Ozone Hole Discovery (1985): CFCs and the Antarctic Ozone Layer

The discovery of the ozone hole was announced in May 1985, when scientists of the British Antarctic Survey reported that ozone in the stratosphere above Antarctica was falling sharply each southern spring. Their findings showed that a layer protecting all life on Earth from harmful ultraviolet radiation was being damaged far faster than expected. The discovery turned a scientific warning about chlorofluorocarbons into a global emergency and led to the Montreal Protocol of 1987, often described as the most successful environmental treaty ever agreed.

Background: the ozone layer

Ozone is a form of oxygen made of three oxygen atoms. Most of it lies in the stratosphere, between about 15 and 35 kilometres above the ground, where it absorbs most of the Sun’s ultraviolet-B radiation. Without it, rates of skin cancer, cataracts and damage to crops and marine life would rise sharply.

Scientists have measured ozone from the ground since the 1920s, when the British physicist G. M. B. Dobson designed the spectrophotometer that still bears his name. The total amount of ozone above a point is measured in Dobson units. In 1956, in preparation for the International Geophysical Year of 1957-1958, British scientists began regular ozone measurements at Halley Bay, a research station on the Brunt Ice Shelf in Antarctica.

The CFC warning

Chlorofluorocarbons (CFCs) were invented in the late 1920s as safe, non-toxic and non-flammable chemicals for refrigerators. By the 1970s they were used worldwide in aerosol sprays, air conditioning, foams and cleaning solvents. Because they are so stable, they do not break down in the lower atmosphere.

In 1974 the chemists Mario Molina and F. Sherwood Rowland of the University of California, Irvine, published a paper in Nature arguing that CFCs would slowly drift into the stratosphere. There, ultraviolet light would break them apart and release chlorine atoms, and each chlorine atom could destroy many thousands of ozone molecules in a chain reaction. Earlier work by the Dutch scientist Paul Crutzen had shown that nitrogen oxides could also destroy ozone.

The theory caused public concern, especially in the United States, which banned CFCs in most non-essential aerosol sprays from 1978. Canada, Norway and Sweden took similar steps. But industry disputed the science, and models in the early 1980s predicted only slow, modest ozone loss over the coming century.

The discovery at Halley Bay

At the British Antarctic Survey (BAS), Joseph Farman, Brian Gardiner and Jonathan Shanklin had continued the routine ozone measurements made with Dobson instruments at Halley Bay and at the Argentine Islands station. From the late 1970s they noticed that ozone levels in October, the Antarctic spring, were becoming unusually low.

At first the team suspected an instrument fault. They checked their equipment, obtained a new spectrophotometer and compared the Halley Bay readings with those from the Argentine Islands on the Antarctic Peninsula. The decline persisted and grew each year. By the early 1980s October ozone values over Halley Bay were roughly a third lower than those of the 1960s.

Their paper, “Large losses of total ozone in Antarctica reveal seasonal ClOx/NOx interaction”, was published in Nature on 16 May 1985. It linked the losses to rising concentrations of chlorine in the atmosphere, although the exact chemistry was not yet understood. The term “ozone hole” soon came into use to describe the region of severely depleted ozone, though it is a thinning rather than a complete absence.

Confirmation from space

NASA’s Nimbus 7 satellite had carried a Total Ozone Mapping Spectrometer (TOMS) since 1978. Its data processing flagged very low ozone values as possible errors for further checking. After the BAS paper, NASA scientists re-examined the satellite record and confirmed that the depletion covered the whole Antarctic continent. Their colour maps of the hole, first presented in 1985 and 1986, became among the most widely reproduced scientific images of the century.

Explaining the chemistry

The size and speed of the loss surprised scientists, because existing models of gas-phase chemistry could not explain it. In 1986 the American atmospheric chemist Susan Solomon and colleagues proposed that reactions on the surfaces of polar stratospheric clouds, which form only in the extreme cold of the Antarctic winter, converted relatively harmless chlorine compounds into reactive forms. When sunlight returned in spring, these would destroy ozone very rapidly.

Expeditions tested this explanation. The National Ozone Expedition to McMurdo Station in 1986 and the Airborne Antarctic Ozone Experiment of 1987, which flew high-altitude research aircraft from Punta Arenas in Chile into the polar vortex, measured high levels of chlorine monoxide exactly where ozone was disappearing. This was widely seen as the “smoking gun” linking the ozone hole to man-made chlorine.

The explanation also accounted for the timing of the hole. During the dark polar winter, the polar vortex traps very cold air over Antarctica and the clouds prepare large amounts of reactive chlorine. When sunlight returns in late August and September, it drives rapid ozone-destroying reactions, and the hole reaches its greatest extent around late September or early October. As the vortex breaks up in late spring, ozone-rich air from lower latitudes flows back in and the hole closes until the next year. Bromine from halons, used in fire extinguishers, and from the pesticide methyl bromide was found to make the process even more efficient.

In 1995 Paul Crutzen, Mario Molina and F. Sherwood Rowland shared the Nobel Prize in Chemistry for their work on the formation and decomposition of ozone.

Global action

Diplomatic efforts had begun before the discovery. The Vienna Convention for the Protection of the Ozone Layer, a framework agreement with no binding cuts, was signed on 22 March 1985, two months before the BAS paper appeared. The shock of the ozone hole added urgency to negotiations, led by the UN Environment Programme.

On 16 September 1987 governments adopted the Montreal Protocol, which required cuts in CFC production. As the science grew stronger, amendments agreed in London in 1990, Copenhagen in 1992 and later meetings turned the cuts into a complete phase-out of CFCs, halons and other ozone-depleting substances. In 2009 the Vienna Convention and Montreal Protocol became the first treaties in UN history to be ratified by every member state. Industry positions changed quickly once the evidence became clear. In March 1988 DuPont, the largest producer of CFCs, announced that it would stop making them, and manufacturers developed substitutes such as hydrochlorofluorocarbons (HCFCs), which do less damage, and later hydrofluorocarbons, which contain no chlorine. Developing countries were given longer to comply, and a Multilateral Fund created in 1990 helped them pay for the transition. The Kigali Amendment of 2016 extended the protocol to hydrofluorocarbons (HFCs), CFC substitutes that are powerful greenhouse gases.

By the numbers

Myths and facts

• Myth: the ozone hole is a literal hole in the sky. It is a region where ozone falls well below normal levels, commonly defined as below 220 Dobson units, mainly from September to November.
• Myth: NASA’s computers threw the data away. Low satellite values were flagged for checking rather than simply discarded. The ground-based British record, with decades of careful calibration, gave scientists the confidence to report the trend first.
• Myth: the problem has been solved. Ozone-depleting chemicals last for decades in the atmosphere, so the hole still forms every spring and recovery is expected to take until the second half of the twenty-first century.

Timeline

• 1956-1957: Regular ozone measurements begin at Halley Bay.
• 1974, June: Molina and Rowland publish their theory that CFCs destroy ozone.
• 1978: The United States bans CFCs in most non-essential aerosol sprays.
• 1985, 22 March: The Vienna Convention for the Protection of the Ozone Layer is signed.
• 1985, 16 May: Farman, Gardiner and Shanklin report Antarctic ozone loss in Nature.
• 1986: Susan Solomon and colleagues propose the polar stratospheric cloud mechanism.
• 1987, 16 September: The Montreal Protocol is adopted.
• 1995, October: Crutzen, Molina and Rowland are awarded the Nobel Prize in Chemistry.
• 2016, October: The Kigali Amendment adds HFCs to the protocol.

Legacy

The discovery of the ozone hole is often cited as a model for how science can inform policy. A long-term record kept by a small team led to a finding that governments acted on within a few years, even before the chemistry was fully understood. The response built on the environmental awareness promoted by earlier landmarks such as Silent Spring and the first Earth Day.

Atmospheric measurements show that concentrations of ozone-depleting substances peaked around the late 1990s and have since declined. Scientists also note that phasing out CFCs, which are powerful greenhouse gases, has slowed climate change more than many policies aimed directly at it.

Why it matters

The ozone hole showed that human activity could alter the global atmosphere in unexpected ways and within a few decades. The international response, from the 1985 discovery to the near-universal phase-out of CFCs, remains the clearest example of countries cooperating successfully to solve a planetary environmental problem.