Remember the hole in the ozone layer? It still opens every spring and this year’s hole was a doozy. But overall things are getting better.

New data suggests that despite the reduction of ozone depleting substances, this year’s hole was large. However, things are getting better. Image: sandwich/Flickr, CC BY-NC-ND

We’re guessing you’ve heard about the hole in Earth’s ozone layer. But you might be thinking, “didn’t we fix that?”

Thanks to a not-so-little thing called the Montreal Protocol, we’ve come a long way over the last three decades towards phasing out the production and consumption of nearly 100 ozone depleting substances (ODSs). But as data on the 2020 ozone hole shows, the problem is not ‘fixed’ just yet.

Ozone depleting substances: what, why, when and for how long?

As their name suggests, the release of ozone depleting substances into our atmosphere reduces the amount of ozone in Earth’s stratosphere.

This is a problem because the ozone layer acts like a sunscreen for Earth and all its inhabitants (i.e. you). Without an ozone layer, more ultra-violet (UV) radiation from the sun passes through our atmosphere. As sun-loving Australians, we’re all too familiar with the damage UV can cause.

During the 1980s, scientists discovered that key ODSs known as chlorofluorocarbons (CFCs) caused significant damage to the ozone layer. CFCs were used in aerosol spray cans, as blowing agents for foams and packing materials, as solvents, and as refrigerants. The damage was so severe that by the 1990s, the concentration of ozone over Antarctica was dropping to less than half the minimum level observed prior to 1979. This resulted in the so-called ‘hole in the ozone layer’.

Enter the Montreal Protocol. This landmark international agreement saw the world’s countries sign on to phase out the use of CFCs. For the most part, this has been a success. However, ODSs are so long lasting we still feel their effects now and will so for decades to come.

Fun fact: The Montreal Protocol is, to date, the only United Nations (UN) treaty that has been ratified by all UN Member States. It is also the most successful global protocol to mitigate climate change, as ODSs are also very powerful greenhouse gases.

Holy ozone, Batman!

Since 1979, scientists have been tracking the size and depth of the ozone hole. It’s important to point out that the ‘hole’ isn’t literally a hole (i.e. an area totally devoid of ozone). Scientists use the term hole to describe the area in which the concentration of ozone drops below the historical threshold of 220 Dobson Units (DU). If you want to learn more, our friends at NASA have a great explanation.

Here in Australia, we compile weekly Antarctic Ozone Hole reports. It begins when the Antarctic hole forms (usually in early August) and continues until it recovers (usually by mid-December). Our scientists draw on several sources of information to develop these reports. This includes long-term satellite ozone data and meteorological data from NASA, Netherlands’s Agency for Aerospace Programs (NIVR), the Finnish Meteorological Institute (FMI), and the Royal Netherlands Meteorological Institute (KNMI). However, we also use our own measurements of ODSs from the Cape Grim observatory located on the north-west tip of Tasmania.

Graph showing the steep rise in the size of the ozone hole from early August through to early September and then the steady decline through to December.

Ozone hole area based on OMPS (data up to 19 December 2020) and OMI (data up to 19 December 2020) satellite data. The black line shows the area of the 2020 ozone hole in comparison to the previous five years.

The ozone hole in 2020

Although this year’s hole fell short of ‘unprecedented’, it did a decent job of living up to our expectations for 2020!

Ozone levels dropped early this year, briefly dipping below the 220 DU threshold on 22 to 26 July. Levels have sat below 220 DU since 7 August, with the ozone hole completely enclosing the polar night late August and early September.

The first week of September saw the daily ozone hole area increase rapidly by about 10 million km2, reaching a peak of 24.7 million km2 on 20 September. To put this in perspective, that’s an area a bit over three times the size of Australia. This was bigger than the ozone hole from the previous five years for this time of year. It’s also the 18th largest ozone hole out of 41 years of satellite data.

As usual in December, the hole is now well and truly in recession, measuring at 7.5 million km2 on 19 December. But it went out with a bang. It racked up the 2nd largest integrated ozone deficit for a single year on record. This means the amount of ozone lost during the 2020 ozone hole season was the second highest amount on record.

Graphic showing the size and location of the ozone hole at its maximum area and minimum ozone column amount in 2020.

The 2020 Antarctic ozone hole on the date of its largest areal extent (left) and lowest (minimum) ozone level (right). The red line shows the 220 DU contour (the edge of the hole).

Why the big hole?

So, if we’ve all signed up to the Montreal Protocol, why was there such a sizable ozone hole this year?

As mentioned above, CFCs are particularly long-lived chemicals in the atmosphere. So we’ll feel their effects for at least a few more decades. But there are a number of additional factors that contribute to the size and depth of the ozone hole in any one year. These include the temperature in the stratosphere and the size and strength of the circumpolar winds (also known as the polar vortex).

This year, the Antarctic region has seen a very persistent, relatively large and strong polar vortex. This resulted in record low stratospheric temperatures in November and December this year. As a result, this year’s ozone hole, while modest in its overall area, has seen a very persistent ozone hole. There were record levels of ozone depletion in November and December. Additionally, the recovery date for the 2020 ozone hole will be one of the latest on record, so far lasting well into December.

Buildings surrounded by snow and ice.

Long-term monitoring of ozone levels above Australia’s Davis research station (pictured) has been occurring since 2003. Credit: Greg Stone, Australian Antarctic Division.

And finally, the good news.

Encouragingly, our measurements and research show that the levels of ODSs in the atmosphere are decreasing. They’ve fallen by approximately 12 per cent in the stratosphere since their peak values, which our measurements estimate occurred around the year 2000. In line with this, we’re also seeing an overall recovery trend in the Antarctic ozone hole. However, this is a slow process. Researchers don’t expect a full recovery until around 2060-2070.

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