Oct. 7, 1998 NASA and NOAA satellites show that the Antarctic ozone thinning covers the largest expanse of territory since the depletion developed in the early 1980s. The measurements were obtained this year between mid-August and early October using the Total Ozone Mapping Spectrometer (TOMS) instrument aboard NASA's Earth Probe (TOMS-EP) satellite and the Solar Backscatter Ultraviolet Instrument (SBUV) aboard the NOAA-14 satellite.
"This is the largest Antarctic ozone hole we've ever observed, and it's nearly the deepest," said Dr. Richard McPeters, Principal Investigator for Earth Probe TOMS.
Preliminary data from the satellites show that this year's ozone depletion reached a record size of 10.5 million square miles (27.3 million square kilometers) on Sept. 19, 1998. The previous record of 10.0 million square miles was set on Sept. 7, 1996.
The ozone level fell to 90 Dobson units on Sept. 30, 1998. This nearly equals the lowest value ever recorded of 88 Dobson Units seen on Sept. 28, 1994, over Antarctica.
Scientists are not concerned that the hole might be growing because they know it is a direct result of unusually cold stratospheric temperatures, though they do not know why it is colder this year. The decrease in ozone, however, could result in more acute solar or ultraviolet radiation exposure in southern Chile and Argentina if the ozone hole were to pass over that region. One of the primary concerns with an ozone hole of this size is that as the hole "breaks up," the ozone-depleted air will diffuse and reduce the overall ozone levels in the mid-latitudes of the southern hemisphere.
These ozone losses are caused by chlorine and bromine compounds released by chlorofluorocarbons (CFCs) and halons. Year-to-year variations of size and depth of the ozone hole depend on the variations in meteorological conditions. Scientists believe that the decrease in Antarctic ozone is attributed to unusually cold (by 5-9 degrees Fahrenheit) temperatures in the southern middle and polar latitudes. "This year was colder than normal and therefore enables greater activation of reactive chlorine that ultimately causes more ozone loss and lower ozone levels," said Dr. Alvin J. Miller of the National Centers for Environmental Prediction (NCEP).
This decrease in ozone was observed earlier than usual with the hole opening in mid-August about two weeks before a typical year. This is consistent with expectations, since chlorine levels have slightly increased since the early 1990s.
As a result of international agreements known as the Montreal Protocol on ozone-depleting substances (and its amendments), chlorine levels from CFCs already have peaked in the lower atmosphere and should peak in the Antarctic stratosphere within a few years. As we move into the next century, chlorine-catalyzed ozone losses resulting from CFCs and other chlorine-containing species will be reduced.
"An ozone hole of substantial depth and size is likely to continue to form for the next few years or until the stratospheric chlorine amount drops to its pre-ozone hole values," said Dr. Paul Newman at NASA's Goddard Space Flight Center (GSFC), Greenbelt, MD. "The decrease in chlorine in our atmosphere is analogous to using a small air cleaner to recycle all of the air in a large indoor sports stadium -- it will take a very, very long time."
Scientists and others have a keen interest in ozone depletion, given that the increased amounts of ultraviolet radiation that reach the Earth's surface because of ozone loss have the potential to increase the incidence of skin cancer and cataracts in humans, harm some crops, and interfere with marine life.
NASA and NOAA instruments have been measuring Antarctic ozone levels since the early 1970s. Since the discovery of the ozone hole in 1985, TOMS and SBUV have been key instruments for monitoring ozone levels over the Earth.
Analysis of TOMS and SBUV data have traced in detail the annual development of the Antarctic "ozone hole," a large area of intense ozone depletion that occurs between late August and early October. Analysis of the historical data indicated that the hole has existed since at least 1979.
A Dobson unit measures the physical thickness of the ozone layer at the pressure of the Earth's surface. The global average ozone layer thickness is 300 Dobson units, which equals three millimeters or 1/8th of an inch, and while not uniform, averages the thickness of two stacked pennies. In contrast during these annual occurrences, the ozone layer thickness in the ozone hole is about 100 Dobson units (1/25th of an inch or 1 millimeter thick), approximately the thickness of a single dime.
Ozone shields life on Earth from the harmful effects of the Sun's ultraviolet radiation. The ozone molecule is made up of three atoms of oxygen; ozone comprises a thin layer of the atmosphere which absorbs harmful ultraviolet radiation from the Sun. Most atmospheric ozone is found in a thin layer between 6-18 miles up.
TOMS ozone data and pictures are available on the Internet at the following URL:
or through links at URL:
TOMS-EP and other ozone-measurement programs are key parts of a global environmental effort of NASA's Earth Science enterprise, a long-term research program designed to study Earth's land, oceans, atmosphere, ice and life as a total integrated system. Goddard developed and manages the operation of the TOMS-EP for NASA's Office of Earth Science, Washington, DC.
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