What Happened To The Once-Feared Ozone Layer Depletion?

Being a school kid in the 1990s meant that you feared, with an unchildlike seriousness, the depletion of the ozone layer.

“A hole in the ozone layer!” was a scary refrain just before the turn of the last century, but one that gradually turned sparse and faded out over the two decades that followed.

Ozone loss is no longer a major cause for worry because, for a change, Earth banded together for a rescue mission on an environmental matter. And it met with success.

Now, the once-jeopardised ozone layer is said to be healing and expected to make a full recovery by the middle of this century.

Ozone Basics

The Earth’s atmosphere is composed of many layers. One of them is the stratosphere — a section extending from 10 km to 50 km above the Earth’s surface.

A part of this layer, from 15 km to 35 km up, happens to host a lot of atmospheric ozone (about 90 per cent).

Ozone is a molecule — a highly reactive gas — made up of three oxygen atoms. It can either be natural or manmade, and can arise from various processes.

The ozone in “the ozone layer” is formed naturally through the interaction of solar ultraviolet (UV) radiation with molecular oxygen (O₂).

Since ozone absorbs harmful UV light — the sunburn-causing UV-B in particular, the ozone layer protects all life on Earth by cutting down the particularly harsh radiation beaming down from the Sun.

Naturally, then, if the ozone layer is, say, thinning or disappearing, it means that more harmful radiation is reaching life on Earth unimpeded, and the results are not pretty.

Concrete effects of the Sun's UV radiation passing through without the ozone layer 'sunscreen' include damage to crops, microbes, and marine life, and higher skin cancer and cataract rates, in addition to immune deficiency disorders, among humans.

Ozone has no sense of permanence in the stratosphere; the molecules are constantly formed and destroyed. The concentrations also vary naturally with sunspots, seasons and latitude. And scientists already know of the natural rise and fall in ozone levels.

However, data from the 1970s began to suggest that there was ozone loss well beyond what could be explained by natural processes alone.

In fact, as we learnt later, the ozone layer was reduced by about 4 per cent per decade from the 1980s to the 2000s.

Ozone Depletion

The chief culprit was found to be chlorine.

Recall the earlier point about ozone impermanence? Turns out, ozone can be destroyed much faster than it is naturally created.

The free chlorine atom partakes in chemical reactions that destroys ozone and gets returned to the atmosphere only to destroy more ozone in what is known as a catalytic reaction.

Where was all the ozone-destroying chlorine coming from?

It was originating from commonly known applications like refrigerators, air conditioners, fire extinguishers, aerosol sprays, and inhalers used for asthma — broadly, refrigeration and propellant devices and processes.

More specifically, the compounds that would release chlorine, which ultimately destroys ozone in the stratosphere upon interaction with UV light, were chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), carbon tetrachloride, and methyl chloroform, among others.

What was different about these compounds from those other chlorine sources that didn’t destroy ozone was that these were lasting and stable, and gradually ascended to the stratosphere over a span of two to five years or more until their fateful meeting with UV light.

Over a long period of time, the result has been not so much an “ozone hole”, for there isn’t really an absence of ozone in the stratosphere, but rather a zone of exceptionally depleted ozone, such as in Antarctica.

"Ozone Hole" In The Antarctic

Ozone loss is at its worst at the South Pole.

Satellite sensors and other ozone-measuring devices, whether on the ground or in the air, help relay regular information about ozone concentrations over the Antarctic region. It provides the basis for monitoring changes in the ozone level.

The measurement is made and expressed in Dobson units, thanks to the use of the “Dobson spectrophotometer” — a ground-based instrument that measures changes in atmospheric ozone.

Explaining a Dobson unit, the American space agency’s NASA Ozone Watch says, “a column of air with an ozone concentration of 1 Dobson Unit would contain about 2.69x10¹⁶ ozone molecules for every square centimeter of area at the base of the column.”

The average thickness of the ozone layer is about 300 Dobson units or a layer that is 3 millimetres thick.

Historical data shows that 1979 was a point of deviation from normal ozone layer size. It was after this year that the average amount of ozone over the Antarctic fell unprecedentedly below 220 Dobson units.

This specific measure, therefore, came to mark the boundary of the region representing ozone loss. Less than 220 Dobson units and we're talking catalysed ozone loss from chlorine and bromine compounds.

In Antarctica, unusual atmospheric conditions from winter to spring, involving polar darkness, extremely cold weather, and the polar vortex, cause chemical reactions around chlorine and bromine atoms that lead to severe ozone depletion.

Conditions ease up after spring, leading to the stabilisation of the ozone layer. This has become an annual affair.

The study of springtime losses of ozone over Antarctica was first published in 1985 in the journal Nature. Over the next few years, further studies advanced our scientific understanding of ozone depletion in the region.

It came to light that from 1979 to 1989, more than half of the ozone layer over Antarctica was destroyed.

The World Got To Work

Evidence was right before the world‘s eyes during the 1980s. Satellite and aircraft measurements had confirmed the link between CFCs, chlorine, and ozone loss.

If nothing was done, “we’d be in dead trouble” with a “blind, burned population,” as was described in the Sir David Attenborough-narrated short documentary, The Hole.

Governments were refreshingly quick to act against the causes of ozone depletion despite some industry resistance early on. It was agreed that ozone-depleting substances emitted by human activities had to be phased out.

As a first response, the Vienna Convention for the Protection of the Ozone Layer was initiated in 1985. It took effect three years later and reached universal ratification in 2009.

Notably, it became the first convention of any kind to be signed by every country involved.

While the Vienna Convention was more in the realm of creating and sharing scientific knowledge, what followed next demanded specific global action to protect the ozone layer.

The landmark global agreement Montreal Protocol on Substances that Deplete the Ozone Layer was signed on 16 September 1987 and entered into force two years later.

In the years that followed, CFCs were painstakingly phased out in favour of HCFCs and then HFCs.

However, recognising the danger of HFCs to world climate, a 2016 update to the Montreal Protocol, known as the Kigali Amendment, called for a switch to more environmentally friendly alternatives. The amendment came into effect in 2019.

Thus far, 98.6 per cent of the ozone-depleting substances that are controlled under the Montreal Protocol have been phased out. The remaining 1.4 per cent is said to be mainly HCFCs.

Without the Montreal Protocol, humans were staring down the prospect of severe ozone holes occurring every year over the Arctic and Antarctic until it spread all across the planet, including the tropics, by the mid-twenty-first century.

The situation is looking better today. While total ozone declined over most of the globe during the 1980s and early 1990s, with a recorded minimum in 1993, not helped by the eruption of Mount Pinatubo, total ozone has increased in the range of 0.3 to 1.2 per cent per decade since 1997, according to the United Nations Environment Programme.

In the mid-latitudes in particular, upper stratospheric ozone has increased by 1 to 3 per cent per decade since 2000.

According to a recent study, “In early 2022, the overall concentration of ozone-depleting substances in the mid-latitude stratosphere had fallen just over 50 percent back to levels observed in 1980, before ozone depletion was significant.”

Finally, ozone is set to return to 1980 values by the 2060s. The estimated recovery date may seem well ahead of us, but that’s because of the long lifespan of ozone-depleting molecules in the atmosphere. They don’t go down without a fight.

This is why the world must keep fighting, too, by reviewing consumption regularly and choosing better alternatives to ozone-depleting substances.

Climate Change

Cutting down ozone-depleting substances has thankfully had a positive impact on the climate as well. This is because these harmful substances make for potent greenhouse gases that only further global warming.

“Some of them have a global warming effect up to 14000 times stronger than carbon dioxide (CO2), the main greenhouse gas,” the Directorate-General for Climate Action of the European Commission states.

"Replacing them (CFCs) has slowed climate change by at least a decade. Had we not acted, the world could have suffered this year’s extreme weather 10 years ago," David Doniger of the Natural Resources Defense Council writes.

A study published in August last year has estimated that if not for the Montreal Protocol, an additional warming of global-mean surface temperature by 0.5 to 1 degrees Celsius was on the cards.

Additionally, the Kigali Amendment, by further reducing the use of ozone-damaging substances, will avoid around 0.4 degrees Celsius of global heating over this century.

It’s safe to say that ozone protection efforts are helping mitigate climate change. Perhaps, ongoing global efforts to combat the climate crisis would do well to take inspiration from this most successful, yet still surprisingly underrated, environmental endeavour.