The size of this year’s Antarctic ozone hole is slightly above the 10-year average in both depth and overall area, NOAA scientists announced Thursday. Last year’s ozone hole broke records for both ozone loss within the critical layer and for the size of the area affected. This year marks the 20th anniversary of the signing of the Montreal Protocol, which set international limits on the emission of ozone-depleting compounds, and is credited with beginning the process to recover the Antarctic ozone hole.
“This year’s ozone hole is no giant—and that’s good news,” said Earth System Research Laboratory scientist Bryan Johnson, project leader for NOAA’s balloon-borne ozone monitoring at the South Pole. “In spite of variations caused by temperature, atmospheric dynamics, and other natural factors, we expect the ozone hole to gradually appear smaller and smaller over time and eventually not form at all.”
The primary cause of ozone depletion is human-produced compounds that release chlorine and bromine gases into the stratosphere. The concentrations of these gases peaked in the lower atmosphere in 1995 and in the stratosphere in 2001. The Montreal Protocol and other regulations banning ozone-depleting substances have reversed the destructive trend first noticed in the late 1980s. Because ozone-depleting gases typically last 40 to 100 years in the atmosphere, full recovery is not expected to be reached before 2070.
Scientists from ESRL measured this year’s ozone minimum of 125 Dobson units Oct. 8. Dobson units are a measure of total ozone in a vertical column of air, in this case between the ground and the top of the atmosphere.
In the critical layer between 8 and 13 miles (14 and 21 kilometers) above the surface, the team found nearly complete ozone destruction. This degree of depletion has become typical for those altitudes at this time of year, though slightly more ozone remained this year compared to last year.
The ozone hole reached a maximum area of 9.3 million square miles (24 square kilometers) Sept. 13, well within the average range for this stage of recovery, but still an area nearly as large as North America.
Last year’s record-breaking ozone loss plunged to 93 Dobson units Oct. 9 and covered nearly 11.4 million square miles at its peak. Scientists blamed colder-than-usual temperatures in the stratosphere for last year’s unusual size.
The ozone hole, located above Antarctica, is a thinning of upper atmospheric ozone, which protects life on Earth by blocking harmful ultraviolet-B rays from the sun. UV-B can cause skin cancer and cataracts in humans, harm plant life, and degrade certain materials. In the southern oceans, ultraviolet light is most dangerous to phytoplankton, the base of the oceanic food chain.
NOAA scientists measure the amount of ozone vertically using a balloon-borne instrument called an ozonesonde. A team based at the South Pole launches balloons several times a week during the early Southern Hemisphere spring.
The ESRL balloon measurements showed the temperatures in the critical depletion layer were below average in early September, but warmed to above average values in mid-September, bringing the ozone reading back up to slightly above average.
From mid-September to mid-October, the ozone hole area hovered about the average size for the past 10 years of observations, according to NOAA scientist Craig Long, who analyzes ozone satellite data. It could take another decade of recovery before the observations detect a real trend of progressively smaller ozone holes. Although the amount of ozone-depleting substances has remained fairly constant during this period, the size of the ozone hole has varied widely. Scientists attribute these variations to the Southern Hemisphere's polar winter circulation and temperatures. NOAA’s Solar Backscatter Ultraviolet 2 instrument aboard the NOAA-18 satellite provides the area data.