Climate Change makes Arctic Ozone Loss Worse

On 12 October 2020, when Polarstern returned to Bremerhaven from MOSAiC – the largest Arctic expedition in history – she brought with her troubling news: broad expanses of open water to the north of Greenland, and sea ice riddled with meltwater pools at the North Pole, bore witness to the dramatic effects of global warming that could already be seen in the Arctic. But the rapid melting wasn’t the only thing that worried Expedition Leader Markus Rex: “Readings taken during the expedition showed that the chemical loss of ozone over the Arctic in the spring of 2020 was worse than ever before. A comprehensive analysis has now determined that this was also due to climate change. Unfortunately, our work indicates that, if climate change continues unchecked, loss of ozone over the Arctic will likely continue to worsen through the end of the 21^st century, despite the global ban on ozone-destroying substances.” During MOSAiC, at the altitude range of maximum ozone concentration, approx. 95% of the ozone had been destroyed. As a result, the thickness of the ozone layer was reduced by more than half, even though the concentration of ozone-destroying substances has declined since the turn of the millennium – a success of the international efforts to protect the ozone layer.

The complex underlying mechanism is at least partly understood: the same gases that result in global warming at the Earth’s surface (like CO₂) also promote cooling of higher atmospheric layers in the stratosphere, where the ozone layer is located. “In addition, due to climate change we notice changes in the prevailing wind systems. We suppose that these changes also contribute to lower temperatures in the polar vortex. Up to now, the trend towards colder winters in the Arctic stratosphere was debated among researchers. Even when assuming such a trend it was believed that climate change would, in the worst case, prolong the recurring ozone depletion over the Arctic by a few years,” Peter von der Gathen explains. “By then at the latest – that’s what we assumed, too – the falling concentrations of CFCs would become such a dominating factor that the ozone loss would continually fade. But, according to our new calculations, things could also turn out very differently in the Arctic.”

The team’s analysis shows that the future chemical loss of ozone in the Arctic will greatly depend on the amount of greenhouse gases emitted by the end of the century. If emissions are substantially reduced in the coming decades, the study actually predicts an early and then steady decline in the degree of ozone loss; but for less optimistic scenarios characterised by rising greenhouse-gas emissions, the opposite could happen. “If we don’t reduce our greenhouse-gas emissions rapidly and decisively, loss of ozone in the Arctic stratosphere could, despite the great success of the Montreal Protocol, continue to worsen through the end of this century, instead of following the generally expected path towards a full recovery of the ozone layer” explains Markus Rex. “This represents a fundamental paradigm shift in our assessment of the future of the Arctic ozone layer. And the future of the Arctic ozone layer is relevant for life in Europe, North America and Asia: the Arctic polar vortex occasionally drifts south, which can lead to a few days with a reduced ozone layer over populated areas in spring, increasing levels of UV radiation and ultimately a greater risk of sunburn and acquiring skin cancer during these periods.”

For Markus Rex, the message from the Arctic is clear: “There are plenty of good reasons to quickly and comprehensively reduce greenhouse-gas emissions. Now we can add the risk of intensified ozone depletion over the Arctic to the list.”