Q&A: Montreal and the fall and rise of stratospheric ozone

What is ozone, and where is it found?

Ozone (O₃) is a naturally occurring, colourless, pungent gas composed of three oxygen atoms. It is sparsely scattered throughout the atmosphere, but the greatest concentrations lie in a thin layer (the 'ozone layer') within the stratosphere, 20–30 kilometres above the Earth's surface.

Why is ozone significant?

In the lower atmosphere, ozone is a highly-corrosive pollutant that can cause respiratory harm. The chemical ingredients to make ozone come from vehicle and factory emissions, which means local concentrations are quite variable.

In the stratosphere, however, ozone is a lifesaver. The ozone layer absorbs much of the sun's UVB ultraviolet radiation – a catalyst for skin cancer and a range of other health problems.

Without stratospheric ozone, then, little could survive on Earth. That's why scientists were alarmed to discover in 1985 that a gaping hole was appearing in the ozone layer above Antarctica each spring.

What causes depletion of the ozone layer?

During the 1970s, University of California scientists made the connection between ozone depletion and the presence of chlorinated fluorocarbons (CFCs) and halons in the stratosphere. They also recognised the serious implications of ozone depletion for UVB levels at Earth's surface.

CFCs and halons are stable, manmade compounds invented in the 1930s for use as 'safe' refrigerants, propellants, foaming agents and cleaning solvents. CFCs comprise chorine, fluorine and carbon. Halons are similar, but also contain bromine or iodine.

CFCs and halons are very stable, so are not destroyed in the lower atmosphere but, over decades, waft slowly upwards to the stratosphere, where only harsh UV radiation finally breaks them down, releasing their chlorine and/or bromine atoms. Each unleashed atom can destroy more than 100,000 ozone molecules, much faster than they are naturally created.

What is the Montreal Protocol?

The gravity of these discoveries was not lost on the wider scientific world, nor on policymakers. In 1985, 20 nations – including most major CFC producers – signed the Vienna Convention, which established a framework for negotiating international regulations on ozone-depleting substances. After the discovery of the Antarctic ozone hole later in the same year, it took just 18 months to reach a binding agreement in Montreal.

That agreement – the Montreal Protocol on Substances that Deplete the Ozone Layer – sets binding targets for phasing out, and eventually eliminating, the production and use of CFCs and other ozone-depleting substances. It was opened for signature on 16 September 1987, and came into force on 1 January 1989.

Find out more about the Montreal Protocol on Substances that Deplete the Ozone Layer

Is New Zealand playing its part?

New Zealand was present at negotiations, and an early signatory. Our obligations under the Protocol are articulated in the Ozone Layer Protection Act, 1996.

The import of all ozone-depleting substances into New Zealand is being phased out in accordance with – or in some cases ahead of – the Protocol's timetable.

Are we out of the woods yet?

Since the Montreal Protocol came into effect, concentrations of the most significant CFCs and related chlorinated hydrocarbons have either leveled off or decreased. Halon concentrations have continued to increase, as they are released from storage in equipment such as fire extinguishers. But their rate of increase has slowed. 

If all 197 signatories continue to meet their obligations under the Protocol, it is expected that stratospheric ozone (and hence surface UVB) will return to pre-1980 levels by about 2050. There are already signs of a slow restoration.

However, there is still much work to be done. CFCs have a very long lifespan in the stratosphere, so ozone-depleting chlorine levels will decrease only very slowly. Other ozone-depleting substances, such as nitrous oxide, are not covered by the Protocol, and continue to proliferate. Roughly a third of nitrous oxide emissions are man-made, with agriculture by far the largest source. Though less destructive than CFCs, they nevertheless comprise the greatest ozone-depleting emission for at least the rest of this century.

There are other influences on stratospheric ozone. Volcanic eruptions, such as Mt Pinatubo in 1991, can alter atmospheric chemistry, depleting ozone locally. In addition, greenhouse gases warm the lower atmosphere, leading to cooling of the stratosphere, increasing the probability that stratospheric clouds will form – an accelerator of ozone depletion.

Researchers from NIWA are monitoring changes to ozone and UV radiation to predict how these might change in the coming years.


Principal Scientist - Atmosphere and Climate