All life on Earth depends on the extraordinarily thin layer of livable atmosphere which envelopes the biosphere in our Earth system. But above the atmosphere in the high atmosphere, roughly ten to fifty kilometers above ground, we have the stratospheric ozone layer.

And the stratospheric ozone layer is a protective shield that enables life on Earth by reflecting back harmful ultraviolet radiation from the Sun. So clearly the ozone layer is a planetary boundary enabling human prosperity and development on Earth.

The stratospheric ozone layer has for a long time been understood as being absolutely essential for living conditions on Earth. And in the early ’80s scientists started to observe something absolutely extraordinary namely a rapid, abrupt drop in the thickness of the ozone layer.

This was a huge surprise, in fact scientists even thought it was an error in the scientific observations. But through fantastic research by top scientists in the interface between atmospheric research and chemistry soon it was proven that the reason for this depletion was that certain chemicals that we used as refrigerants, as solvents, propellants, the whole family of chlorofluorocarbons were moving up the atmosphere through high winds and reacting with ozone and – and breaking these molecules apart and thereby depleting the stratospheric ozone layer, threatening life on Earth, and particularly health for humans, by risks of rising skin cancer, cataracts and damage also on vegetation, food production systems on Earth.

This led in the mid-1980s to the extraordinary step where the world gathered around a protocol, the famous Montreal Protocol, to ban chlorofluorocarbons from use in refrigerators. And this in turn has led to a success story where a boundary of ozone depletion was transgressed in the early ’90s and now we’re actually moving into a safe operating space, showing that humanity in fact can collectively as all nations on Earth work together to operate within a safe operating space.

So we are moving in the right direction on ozone, but what is very important to recognize is that we’re still observing an ozone hole, particularly over the polar regions, and the classic ozone hole is in Antarctica, which is due to the combination of ozone depleting substances, continued emissions of chemicals, but also the fact that the sins of the 1980s are still haunting us because of the delay time in much of these chemical reactions, which is also a reminder that, uh, we need to apply the core thinking of planetary boundary theory which is a precautionary principle, because what we do today, which we sometimes do not even understand, can have a harmful effect on the Earth system, can actually come back and hit us many, many decades later.

I’ll give you a small example that comes from the Nobel Laureate Paul Crutzen, who was one of the three scientists observing the depletion of the ozone layer in the early 1980s. The industry at the time had a choice of two molecules to develop the refrigerating chemicals that were used worldwide, either chlorine or bromine. And just so happened by pure coincidence that the industry chose chlorine. That was very lucky for humanity because it just so happens that chlorine has several magnitudes lower harmful effect on ozone. If the industry in the early ’80s instead has chosen, or rather in the early ’60s all the way up to the ’80s when we banned the chemicals, had chosen bromine as the carrier of refrigerating systems across the world we most likely would have had a catastrophic tipping point that would have undermined human development on Earth.

So that’s an example of how close we were of what we can call a planetary scale disaster, and why thinking in terms of defining planetary boundaries is so essential. Science has come to a point where we are at a position where we can define a control variable, which we have chosen as the thickness of the column of ozone across the planet.

And this gives us a very good, robust, science-based definition of how much we must maintain in terms of ozone, and thereby also translating that to avoiding chemicals that can destroy the ozone layer.

Is the problem finally resolved? Well the answer is unfortunately no. The most damaging chemicals used in the early ’80s are not on the market any longer, but we’re using other types of refrigerants, and methane is a compound that also poses a threat to the stratospheric ozone layer, and we see other emerging novel entities that could actually threaten the ozone layer, reminding us that planetary boundary processes do interact, and one very strategic way of protecting the ozone layer is also to have a strong boundary on chemical pollution.