A recent study spearheaded by MIT verifies that the Antarctic ozone layer is recuperating, thanks to international initiatives aimed at minimizing ozone-depleting substances.
Researchers, including the MIT group, have noted indications of ozone restoration previously. However, this new research represents the first instance of demonstrating, with substantial statistical assurance, that this restoration is primarily attributed to the decrease of ozone-depleting substances rather than other factors like natural weather fluctuations or rising greenhouse gas emissions in the stratosphere.
“There’s been ample qualitative proof indicating that the Antarctic ozone gap is improving. This is genuinely the first research that has quantified trust in the recovery of the ozone gap,” states study contributor Susan Solomon, the Lee and Geraldine Martin Professor of Environmental Studies and Chemistry. “The conclusion is, with 95 percent confidence, it is recovering. Which is fantastic. It demonstrates that we can indeed tackle environmental challenges.”
The new research is published today in the journal Nature. Graduate student Peidong Wang from the Solomon team in the Department of Earth, Atmospheric and Planetary Sciences (EAPS) serves as the lead author. His co-authors include Solomon and EAPS Research Scientist Kane Stone, along with partners from several other institutions.
Origins of ozone recovery
Ozone, a gas naturally present in the Earth’s stratosphere, functions like a sunscreen, shielding the planet from harmful ultraviolet rays from the sun. In 1985, researchers detected a “gap” in the ozone layer over Antarctica that appeared during the austral spring, between September and December. This seasonal depletion of ozone was unexpectedly permitting UV rays to reach the surface, resulting in skin cancer and other negative health consequences.
In 1986, Solomon, then affiliated with the National Oceanic and Atmospheric Administration (NOAA), spearheaded expeditions to Antarctica, where she and her colleagues collected evidence that swiftly validated the cause of the ozone gap: chlorofluorocarbons, or CFCs — chemicals that were in use for refrigeration, air conditioning, insulation, and aerosol propellants at the time. When CFCs ascend into the stratosphere, they can decompose ozone under specific seasonal circumstances.
The subsequent year, these insights led to the formulation of the Montreal Protocol — an international agreement aimed at phasing out the manufacture of CFCs and other substances that deplete ozone, in hopes of mending the ozone hole.
In 2016, Solomon conducted a study reporting significant indications of ozone recovery. The ozone gap appeared to be diminishing with each passing year, particularly in September, the period of its opening. Nevertheless, these findings were qualitative. The research revealed considerable uncertainties regarding the extent to which this recovery was linked to concerted efforts to decrease ozone-depleting substances or if the shrinking ozone gap was a consequence of other “forcings,” such as year-to-year climatic variability from El Niño, La Niña, and the polar vortex.
“While identifying a statistically significant rise in ozone is comparatively straightforward, associating these alterations to specific causes is more intricate,” explains Wang.
Human-induced recovery
In their latest study, the MIT researchers employed a quantitative methodology to pinpoint the cause of Antarctic ozone recovery. The scholars utilized a technique known as “fingerprinting,” originally developed by Klaus Hasselmann, who received the Nobel Prize in Physics in 2021 for this methodology. In climate science, fingerprinting refers to a technique that distinguishes the impact of specific climatic factors from natural meteorological noise. Hasselmann used fingerprinting to recognize, validate, and calculate the human influence of climate change.
Solomon and Wang aimed to apply the fingerprinting technique to detect another human-induced signal: the impact of human reductions in ozone-depleting substances on the recuperation of the ozone hole.
The researchers initiated simulations of the Earth’s atmosphere and created multiple “parallel worlds,” or simulations of the same global atmosphere, under varying initial conditions. For example, they conducted simulations that presumed no rise in greenhouse gases or ozone-depleting substances. Under such conditions, any alterations in ozone should result from natural climatic variability. They also executed simulations featuring solely increasing greenhouse gases, as well as those with only decreasing ozone-depleting substances.
They compared these simulations to examine how ozone in the Antarctic stratosphere varied, both seasonally and across various altitudes, in response to different initial conditions. From these simulations, they charted the times and altitudes where ozone recovered from month to month over several decades, discovering a significant “fingerprint,” or pattern, of ozone recovery specifically associated with declining ozone-depleting substances.
The team then sought this fingerprint in actual satellite data of the Antarctic ozone gap from 2005 to the present. They observed that, over time, the fingerprint identified in simulations became progressively clearer in the observations. By 2018, the fingerprint was at its most robust, allowing the team to assert with 95 percent confidence that ozone recovery was primarily attributable to reductions in ozone-depleting substances.
Should this trend persist, and the fingerprint of ozone recovery becomes increasingly pronounced, Solomon predicts that soon there will be certain years when the ozone layer remains completely intact. Eventually, the ozone hole is expected to be permanently closed.
This research received funding, in part, from the National Science Foundation and NASA.