Temps de lecture :4 minutes
It’s not the sort of record you like to see broken, particularly if you live in the Southern Hemisphere.
Scientists from various agencies and universities measuring ozone depletion over Antarctica say this year’s annual ozone hole not only matches the largest hole in area on record, but it is also the deepest that’s ever formed. On Sept. 24 of this year, the Antarctic ozone hole reached a one-day record of 29.5 million square kilometres, an area that spans the entire continent and spills over into parts of Australia and South America. That’s roughly the same size as the record-setting hole that appeared over the region in 2000. […]
“There is a huge section this year that is completely depleted to zero,” said Jennifer Mercer, co-principal investigator (PI) from the University of Wyoming, who is leading the group carrying out balloon-borne and laser measurements of the annual ozone depletion event. The ozone hole, a region in the lower stratosphere centered more or less over the Antarctic continent, has suffered nearly total depletion in previous years but never to this vertical extent.[…]
The University of Wyoming was one of two universities, in addition to NASA and the National Oceanographic and Atmospheric Administration (NOAA), which first came to Antarctica 20 years ago to find the cause behind the ozone hole after its initial discovery in 1985. State University of New York at Stoneybrook rounded out the team.
That group – the National Ozone Expedition – eventually determined through balloon-borne, surface-based and laboratory measurements that chlorine released from chlorofluorocarbons (CFCs) was the main culprit responsible for stealing ozone from the atmosphere.
David Hofmann was the PI for the University of Wyoming in 1986. Today, he’s the director of the Global Monitoring Division at NOAA’s Earth System Research Laboratory in Boulder, Colo. In that role, he’s the PI for a South Pole project that measures atmosphere constituents, including ozone-depleting chemicals. Balloon-borne instruments are launched from South Pole station year-long. […]
Ozone refresher (see ozone team)
Ozone is a somewhat unstable molecule made up of three oxygen atoms instead of two. It’s found throughout the Earth’s atmosphere, with the highest levels occurring in the lower stratosphere, a region commonly referred to as the ozone layer, between about 10 and 30 kilometres above the planet’s surface. (the layer just below that is the troposphere, where we live and breathe). The ozone layer blocks harmful ultraviolet rays that have been linked to skin cancer.
The ozone hole over the Antarctic waxes and wanes every year between August and November as the Antarctic summer begins (a similar but far smaller hole occurs above the Arctic during springtime there). The hole is the result of the input of CFCs and other chlorine- and bromine-containing gases that interact with two naturally occurring events. One is the polar vortex, a sort of atmospheric cyclone that is strongest in winter when temperatures are below negative 80 degrees Celsius. The other is polar stratospheric clouds (PSCs), or nacreous clouds when visible, that also form in the extreme polar winter.
The vortex is a closed system that can circulate the ozone-destroying chemicals quickly. The PSCs provide an excellent surface for setting chlorine and bromine free to run amok and destroy ozone in the presence of sunlight. […]
Scientists say they expect these monstrous-sized ozone holes to continue for at least the next several years. Real improvements may not come until the year 2010, and continued variability may mask the healing process for quite some time. Estimates for recovery range from 2040 to 2080, though NOAA and NASA anticipate recovery by 2065.
Researchers are measuring slightly less ozone-destroying chemicals despite this year’s record ozone hole.
“The chlorine maximum [in the stratosphere] was reached about the year 2000, so we should start to see improvements,” Mercer said.
Hofmann said chlorine and bromine levels reached their maximum concentration at the Earth’s surface around 1994. “It takes a long time for these gases to get into the stratosphere, especially to get into the Antarctic stratosphere,” he said, adding that these molecules can take six years to travel here from the northern hemisphere.
Whether the ozone layer can make a full recovery to pre-1980s levels when CFCs and other chemicals first started their assault is uncertain at this time. That’s partly because not all production of ozone-destroying chemicals has ceased.
In fact, United States chemical companies have more than 20 million pounds of methyl bromide, a pesticide, stockpiled. In addition, the U.S. Environmental Protection Agency allowed production of another 15 million pounds this year, according to a Los Angeles Times article.
Hofmann noted that atmospheric measurements, such as those NOAA conducts at the South Pole, have found declining levels of these pesticide compounds in the last five or six years.
Global warming, while unrelated to ozone destruction, may play a role in the severity of the annual ozone hole and the ozone layer’s eventual recovery. That’s because greenhouse gases that trap heat in the troposphere slow the heat transfer to the stratosphere, causing it to remain colder for longer periods of time.
“If you have a colder stratosphere, then you could have much longer, colder polar vortices every year,” Mercer explained. “This would allow for [a] larger or longer ozone hole because the ozone hole area is controlled by the size of that vortex.”
The variability in climate change models makes it hard to predict how the greenhouse effect may impede ozone hole recovery, the scientists say.
A model solution?
Despite these apparent setbacks, scientists remain optimistic that the ozone layer will eventually heal. They credit the quick reaction of the international community, which adopted the 1987 Montreal Protocol. The agreement, and its subsequent amendments, provided for the protection of the ozone layer by phasing out the substances causing depletion.
The protocol is an example of how nations could cooperate on tackling a potentially more serious problem: global warming and the pervasive use of technologies that emit carbon dioxide, the key greenhouse gas, according to Hofmann. […]