Science Briefing – The ozone hole
It’s over 25 years since the
discovery of the ozone hole drew world attention to the impact of human activity
on the global environment.
Why is the ozone layer important?
The ozone layer is the Earth’s
natural sunscreen that protects humans, plants and animals by filtering out
harmful UV-B radiation.
In the 1970s concern about the
effect of man-made chemicals, especially chlorofluorocarbons, on the ozone layer
were raised by Paul Crutzen, Mario Molina and Sherwood Rowland. Their pioneering
work was recognised in 1995 by the award of the Nobel Prize in Chemistry.
How was the ozone hole
Scientists from British Antarctic
Survey began monitoring ozone during the International Geophysical Year of
1957–58. In 1985 scientists discovered that since the mid 1970s ozone values
over Halley and Faraday research stations had been steadily dropping when the
sun reappeared each spring. Something in the stratosphere (about 20km above
Earth) was destroying ozone.
So what was wrong with the ozone
We now know that during the polar
winter, clouds form in the Antarctic ozone layer and chemical reactions on the
clouds activate ozone destroying substances. When sunlight returns in the
spring, these substances (mostly chlorine and bromine from compounds such as
chlorofluorocarbons and halons) take part in efficient catalytic reactions that
destroy ozone at around 1% per day. This discovery changed the world.
What happened after the hole was
NASA scientists used their
satellite data to confirm that not only was the hole over British research
stations but it covered the entire Antarctic continent. International efforts by
scientists and politicians then led to steps to control the production and use
of CFCs and other ozone depleting chemicals. The resulting Montreal Protocol
(1987) and its subsequent amendments is a successful example of leadership by
all the world's governments in tackling a global environmental issue.
How successful is the Montreal
The Protocol is having a clear
effect and the amount of ozone destroying substances in the atmosphere is
beginning to go down. Nevertheless, the original compounds are so stable and
long-lived that an ozone hole will exist each Antarctic spring for at least
another 50 years.
Is there a hole over the Arctic?
Unlike Antarctica, which is a
continent surrounded by oceans, the Arctic is an ocean surrounded by mountainous
continents. This means that the stratospheric circulation is much more
irregular. Because it is not as cold as the Antarctic, stratospheric clouds are
less common. So a deep ozone hole over the North Pole is unlikely, but limited
ozone depletion can occur above parts of the Arctic, though it lasts only a few
days at a time.
And what about elsewhere?
In 1996 stratospheric clouds were seen widely over the UK and a small, short-lived ozone hole passed over the country. Elsewhere in the Northern Hemisphere, winter stratospheric ozone amounts have fallen by 10 to 15%. Depletion is generally even greater in the Southern Hemisphere as a direct consequence of the deep Antarctic ozone hole. There has been little ozone depletion over the tropics and globally the depletion averages out at about 4%.
Is the ozone hole linked to
A side effect of global warming
is that the temperature of the ozone layer is falling slightly. This means that
more of the clouds can form over Antarctica, and hence delay the recovery of the
ozone hole. Elsewhere however the same cooling is likely to slightly thicken the
ozone layer. The ozone hole can also help amplify global warming by changing
where solar energy is absorbed in the atmosphere. Another link is that the ozone
depleting chemicals are greenhouse gasses, so reducing their amount has
significantly helped in combating climate change.
Isn't the ozone hole caused by
No. Cosmic rays can affect the
upper most part of the ozone layer, which contains less than 10% of the total
ozone. During the ozone hole however, over 60% of the total ozone disappears and
this can only be explained by chemistry involving ozone depleting substances
such as CFCs.
What can we learn from the ozone
The ozone hole formed within less
than a decade, and shows us just how sensitive our planet is to human
activities. Other signals of the planet’s health, which today are just beginning
to be detected, may develop equally rapidly. The long series of careful
measurements obtained by the British Antarctic Survey show how important it is
to have a good baseline from which to measure changes.