The Ozone Layer

The ozone layer is a thin region in the upper atmosphere (around 20 to 30 km up, in the stratosphere) that contains higher amounts of ozone gas (O3). Ozone absorbs almost all the dangerous ultraviolet (UV) radiation from the Sun before it can reach the ground. Without the ozone layer, life on land would be impossible: UV rays would damage DNA, cause skin cancer at a much higher rate, and damage most surface ecosystems. The ozone layer is one of the most important things keeping the planet liveable, and one of the great environmental success stories has been the global effort to save it from chemical damage.

  • Where it is20 to 30 km upIn the stratosphere
  • What blocksUV-B and UV-CMost dangerous parts of UV
  • Ozone (O3) structureThree oxygen atomsCompared to normal O2 (two atoms)
  • Discovered ozone hole1985British Antarctic Survey
  • Treaty banning CFCsMontreal ProtocolSigned 1987
  • Expected ozone recoveryAround 2070Hole shrinking already

What is ozone?

Ordinary oxygen gas (what we breathe) is made of two oxygen atoms joined together (O2). Ozone is made of three oxygen atoms joined together (O3). It is unstable: ozone molecules constantly break apart and re-form. In the upper atmosphere, this constant cycle is exactly what blocks UV radiation. Each time a UV photon hits an ozone molecule, the energy is absorbed and the molecule splits apart, then quickly recombines. The UV never reaches the ground.

Good ozone vs bad ozone

Ozone is good or bad depending on where it is.

  • "Good" ozone in the stratosphere (20 to 30 km up): blocks dangerous UV, protects life on the ground.
  • "Bad" ozone near the ground: a pollutant that irritates lungs, damages crops and is part of smog. Mostly produced by reactions between vehicle exhaust and sunlight in cities.

The same molecule, in totally different positions, gives completely opposite effects. We want lots of ozone high up and very little down here.

The ozone hole

In 1985, scientists at the British Antarctic Survey announced that they had detected a big hole in the ozone layer over Antarctica. Every spring, ozone over the South Pole was dropping by more than 50% compared to historical levels. The discovery was a shock: many people had not known we could damage the atmosphere on a planetary scale.

The cause turned out to be chlorofluorocarbons (CFCs): man-made chemicals that were widely used as refrigerants, aerosol propellants, and in foam plastics. CFCs are extremely stable on the ground (which is why they were useful), but they slowly drift up to the stratosphere over years or decades. There they are broken apart by UV light, releasing chlorine atoms. A single chlorine atom can destroy thousands of ozone molecules before being neutralised. Decades of CFC release had built up enough chlorine in the stratosphere to chew a hole in the ozone layer.

How the world fixed it

The world reacted faster than anyone expected. Just two years after the ozone hole was confirmed, in 1987, almost every country in the world signed the Montreal Protocol: an international treaty to phase out CFCs and other ozone-damaging chemicals. The agreement has been ratified by every country in the United Nations, the only treaty in UN history to achieve this.

By the late 1990s, CFC production had essentially ended. Existing CFCs continue to leak out of old equipment and slowly drift up to the stratosphere, but levels are dropping. The ozone hole is slowly shrinking. Scientists now expect the ozone layer to be fully recovered by around 2070. The Montreal Protocol is considered one of the most successful environmental treaties ever signed.

Fact The Montreal Protocol that fixed the ozone problem also accidentally became one of the most effective climate treaties ever. CFCs are also powerful greenhouse gases (often thousands of times more potent than CO2 per molecule). Phasing them out has prevented an estimated 0.5 °C of additional global warming. So saving the ozone layer also slowed climate change as a side effect.

What UV does to living things

UV light is dangerous because it has enough energy to damage DNA. Different parts of UV are more or less dangerous.

  • UV-A (long wavelength): mostly reaches the surface. Causes tanning, ageing of skin and some skin cancer.
  • UV-B (medium wavelength): mostly blocked by the ozone layer, but a small amount gets through. Causes sunburn and most skin cancer.
  • UV-C (short wavelength): the most dangerous, completely blocked by the ozone layer. Used in UV sterilisers to kill germs.

Without the ozone layer, all three would reach the surface unfiltered. Skin cancer rates would skyrocket. Crops would be damaged. Microbes in the soil and surface waters would die. Life on land (and in shallow water) would be very different and probably much less abundant.

Did you know? The total amount of ozone in the atmosphere is surprisingly small. If you collected all the ozone in the stratosphere and brought it down to sea-level pressure, it would form a layer only about 3 mm thick across the whole planet. The "ozone layer" is mostly empty space with just enough ozone scattered through it to do its vital job.
Deeper dive: how scientists worked out the ozone hole was real

The ozone hole was almost missed by science. NASA had a satellite called Nimbus 7 that had been measuring ozone over Antarctica since 1978. The data showed a dramatic drop in ozone in the early 1980s, but the satellite's software had been programmed to flag any readings below a certain threshold as "errors" and throw them out. The team thought the satellite was broken.

The real discovery came from a small team of British scientists taking old-fashioned ground-based measurements with the Halley Research Station in Antarctica. They had a vintage instrument called a Dobson spectrophotometer that they used to measure ozone every spring. When they noticed levels dropping year after year, they were so surprised they ordered a new instrument from the UK, thinking the old one was faulty. The new one showed the same drop. After three years of careful measurements, they published their results in Nature in May 1985.

NASA quickly went back and checked the satellite data. Sure enough, when they re-included the "error" readings, the satellite had seen the hole all along: just below the threshold the programmers had set. The lesson was painful: never automatically reject data just because it does not match what you expect.

The story is a great example of how rigorous, patient ground-based science can sometimes spot things that high-tech systems miss. The British team's slow careful work led directly to the Montreal Protocol and saved the ozone layer.

For more, see layers of the atmosphere and the greenhouse effect.