the biosphere, we can expect changes in their activities to alter biogeochemical cycles. For example, reducing populations of phytoplankton would significantly impact the worlds carbon cycle, because phytoplankton store huge amounts of carbon in the ocean.
Much of scientists work to determine the effects of increased UV-B on the marine biosphere has focused around Antarctica because the stratospheric ozone depletion there has been so dramatic, and because phytoplanktonwhich grow in abundance around Antarcticaform the basis of the marine food chain. Largely because of phytoplankton, oceans are responsible for the production of at least half of the organic material in the biosphere.
In the Antarctic, increased exposure to UV-B radiation due to the appearance of the ozone hole commonly results in at least a 6-12 percent reduction in photosynthesis by phytoplankton in surface waters. In a study of California coastal waters, effects of current levels of UV-B radiation compared to historical levels range from 40 percent reduction of photosynthesis by phytoplankton to a 10 percent increase. In fact, phytoplankton off the California coast sometimes turn out to be more susceptible to UV-B radiation than phytoplankton in Antarctica, to the surprise of biologists.
Communities of plants, animals, and microorganisms may be more resilient than we yet know. In spite of increased ultraviolet exposure in Antarctica over the last decade or so, no catastrophic events have occurred at the ecosystem level. However, the reason for this may be that the large ozone hole lasts only from September to December and covers a small geographic region relative to the entire globe. If the ozone hole should remain for longer time periods, or if ozone were to be reduced over a wider area every year, sooner or later, we could expect to see major ecosystem changes. So many studies in both the laboratory and the field have demonstrated serious consequences of increased UV-B radiation on the biosphere that we need to improve our understanding of the complex Earth environment and its responses to that radiation.
What Determines How Much Ultraviolet Radiation Reaches the Earths
Ozone in the Stratosphere
Ozone depletion is greater at higher latitudes, (toward the North and South Poles) and negligible at lower latitudes (between 30 degrees N and 30 degrees S). This means that decreases in ozone over Toronto are likely to be greater than those over Boston, and those over Boston greater than those over Los Angeles, while Miami will typically see the least ozone depletion of the four cities. However, cities at lower latitudes generally receive more sunlight because they are nearer the equator, so UV levels are higher even in the absence of ozone depletion. If ozone were to decrease at lower latitudes, southern cities would experience a greater absolute increase in UV-B than cities in the north for the same amount of ozone depletion.
Oblique angle of sunlight reaching the surface
While the presence of aerosols anywhere in the atmosphere will always scatter some UV radiation back to space, in some circumstances, aerosols can contribute to an increase in UV exposure at the surface. For example, over Antarctica, cold temperatures cause ice particles (Polar Stratospheric Clouds) to form in the stratosphere. The nuclei for these particles are thought to be sulfuric acid aerosol, possibly of volcanic origin. The ice particles provide the surfaces that allow complex chemical reactions to take place in a manner than can deplete stratospheric ozone.
Reflectivity of the Earths Surface
How Much Ultraviolet (UV-B) Radiation Are We Getting?
The second way to determine UV-B irradiance at the surface is by making estimates based on satellite measurements of ozone, cloud cover, and the other parameters described in What Reaches Earths Surface. Such estimates must take into account changes in the amount of radiation coming from the sun to the top of the atmosphere. To understand how researchers arrive at estimates of UV-B radiation reaching the Earths surface, one must first visualize a column of air that extends from the ground to the spacecraft above the atmosphere. Instruments on satellites orbiting the Earth (such as TOMS and OMI/Aura) measure the amounts of ozone, cloud cover, and aerosols in that column. Researchers can accurately calculate how much UV-B radiation there should be at the ground based on those measurements and on other conditions described earlier in this article (elevation, angle of sunlight, etc.). These values for each satellite field of view are incorporated into a global visualization of the data.
Satellite measurements are critical to our understanding of global change such as increases in UV radiation. Their importance derives from their superior calibration over long periods, their ability to observe remote or ocean-covered regions, and their capability of providing consistent global coverage. We also need well-maintained, strategically located ground-based instruments to continue to verify the accuracy of satellite-derived estimates of surface UV exposure over the globe.
Determining very long-term global trends still remains a problem because we have little historical data available before 1978, when NASAs TOMS was first launched. Our need for historical data to detect and understand change underscores the critical importance of monitoring the Earthws processes for a long period of time, an objective to which NASA has committed in its Earth Observing System (EOS) program.
In September and October over Antarctica, loss of ozone and consequent increased levels of UV-B radiation at the surface are now commonly twice as high as during other times of the year. High UV-B exposures occur in nearby regions at both poles, including some regions where people live, such as Scandinavia, most of Europe, Canada, New Zealand, Australia, South Africa, and the southern region of South America. Exposures get especially high in regions of elevated altitude, such as in the Andes Mountains, and in places that are relatively free of clouds at certain times of the year, such as South Africa and Australia during their summer (December to February). In July, very high exposures appear over the Sahara, Saudi Arabia, southwestern United States, and the Himalayan Mountain regions in northern India and southern China. The equatorial regions have their maximum exposure in the spring and autumn, with higher values during the autumn due to decreased cloud cover.
We have no reliable long-term record of actual UV-B exposure from ground-based measurements, but we do have accurate short-term estimates of decreasing ozone, which we know leads to an increase in UV-B exposure at the surface. In Scientific Assessment of Ozone Depletion: 1998, the World Meteorological Organization states that during 1998 at mid-latitudes in the north, between 35 and 60 degrees N, average ozone abundances were about 4 percent (per satellite measurements) or 5 percent (per ground-based measurements) below values measured in 1979, with most of the change occurring at the high end of that latitude zone. That means that recent UV-B radiation doses are correspondingly higher at those latitudes than historical levels (by amounts that depend on specific wavelengths). In the tropics and mid-latitudes, between 35 degrees S and 35 degrees N, both satellite data and ground-based data indicate that total ozone does not appear to have changed significantly since 1979.
U.S. Department of Agriculture UV-B Monitoring Program
National Science Foundation, Office of Polar Programs Ultraviolet Monitoring Network
U.S. Environmental Protection Agency (EPA)
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Predictions and Monitoring
Data from NASAs satellites, coupled with observations on the ground, are essential to resolve critical questions about the impacts of increased ultraviolet radiation due to ozone depletion. The suite of TOMS (Total Ozone Mapping Spectrometer) missions will provide us with ozone and UV-B surface exposure data. NASAs Aura mission, to be launched in 2003, will monitor the status of stratospheric ozone and will enable the scientific community to determine whether or not the ozone layer is recovering as scientific models predict. Until the ozone layer recovers, Aura will help us to better predict how much UV-B exposure we can expect to receive at the surface.
Krotkov, N.A., P.K. Bhartia, J. Herman, Z. Ahmad, V. Fioletov. 2001. Satellite estimation of spectral surface UV irradiance 2: Effect of horizontally homogeneous clouds and snow, Journal of Geophysical Research, 106
Prézelin, Barbara B.; Moline, Mark A.; and Matlick, H. Allen. 1998. Icecolors 93: Spectral UV Radiation Effects on Antarctic Frazil Ice Algae. Antarctic Sea Ice Biological Processes, Interactions, and Variability. American Geophysical Union, Antarctic Research Series, Vol. 73
U.S. Department of Agriculture at Colorado State University. 2001. Managing Editors James R. Slusser and Wei Gao. The USDA UVB Monitoring and Research Program. (Colorado State University, Natural Resource Ecology Laboratory)
Vasilkov, Alexander, et al. 2001 Global mapping of underwater UV fluxes and DNA-weighted exposures using TOMS and SeaWiFS data products Journal of Geophysical Research, 106
World Meteorological Organization, Christine A. Ennis, Coordinating Editor. 1998. Global Ozone Research and Monitoring ProjectReport No. 44. Scientific Assessment of Ozone Depletion.
Jay R. Herman
Nikolay A. Krotkov
Sasha Madronich, Senior Scientist
Raymond C. Smith