Global warming of the lower atmosphere may encourage the growth of an
Arctic ‘ozone hole’. John Austin of Britains’s Meteorological Office, Neal
Butchart of the Hadley Centre for Climate Prediction and Keith Shine of
the University of Reading calculate that if the amount of carbon dioxide
in the atmosphere doubles – as is likely to happen by the middle of the
21st century – almost all the ozone in the lower stratosphere above the
Arctic will be destroyed (Nature, 19 November, p 221).
The role of carbon dioxide in depleting ozone was worked out in 1988,
shortly after scientists identified the cause of the Antarctic ozone hole.
Carbon dioxide, and other greenhouse gases, trap heat near the surface of
the Earth. This makes the lower atmosphere warmer but robs the upper atmosphere,
or stratosphere, of warmth. The temperature of the stratosphere is one of
the key factors in the springtime depletion of ozone above the Antarctic
where in winter it gets colder than anywhere else on Earth, encouraging
icy particles to form in polar stratospheric clouds. Reactions on the surfaces
of these particles then release chlorine, which attacks ozone.
Since the discovery of the ozone hole above the Antarctic, scientists
have been studying the Arctic stratosphere intensively to determine how
primed it is for ozone destruction. The Arctic stratosphere, even in winter,
does not get as cold as air over Antarctica. This is partly because the
Arctic air is not cut off by the circumpolar vortex, a winter-time wall
of wind which blows unobstructed over the southern ocean. Equivalent winds
in the north provide a less effective barrier because they are deflected
Austin and his colleagues say that ‘the appearance of an Arctic ozone
hole would require a considerable change in the atmospheric circulation’.
But they point out that ‘increases in carbon dioxide anticipated over the
next 50 years are expected to produce just such a change by cooling the
Their new analysis is based on a computer simulation of the changes
in temperature of the atmosphere associated with greenhouse warming. They
find that even if ozone-damaging chemicals (chiefly CFCs) are phased out
in line with current international agreements, the amount of chlorine-bearing
material in the stratosphere will continue to increase for several decades.
The concentration of carbon dioxide and other greenhouse gases in the air
will also increase for the foreseeable future. Allowing for both effects,
the team concludes that ‘a combination of conditions will occur during the
next 50 years which will give rise to an Arctic ozone hole’.
However, even this may be on the optimistic side. When ozone is lost
from the stratosphere, it is not available to absorb incoming ultraviolet
light from the Sun; so the stratosphere cools. ‘A long-term decrease in
ozone might systematically affect stratospheric temperatures so that the
beginning of winter could be colder than in our simulation,’ say Austin
and his colleagues.
This possibility is set in a disturbing context by the latest measurements
of stratospheric ozone by the World Meteorological Organization. These show
that just such a long-term decrease is taking place, and that there were
unusually low values of ozone concentration in 1992 between latitudes 50
degrees North and 60 degrees North, covering Northern Europe, Russia and
Canada. The ozone level was 12 per cent below normal, lower than at any
time in the 35 years of continuous monitoring.
This year, the Antarctic hole itself has been worse than ever before,
partly because of damage caused by the eruptions of Mount Pinatubo and
Mount Hudson (see below). The WMO reports the lowest ever measurements
of Antarctic stratospheric ozone – 105 Dobson units – from the South Pole
a few weeks ago. They also report that there was a 50 per cent loss of
the ozone above southern Argentina and southern Chile for a few days in
early October. This resulted in a 100 per cent increase in solar ultraviolet.