The chance discovery of the ozone hole

by R. PRASAD


A British team discovered in 1985 the thinning of ozone in the atmosphere above the Halley Research Station in Antarctica

Twenty-five years ago British scientists — Joseph Farman, Brian Gardiner and Jonathan Shanklin — discovered the thinning of ozone in the atmosphere above the Halley Research Station in Antarctica.

Though man-made chlorofluorocarbons (CFCs) were suspected to have caused the destruction of the ozone, there was no conclusive evidence to prove it. But that changed when the British team which discovered the hole published its findings in Nature in 1985.

CFC-ozone hole link

The scientists provided evidence that the amount of ozone in the Southern Hemisphere during spring had declined sharply since the late 1970s. And for the first time they proved that the reduction was indeed due to CFCs.

Their paper reported that the lowest values of ozone were during mid-October; a drop of 40 per cent during the short period between 1975 and 1984 was seen.

Dramatic discovery

If the depletion of ozone was dramatic, so was the discovery of the ozone depletion by the British team. To start with, the British Antarctic Survey (BAS) where the scientists worked was not originally meant for monitoring long-term ozone changes. Its real purpose was to improve weather forecasting and verify theories of atmospheric circulation.

Writing in the Opinion piece in Nature today (May 6), Dr. Shanklin notes that, in his perspective, luck played a part. But he sees the role of luck in a totally different way.

Capitalising on luck

“The story provides an example of how to capitalise on good luck in science,” he writes. He then goes one step further to remind his fellow scientists: “Researchers should be reminded to question their perceptions, for example, to ensure that people don't see only what they are looking for, even when it seems to yield no immediate insights or benefits.”

His is a classic case of doing the best despite no background or knowledge of the subject. “I had no background in meteorology, and no preconceived ideas of how the atmosphere behaved,” he writes.

But in retrospect, his lack of knowledge and ignorance of atmospheric sciences was a blessing in disguise. He had an open mind.

There are numerous examples of how scientists get strapped to preconceived ideas and refuse to see things in a new light.

That the chlorine present in CFC s would affect the ozone layer by photocatalytic decomposition was already known. And that chorine's effect would be pronounced in the tropical stratosphere was also well known. Studies by Dr. Farman (one of the authors of the landmark 1985 Nature paper) and another researcher had also shown that ozone measurements around the end of January were the least variable year after year.

But none had ever looked at long-term patterns. “No one was searching for long-term patterns in springtime data for the Antarctic,” he writes. Dr. Shanklin was not aware of this lacuna either.

Dr. Shanklin's task at British Antarctic Survey was to write computer programs to process observations made by using a Dobson ozone spectrophotometer which measures atmospheric ozone.

Blessing in disguise

Converting the results into the amount of ozone in the column of air above was done manually. And this meant a backlog was building up.

But this backlog was indeed a blessing in disguise — “it covered the crucial decade when ozone levels began to drop.”

Trying to assuage the public in 1983 that no significant change in the ozone levels was seen, the team presented data in a meeting that reflected this.

Though the data presented did not distort the truth, there was indeed a decline in the springtime ozone values year after year.

But the prevailing theory in the early 1980s was that springtime values were “highly variable and dependent on short-term weather conditions.” So there was nothing unusual about the spring-time decline.

But what appeared to change all that were the 11-day mean values during springtime.

Steady decline found

They showed a steady decline, and decline was systematic.

This data spurred the team to dig further. Dr. Farman was instrumental in coming up with a chemical explanation for the decline, and Dr. Shanklin's task was minor. He notes that: “my persistence in looking at the data was my real contribution.”

If the expertise of the scientists was crucial, the location of the BAS Halley station played a vital role. Unlike other stations, it had excellent continuous ozone data going back to 1957.

Locational advantage

The relatively northern latitude location of the Halley station allowed the scientists to begin their observations during spring much earlier than other stations located at the South Pole.

Finally, the centre of the ozone hole is offset towards the Atlantic Ocean.

This permitted the Halley station to observe lower ozone values compared to stations located on the Pacific side of the Antarctic.

The location advantage, long-term continuous data and the ability to see what others had missed resulted in the landmark paper being published in the journal Nature in 1985. The rest is history.