Scientists divided over ozone hole depth

In September of this year, the ozone hole over Antarctica appeared earlier than ever before and was the largest since scientists have been measuring it

By Environmental News Network staff

The ozone hole over Antarctica this year garnered a few firsts -- it appeared earlier and spread over a larger area than ever. And for the first time in its 15-year existence it is threatening the health of thousands of residents of southern South America.

But there appears to be a discrepancy between the World Meteorological Organization's measurement of the depth of the hole and that of the United States' weather and space agencies. Those differences are based on how each agency measures the depth of the hole and for how long.

According to National Oceanic and Atmospheric Administration, the ozone hole did not reach its lowest levels of ozone.

"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 NOAA's Climate Monitoring and Diagnostics Laboratory.

On the other hand, ground-based and satellite measurements both indicate that the depth of the Antarctic ozone hole during the last 10 days of September was the greatest on record, the WMO announced Oct. 6.

Regardless of its depth, the enormous size of the ozone hole over the South Pole is reason for concern. It has spilled over the southern city of Punta Arenas, Chile, where health officials warned residents Monday not to go in the sun when it's highest in the sky. Residents of Ushuaia, Argentina, are also at risk from extremely low levels of ozone that are letting harmful ultraviolet radiation leak through to the Earth's surface.

Satellite data revealed the ozone hole covered more than 10.4 million square miles for three days in early September, peaking at more than 11 million square miles.

"This is the largest geographical size on record and comparable to the area of North America", said Lawrence Flynn, a physical scientist at NOAA's National Environmental Satellite, Data and Information Service.

All agencies maintain, and scientists at CMDL concluded, that the September decline in ozone occurred about six days earlier than in any previous year. And the ozone was totally destroyed between about 9 and 13 miles altitude, as observed in recent years.

NOAA, NASA measurements

Scientists have been measuring ozone levels over Antarctica for almost 30 years

NOAA used weather balloons to measure the amount of ozone over the South Pole, as well as satellites in space that measure a 124-square-mile area over Antarctica, but not the South Pole. NOAA defines the ozone hole over the South Pole as the size of the region with total ozone below 220 Dobson units. Total ozone reached a minimum reading of 98 Dobson units on Sept. 29, compared to a minimum 90 Dobson units in 1999.

"We can't claim we saw the minimum value everywhere in the hole," explained Flynn.

The deepest ozone hole recorded by NASA and NOAA reached a record low of 88 Dobson units 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.

Globally, the ozone layer averages approximately 300 Dobson units, which would equal a layer about the thickness of two stacked pennies.

WMO measurements

The WMO measures the depth of the ozone hole as a deviation from pre-ozone hole norms. The depth of the Antarctic ozone hole during the last 10 days of September was the greatest on record, says Michael Proffitt, senior scientific officer at the WMO. Presently, ozone is more than 50 percent below the 1964-76 pre-ozone hole norms throughout most of the ozone hole, he said.

By comparing satellite averages over 10-day intervals throughout the historical record, the WMO found that the lowest ozone values of the season have consistently occurred during Oct. 1-10 or mid month from Oct. 11-20, as was the case in 1998, the previous record year.

The average ozone during the entire month of September was also exceptionally low, says the WMO, with values 50 percent below norms within the hole at certain areas.

"The NOAA position for the South Pole station is apparently based upon four days of ozone measurements from balloons. (These data are on the NOAA CMDL South Pole web site.) This year TOMS satellite measurements are not available for the South Pole until early October, nor are the daily column ozone ground based measurement made by the South Pole scientists. This is due to the South Pole being the last place in Antarctica to see the sun (needed for measurements) after the total 24 hours a day of darkness during the Southern Hemisphere Winter.

"Also, the amount of ozone loss over any one station is not indicative of the entire ozone hole. This can be seen easily from the NASA satellite images where ozone variability is seen as color contours within the hole. And finally, sunlight is required for the ozone loss to take place, therefore, during September the South Pole is not where large losses would occur," explained Proffitt.

What the agencies do agree on is that variations in the meteorological conditions in the stratosphere influence the size of the ozone hole, its depth and its lifetime.

"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," said Hofmann.

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World Meteorological Organization's
Climate Monitoring and Diagnostics Laboratory
  • NOAA CMDL South Poleweb site
  • South Pole ozone data
National Environmental Satellite, Data and Information Service
meteorological conditions and ozone in the polar stratosphere
NOAA's National Environmental Satellite, Data and Information Service
Center for Atmospheric Science
Total Ozone Mapping Spectrometer
  • Dobson units

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