Arctic ozone reaches record level, a positive measure for the climate

Ozone holes over polar regions, where stratospheric ozone levels are significantly reduced, have been a recurring feature of climate change news in recent decades. Anthropogenic chlorofluorocarbons (CFCs) are the main cause, released from household items such as coolants from refrigerators, air conditioners and aerosol cans. Restricting their use has been and remains paramount to recovery from the ozone hole, as their lifespan in the atmosphere extends over several decades.

Policies to combat 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 fill these holes of ozone by 2045 and 2066 in the Arctic and Antarctic respectively. Thus, since the early 2000s, levels of stratospheric ozone-depleting inorganic chlorine and bromine in the Arctic have declined, albeit rather slowly.

Amid these gloomy predictions, a study published in Geophysical research letters hinted at a better future to come. Dr. Paul Newman, chief scientist of Earth Scientists at NASA’s Goddard Space Flight Center, and colleagues have identified March 2024 as a record month for Arctic ozone since the 1970s, following a period of global increase from winter 2023 to 2024.

Above-average ozone levels continued to persist through September 2024. This is significant because previously spring was associated with ozone depletion, when high levels of CFCs coincided with large, cold, rotating low-pressure weather systems known as polar vortices.

The research team highlights the importance of this research as preliminary evidence that CFC levels are now falling, to allow the ozone layer to begin its long recovery. Dr Newman said: “Ozone is Earth’s natural sunscreen. The increase in ozone is a positive story because it is good for the environment and encouraging news that the global agreement of the Montreal Protocol is producing positive results.

To explore this change, Dr. Newman and his colleagues studied satellite meteorological and ultraviolet backscatter data to observe total column ozone (the total amount of ozone from the 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), 6 DU higher than the previous record from March 1979 and 60 DU higher than the average for the study period (1979 to 2023) . Daily record highs for the Arctic were recorded for about half the month, with March 20 recording a high of 499 DU.

They found that the lowest part of the stratosphere (10 to 20 km above Earth’s surface) experienced record temperatures for 23 days of the month, coinciding with these elevated ozone levels due to weather systems. warmer rising from the underlying troposphere towards 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 temperatures and ozone levels in March 2024 is attributed to increased winter eddy heat fluxes from atmospheric Rossby waves. These waves travel through the stratosphere and cause downward motion in the polar regions, resulting in warmer polar temperatures.

The waves also slow the stratospheric nocturnal polar jet stream (polar vortex) around the Arctic, leading to the convergence of mid-latitude air toward the pole, carrying more ozone than normal to the region.

“Arctic ozone is controlled by direct depletion of the ozone layer by chlorine and bromine compounds and by ozone transport,” explains Dr Newman.

“In the first scenario, temperatures were too warm to cause significant depletion. In the second case, waves propagating into the stratosphere from the troposphere move ozone toward the Arctic, warming the polar region and slowing the vortex Polar El Niño events and Siberian snow cover have been studied as a process controlling ozone transport, but do not appear to have a major impact.

“The stronger-than-normal transport appears to be caused by a random weather year with significant Rossby wave propagation in the stratosphere. It is likely that declining levels of oxygen-depleting substances and increasing levels of carbon dioxide of carbon have contributed to further raising Arctic ozone to a record level.

With carbon dioxide levels expected to rise further in the coming years, Dr Newman says it is “very likely” that more record-breaking ozone events will continue to occur.

“Climate change is thought to impact the strength and stability of the stratospheric polar vortex. For example, changes in surface temperature and pressure resulting from sea ice loss may increase the generation of Rossby waves , resulting in a weaker and unstable polar vortex.

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

While the Arctic and Antarctic respond broadly 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). Therefore, the Arctic is warmer, has a more vortex “Because Antarctica has a very cold vortex climate that can contain reactive chlorine, we have deep holes in the ozone layer every year,” says Dr. Newman.

Using this knowledge, coupled chemistry and climate models predict a 10 to 30 DU increase in Arctic ozone between 2000 and 2025, resulting from a reduction in ozone-depleting substances in the atmosphere and high levels of greenhouse gases.

Additionally, by calculating an ultraviolet (UV) index for clear skies at midday based on this data, 2024 Arctic ozone levels further filter out UV, resulting in a 5% reduction in the index UV compared to the average over the study period from 1979 to 2023. .

Ultimately, ozone harvesting is paramount to protecting life on Earth, otherwise increased UV radiation from space can have many consequences, ranging from reduced plant growth (affecting “lungs of the Earth” and agricultural food supply) to the disruption of marine food chains. by affecting growth or primary producers, to an increased incidence of skin cancer and immunodeficiency disorders in humans.

There is now hope that one day these holes in the ozone layer will disappear.

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