Perilous Times
21 April 2011 Last updated at 15:39 ET
Ozone hole has dried Australia, scientists find
Richard Black By Richard Black Environment correspondent, BBC News
The Antarctic ozone hole is about one-third to blame for
Australia's recent series of droughts, scientists say.
Writing in the journal Science, they conclude that the hole has
shifted wind and rainfall patterns right across the Southern
Hemisphere, even the tropics.
Their climate models suggest the effect has been notably strong
over Australia.
Many parts of the country have seen drought in recent years, with
cities forced to invest in technologies such as desalination, and
farms closing.
The scientists behind the new study - led from Columbia University
in New York - added the ozone hole into standard climate models to
investigate how it might have affected winds and rains.
"The ozone hole results in a southward shift of the high-latitude
circulation - and the whole tropical circulation shifts southwards
too," explained Columbia's Sarah Kang.
Of particular interest was the southward migration of the Southern
Hemisphere jet stream.
“There is also the rising trend in carbon dioxide, and that is
acting in the same direction as the ozone hole” - Dr Sarah Kang
Columbia University
These high-altitude winds are key to determining weather patterns,
in both hemispheres. Much of the cold weather felt in the UK over
the last couple of winters, for example, was caused by blocking of
the Northern Hemisphere stream.
The Columbia team found that overall, the ozone hole has resulted
in rainfall moving south along with the winds.
But there are regional differences, particularly concerning
Australia.
"In terms of the average for that zone, [the ozone hole drives]
about a 10% change - but for Australia, it's about 35%," Dr Kang
told BBC News.
Their modelling indicated that global warming due to greenhouse
gas emissions was also a factor - although natural climate cycles
are also thought to be important, as Australia suffered severe
droughts in the era before ozone depletion and before the warming
seen in the late 20th Century.
"This study does illustrate the important point that different
mechanisms of global change are contributing to the climate
impacts we're seeing around the world," observed Professor Myles
Allen of Oxford University, a leading UK climate modeller.
"It's very important to unpack them all rather than assuming that
any impact we see is down simply to greenhouse gas-mediated
warming."
No reverse
Ozone depletion is caused by chemical reactions in the
stratosphere, the upper atmosphere.
The chemicals involved derive from substances such as
chlorofluorocarbons (CFCs) and their relatives, which used to be
staples in air-conditioning, refrigeration and aerosol cans.
Desalination plant Desalination is one of the approaches being
used to combat Australia's dwindling supply of water
Although the UN Montreal Protocol has significantly curbed
emissions of these substances, they endure for decades in the
atmosphere, and so their effects are still being felt.
The ozone layer blocks the Sun's harmful ultraviolet rays, which
can cause skin cancer and other medical conditions.
Earlier this month, the World Meteorological Organization revealed
that the Arctic was experiencing the worst ozone depletion on
record - a consequence of unusual weather conditions.
But the forecast is that even the Antarctic ozone hole - which is
more severe than its Arctic equivalent - should be repaired by
2045-60.
Sarah Kang cautions that this alone will not restore prior climate
conditions to Australia or anywhere else in the Southern
Hemisphere.
"As the ozone hole repairs, it is going to work to reverse this
trend; but there is also the rising trend in carbon dioxide, and
that is acting in the same direction as the ozone hole," she said.
Australia's persistently dry weather has caused major impacts on
communities, farms and nature.
In recent years, the volume of water flowing into the reservoirs
of Perth, the Western Australian capital, has been just one third
of what it was during most of the 20th Century.
The Murray-Darling basin, which lies in the highly populated
southeast, is the subject of a somewhat controversial plan aiming
to distribute water fairly against a backdrop of over-extraction,
prolonged drought, natural climate variability and greenhouse
gas-mediated global warming.