Earth's ozone layer holes over polar regions, where the stratospheric ozone level is significantly depleted, have been a prevalent feature of climate change news in recent decades. Anthropogenic-sourced chlorofluorocarbons (CFCs) are the primary cause, released from household items such as coolants in fridges, air conditioners and spray cans. Restricting their use has been and remains paramount to ozone hole recovery as they have multi-decadal lifetimes in the atmosphere.

Policies to address ozone depletion, such as the 1987 international agreement of the Montreal Protocol, aim to stop the production and consumption of ozone-depleting substances in order to heal these ozone holes by 2045 and 2066 over the Arctic and Antarctic respectively. As such, since early 2000, levels of stratospheric ozone-depleting inorganic chlorine and bromine in the Arctic have declined, albeit rather slowly.

Amidst this bleak forecast, research published in Geophysical Research Letters has hinted at a brighter future to come. Dr. Paul Newman, Chief Scientist for Earth Scientists at NASA's Goddard Space Flight Center, and colleagues identified March 2024 as a record high month for Arctic ozone since the 1970s, following a period of overall increase through winter 2023 to 2024.

Above-average ozone levels continued to persist through September 2024. This is significant as, previously, spring has been associated with ozone depletion when high CFC levels were coincident with large, cold, rotating low-pressure meteorological systems, known as polar vortices.

The research team highlight the significance of this research as preliminary evidence that CFC levels are now declining, in order to allow the ozone layer to begin its lengthy recovery. Dr. Newman said, "Ozone is the Earth's natural sunscreen. Increased ozone is a positive story, since it's good for the environment and encouraging news that the global Montreal Protocol agreement is producing positive results."

To explore this change, Dr. Newman and colleagues investigated meteorological and satellite backscatter ultraviolet data to observe total column ozone (the total amount of ozone from Earth's surface to the top of a particular column of the atmosphere) since 1979.

The March 2024 ozone average peaked at 477 Dobson units (DU), which is 6 DU higher than the previous record in March 1979 and 60 DU higher than the average for the study period (1979 to 2023). Daily record levels for the Arctic occurred for approximately half of the month, with March 20th seeing the maximum of 499 DU.

They found that the lowermost portion of the stratosphere (10–20 km above Earth's surface) experienced record high temperatures for 23 days of the month, coincident with these elevated ozone levels due to warmer weather systems moving up from the underlying troposphere into the stratosphere.

This contrasts with known extreme ozone depletion events in 1997, 2011 and 2020, which occurred during periods of prolonged polar vortices.

The causal mechanism for these anomalous temperature and ozone levels in March 2024 is attributed to enhanced winter eddy heat fluxes from atmospheric Rossby waves. These waves move into the stratosphere and cause a downward motion in the polar regions, leading to warmer polar temperatures.

The waves also slow the stratospheric polar night jet stream (polar vortex) around the Arctic, leading to air from the mid-latitudes converging on the pole, transporting more ozone into the region than normal.

"Arctic ozone is controlled by direct depletion of ozone by chlorine and bromine compounds and ozone transport," Dr. Newman explains.

"For the former scenario, the temperatures were too warm for much depletion. For the latter case, waves that propagate into the stratosphere from the troposphere move ozone into the Arctic, warm the polar region, and decelerate the polar vortex. El Niño events and Siberian snow cover have been examined as controlling processes for ozone transport, but do not appear to have a major impact.

"The stronger than normal transport seems to be caused by a random weather year with significant propagation of Rossby waves into the stratosphere. It is likely that the declining levels of oxygen depleting substances and rising levels of carbon dioxide helped further elevate Arctic ozone to a record level."

Given that carbon dioxide levels are still projected to increase in the years to come, Dr. Newman states that is "highly likely" that more of these record ozone events will continue to occur.

"Climate change is believed to be impacting the strength and stability of the stratospheric polar vortex. For example, changes in surface temperature and pressure that result from sea ice loss can increase generation of Rossby waves, resulting in a weaker and unstable polar vortex.

"In addition, global ozone is expected to slowly increase because of the Montreal Protocol. The combination of these two factors will create favorable conditions for higher polar ozone values."

While the Artic and Antarctic overall respond similarly to the effect of Rossby waves, those in the Arctic are much stronger and therefore, ozone levels are higher than in the southern hemisphere, which experiences more pronounced ozone holes.

"The Arctic has a much stronger source of Rossby waves propagating upward into the stratosphere (quantified by the stronger eddy heat flux). Hence, the Arctic is warmer, has a weaker vortex, and much more ozone. Because the Antarctic has a very cold vortex that can contain the reactive chlorine, we have deep ozone holes each year," Dr. Newman states.

Using this insight, coupled chemistry and climate models project a 10–30 DU increase in Arctic ozone from 2000 to 2025, resulting from reduced ozone depleting substances in the atmosphere and elevated greenhouse gas levels.

Furthermore, calculating an ultraviolet (UV) index for a clear sky at noon based upon this data, the 2024 Arctic ozone levels screen more UV, leading to a 5% reduction in the UV index compared to the average across the 1979 to 2023 study interval.

Ultimately, ozone recovery is paramount to protecting life on Earth, otherwise the increased incoming UV radiation from space can have a plethora of consequences, from reducing plant growth (affecting the 'lungs of the Earth' and agricultural food supply) and disrupting marine food chains by impacting growth or primary producers, to enhanced incidence of skin cancer and immune deficiency disorders in humans.

There is now hope that one day these ozone holes will heal.

© 2024 Science X Network