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A recent study led by MIT researchers has provided compelling evidence that the Antarctic ozone layer is undergoing a significant recovery, thanks to global initiatives aimed at reducing substances that deplete ozone.
The research team, including experts from MIT, has previously noted signs of ozone recovery, but this marks the first study that statistically verifies the primary contribution of reduced ozone-depleting substances as the driving factor behind such recovery, instead of attributing it to natural weather patterns or the effects of increased greenhouse gas emissions in the stratosphere.
“There has been substantial qualitative evidence suggesting that the Antarctic ozone hole is improving, but this study is the first to offer a precise quantification regarding its recovery,” states Susan Solomon, study co-author and the Lee and Geraldine Martin Professor of Environmental Studies and Chemistry. “With a 95 percent confidence level, we can assert that it is indeed recovering, which is fantastic. This illustrates our capacity to address environmental challenges effectively.”
The findings from this study will be published in the journal Nature, with graduate student Peidong Wang from Solomon’s group in the Department of Earth, Atmospheric and Planetary Sciences (EAPS) serving as the lead author. Co-authors include Solomon and EAPS research scientist Kane Stone, along with collaborators from various institutions.
Factors Contributing to Ozone Recovery
Ozone, a naturally occurring component of the Earth’s stratosphere, acts as a protective barrier by absorbing harmful ultraviolet (UV) radiation from the sun. The discovery of an “ozone hole” over Antarctica in 1985 was alarming, as it was most pronounced during the austral spring months of September through December. This significant depletion allowed more UV rays to permeate the atmosphere, resulting in increased risks of skin cancer and other health issues.
In 1986, Solomon, then affiliated with the National Oceanic and Atmospheric Administration (NOAA), led research expeditions to Antarctica that confirmed the cause of the ozone hole: chlorofluorocarbons (CFCs), substances widely used in refrigeration, air conditioning, and aerosol products. These compounds, when released into the stratosphere, can lead to ozone depletion under specific seasonal conditions.
In the following year, these research findings contributed to the establishment of the Montreal Protocol, an international agreement focused on phasing out the production of CFCs and other ozone-depleting substances to facilitate the healing of the ozone layer.
In 2016, Solomon and her team published results indicating early signs of ozone recovery, particularly noting a reduction in the size of the ozone hole during September. However, previous observations remained largely qualitative, with uncertainties around the specific contributions of reduced ozone-depleting substances versus other environmental factors like El Niño and La Niña events.
“Identifying a statistically significant increase in ozone levels is straightforward, but attributing these changes to specific causes is considerably more complex,” Wang explains.
Human-Caused Recovery
The latest study employs a quantitative approach to determine the factors behind the Antarctic ozone recovery. The researchers adapted a methodology known as “fingerprinting,” a technique from climate science that isolates the impacts of specific anthropogenic factors amidst natural meteorological variability. This approach was pioneered by Klaus Hasselmann, who received the Nobel Prize in Physics in 2021 for his work.
Solomon and Wang utilized fingerprinting to discern the human-caused signal of lowered ozone-depleting substances on the recovery process of the ozone layer.
“The atmosphere is characterized by substantial chaotic variability,” Solomon observes. “Our task is to identify the emerging signal of ozone recovery amidst this variability, which is also evident in the stratosphere.”
The researchers synthesized atmospheric simulations, creating “parallel worlds” representing different conditions of the global atmosphere. They conducted simulations under scenarios without increases in greenhouse gases or ozone-depleting substances, where any changes in ozone concentrations would stem purely from natural variability. Other simulations considered increasing greenhouse gases independently, and some focused solely on declining ozone-depleting substances.
By comparing these simulations, the team tracked how Antarctic ozone levels shifted seasonally and across varying altitudes over several decades. This analysis allowed them to isolate a distinct “fingerprint” indicative of ozone recovery linked specifically to the reduction of ozone-depleting substances.
Upon applying this fingerprint to observational data from satellites monitoring the Antarctic ozone hole from 2005 to the present, the researchers noted that the identified pattern grew progressively clearer over time. In 2018, the fingerprint reached its most pronounced state, enabling the team to assert with 95 percent confidence that the recovery was predominantly due to human efforts to decrease ozone-depleting substances.
“After 15 years of compiling observational records, we have observed our signal amidst background noise with 95 percent confidence. This suggests a negligible likelihood that the visible pattern can be attributed to natural variability,” Wang adds. “This encouraging finding reinforces our belief that environmental solutions are achievable. Lessons from this research demonstrate how swiftly countries can collaborate to implement treaties aimed at reducing emissions.”
Should the current trends continue, Solomon predicts that it may soon become routine to encounter years when the ozone layer remains fully intact. Ultimately, this could lead to the permanent closure of the ozone hole.
“By around 2035, we could witness a year without any ozone hole depletion over Antarctica. That would be incredibly exciting for me,” she remarks. “And many of you may live to see the complete disappearance of the ozone hole—an achievement made possible by collective human action.”
This research received partial funding from the National Science Foundation and NASA.
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