OZONE HOLE EARLIER IN 2000 BUT NOT DEEPER
October 5, 2000 NOAA scientists report that measurements of this year's ozone hole at the South Pole show that ozone depletion occurred earlier there but did not reach the very lowest values of some recent years. The ozone hole, a region of ozone depletion over Antarctica, is the result of complex chemical and meteorological processes that occur in the stratosphere. (Click image to see latest graphic.)
Using instrumented balloons to take vertical profiles of the ozone at the South Pole, researchers from NOAA's Climate Monitoring and Diagnostics Laboratory located in Boulder, Colo., reported that the September decline in ozone occurred about six days earlier than in any previous year and, as observed in other recent years, ozone was totally destroyed between about 9 and 13 miles altitude. Total column ozone reached the minimum reading of 98 Dobson units on September 29, compared to 90 Dobson units in 1999. The record low of 88 Dobson units was observed in 1993. A Dobson unit is a unit of measurement that describes the thickness of the ozone layer in a column directly above the location being measured. Prior to the springtime period in Antarctica, when ozone depletion occurs, the normal Dobson unit reading is around 275.
Another measure of the severity of the ozone hole is its total area. Estimates of the area of the ozone hole are made at NOAA's Climate Prediction Center by analysis of measurements made by the Solar Backscatter Ultraviolet (SBUV/2) instruments on NOAA's Polar-orbiting Operational Environmental Satellites operated by NOAA's National Environmental Satellite, Data, and Information Service. Satellite data show the ozone hole area was more than 27 million square kilometers for three days in early September, peaking at 28.5 million square kilometers.
"This is the largest geographical size on record and comparable to the area of North America", said Lawrence Flynn, a physical scientist at NESDIS. The ozone hole is defined as the size of the region with total ozone below 220 Dobson units. NASA released similar measurements using the Total Ozone Mapping Spectrometer (TOMS) aboard NASA's Earth Probe (TOMS-EP) satellite. NOAA and NASA satellites have been measuring Antarctic ozone levels for almost 30 years. Continued monitoring by satellite instruments over the next two months will establish how long-lived the current year's ozone hole will be.
"Even though the geographical size of the ozone hole was the largest on record and the ozone depleted earlier than ever, the severity of the ozone depletion within the hole reached about the same levels as the past few years," said David Hofmann, director of CMDL. "Year-to-year fluctuations in the geographical size of the ozone hole and the timing of the ozone reduction are believed to be related to meteorological factors such as temperature and winds, rather than further increases in ozone-destroying chemicals in the atmosphere," Hofmann said.
Each spring when the sun rises over Antarctica, chemical reactions involving chlorine and bromine from man-made CFCs (chlororfluorocarbons) and bromine-containing compounds, occur in the stratosphere destroying ozone and causing the "ozone hole." The global ozone layer has also deteriorated since 1980 but not to the extent that is observed each spring in Antarctica. An international assessment of the status of the global ozone layer, produced by hundreds of scientists for the World Meteorological Organization and the United Nations Environmental Program in 1998, indicated that the amount of chlorine in the stratosphere from CFCs should soon be reaching a maximum due to regulations on emissions as dictated by the Montreal Protocol on substances that deplete the ozone layer and subsequent amendments.
The amount of chlorine in the atmosphere at the surface of the Earth was measured by CMDL to have begun declining already in 1994. The thinning of the ozone layer is a matter of concern because the ozone layer protects the Earth from the harmful effects of the sun's ultraviolet radiation, which affects life on Earth and contributes to skin cancer and cataracts in humans.
Hofmann says that "since chlorine in the atmosphere has reached nearly constant levels, annual variations in temperature, which affect the rate of the chemical reactions, will be the dominant factor in determining differences in the magnitude of the ozone hole. Total recovery of the ozone hole back to levels observed before 1980 will take at least 50 years, and expected changes in climate, including a cooler stratosphere, could cause a delay in the recovery of the ozone layer."
monitoring of the SBUV/2 measurements