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Scripps Scientist Predicts Global Impact of Eroding Ozone Layer
The steady depletion of the atmospheric ozone layer is causing some areas of the world to be exposed to above-normal levels of ultraviolet radiation, according to a new study conducted at UCSD's Scripps Institution of Oceanography.
The depletion of ozone in the lower stratosphere has been a global environmental concern for two decades, but to date little data have been collected on whether it is causing an increase in the amount of harmful ultraviolet-B radiation reaching the Earth's surface.
While scientists have attempted to estimate the impact of declining ozone, the variability of clouds -- which also screen out ultraviolet radiation -- has been a major stumbling block.
Now Dan Lubin, a research physicist in the California Space Institute at Scripps, and Elsa Jensen, of SeaSpace Corp. in San Diego, have overcome that obstacle by developing a satellite mapping method that takes cloud variability into account. This new method, which includes satellite measurements of global ozone levels and solar radiation, is capable of predicting when the erosion of the ozone layer will supersede variations in cloud opacity in different parts of the world and cause an increase in UV radiation to reach the Earth's surface.
"The scientific community has kind of assumed that any decrease in the ozone layer over temperate regions will automatically bring about an increase in UV radiation at the Earth's surface that has some biological significance," Lubin said. "What the results of this study show is that whether or not there is a biological significance to the UV increase depends very much on where you are. When cloud cover introduces a large natural variability from one year to the next in the UV that a given ecosystem has evolved under, then you can't always assume that ozone depletion is an environmental problem."
Lubin and Jensen's results, published in the Oct. 26 issue of the journal Nature, indicate that as of this year, large parts of North America, most of central Europe, the Mediterranean, Canada, New Zealand, South Africa, and the southern half of Australia, Argentina and Chile will be subjected to significant increases in UV radiation.
Areas of North America that already are seeing increases in UV radiation include the Southwest (including California), the Midwest, the Northeast and Hawaii. The increase in UV radiation is not predicted to exceed that associated with normal cloud variation in the South and the Pacific Northwest, however, for another 20 years.
The British Isles and Ireland are not expected to experience significant increases in UV radiation for another 30 years. Likewise, parts of central Russia, most of China, Japan, Korea and the Indian subcontinent are not expected to see significant increases in UV radiation for 20 to 50 years. Mexico, northern Australia, New Guinea and areas of South America north of São Paulo, Brazil, also are not expected to experience increases for decades.
Exposure to excessive levels of ultraviolet-B radiation is believed to play a role in causing skin cancer, eye damage, and immune system suppression. Increased levels of UV radiation are also thought to cause damage to both terrestrial and aquatic ecosystems.
The concentration of ozone in the Earth's atmosphere is estimated to be declining at an average rate of about 2 1/2 percent per decade. Yet despite the risk increased levels of ultraviolet radiation pose, little is known about its and geographic distributions.
"Unfortunately, the scientific community has been really behind in monitoring UV radiation at the Earth's surface," Lubin said. "It is kind of a strange thing that we've been worrying about ozone depletion as an environmental hazard but the efforts to monitor UV have been very, very sparse."
In order to develop maps of global ultraviolet radiation reaching the Earth's surface, Lubin and Jensen combined data from NASA's Total Ozone Mapping Spectrometer (TOMS), which measured global trends in ozone from 1978 to 1993, with data that measured solar radiation reflected from clouds collected as part of the Earth Radiation Budget Experiment (ERBE) from 1985 to 1990. The ERBE data provided the much needed information on cloud opacity. The scientists analyzed data collected each July over the Northern Hemisphere and each January over the Southern Hemisphere. They then used the physics of radiation transport to map how much UV radiation propagates through both the ozone layer and clouds to reach the Earth's surface.
"Having built a five-year climatology that is sufficient to estimate the variability of UV at any given place in a given month that occurs just due to cloud cover, we then compared trends in UV that we expect from ozone depletion against the natural variability to see how long it takes for the UV trend to become significant," Lubin said.
Scientists believe that the release of manmade compounds into the stratosphere, particularly chlorofluorocarbons (CFCs), is responsible for the net destruction of stratospheric ozone. The level of chlorine in the stratosphere is expected to peak around the end of the century and then gradually return to pre-1980 levels over the next fifty years if the international community complies with new regulations on the releases of CFCs set out in the Montreal protocol.
### Note: Color graphic available showing when ultraviolet increases in various areas of the globe become significant