The largest ozone hole yet detected has opened over Antarctica, measurements taken by US satellites show. It is also one of the deepest ozone depletions but not quite a record, according to the US data.
Scientists watch for ozone loss over the Antarctic each year as the southern hemisphere winter begins to bite. NASA and the US National Oceanic and Atmospheric Administration (NOAA) both have satellites recording ozone levels over the poles.
Preliminary data for mid August to early October indicated that this year's depletion reached a record 10.5 million square miles on September 19th. This area is bigger than North America. The previous record of 10 million square miles was set in 1996.
It is a misnomer to call it a "hole" as there is always ozone present, although at reduced amounts. It is recorded in "Dobson units" and levels fell to 90 Dobson units on September 30th this year. The lowest recorded level was 88 Dobson units in September 1994. The global average thickness is about 300 Dobson units.
"This is the largest Antarctic ozone hole we've ever observed and it's nearly the deepest," according to Dr Richard McPeters, principal investigator for the NASA satellite, the Total Ozone Mapping Spectrometer.
Ozone gas, a molecule which joins three oxygen atoms together, is a key atmospheric gas that helps to shield Earth from solar ultraviolet (UV) radiation, the component of sunlight that causes sunburn. As ozone levels fall, the amount of UV reaching Earth increases.
Ozone is destroyed in a complex chemical reaction involving chlorine and bromine compounds released by chlorofluorocarbons and halons, chemicals formerly in widespread use as refrigerants and propellants in aerosol cans.
These have drifted up into the upper atmosphere to cause profound damage to the protective ozone layer. The reaction is speeded by certain atmospheric conditions, including bitter cold air, which helps the creation of high-altitude ice crystals which offer a platform for the reaction. Stratospheric temperatures were particularly cold this year, according to the researchers, and these low temperatures came two weeks earlier than expected.
There are few residents on the ground in Antarctica to worry about increased UV penetration to the surface, but as the area of depletion breaks up and drifts northward it could increase exposures in more populous southern Chile and Argentina. It also causes an overall reduction of ozone over mid-latitudes as the gas diffuses into the "hole".
Increased UV exposure has the potential to increase the incidence of skin cancer and cataracts in humans. It can harm some crops and it is feared that it could disrupt phytoplankton production, the foundation of the marine food chain, which would in turn affect higher marine life forms.
The increases in ozone depletion were not unexpected, according to the research teams, because upper-atmosphere chlorine levels have continued to rise slowly through the 1990s. They should peak soon, however, because of accords such as the Montreal Protocol which commit countries to reducing the use of chlorofluorocarbons.
"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," according to Dr Paul Newman of NASA. Ozone depletion was confirmed in 1985 but analysis of historical data indicated that it has probably existed since at least 1979.