The ozone layer, a region of Earth’s stratosphere roughly 9 to 22 miles (15 to 35 kilometers) above the surface, serves as a natural shield for our planet. This atmospheric layer contains a relatively high concentration of ozone molecules (O₃), which are composed of three oxygen atoms. Its primary function is to absorb 97% to 99% of the sun’s harmful ultraviolet (UV-B) radiation. This protective absorption is fundamental for safeguarding all life, as excessive UV-B exposure can damage DNA, increase rates of skin cancer and cataracts in humans, and disrupt ecosystems by affecting plants and marine life. The ozone layer’s existence has been crucial for the development of life on land, screening out lethal levels of UV-B radiation.
Understanding Ozone Depletion
The thinning of the ozone layer, known as ozone depletion, primarily results from human-made chemicals. Chlorofluorocarbons (CFCs) and halons, once widely used in refrigerants, aerosol sprays, and fire extinguishers, are major contributors. These stable compounds, when released, slowly rise into the stratosphere.
Once in the stratosphere, intense ultraviolet radiation breaks down CFCs and halons, releasing highly reactive chlorine and bromine atoms. A single chlorine atom can destroy over 100,000 ozone molecules in a chain reaction before being removed from the stratosphere. This process leads to ozone molecules breaking apart faster than they can naturally reform.
The most dramatic manifestation of this depletion was the discovery of the “ozone hole” over Antarctica. In 1985, scientists from the British Antarctic Survey published observations of unusually low ozone levels over Antarctic stations. Satellite data confirmed this thinning covered the entire Antarctic continent.
A Global Success Story
The international community responded swiftly to the discovery of ozone depletion. The Montreal Protocol on Substances that Deplete the Ozone Layer, signed on September 16, 1987, was a landmark agreement. This treaty, which entered into force on January 1, 1989, aimed to protect the ozone layer by phasing out the production and consumption of ozone-depleting substances (ODS) such as CFCs and halons.
The Protocol set binding obligations and timetables for countries to reduce and eventually eliminate ODS, including specific phase-out schedules for developed and developing nations. It also established a financial mechanism to assist developing countries in complying with these control measures.
The Montreal Protocol has been amended multiple times to accelerate ODS phase-outs and include new chemicals. Its success is evident in its near-universal ratification by all countries worldwide, making it a model for international cooperation. This collaborative effort has spurred global investment in alternative technologies and placed the ozone layer on a path toward repair.
The Current State of Recovery
Scientific evidence shows the ozone layer is recovering. Atmospheric concentrations of most controlled ozone-depleting substances have declined significantly over the past two decades. Observations indicate that upper stratospheric ozone outside the polar regions has increased by 1-3% per decade since 2000.
The Antarctic ozone hole, while still occurring annually, is also showing signs of recovery. The 2024 ozone hole was among the smallest recorded since recovery began in 1992. Projections indicate that the ozone layer is expected to return to 1980 levels, a common baseline before significant depletion, around 2040 for most regions.
The recovery of the Southern Hemisphere mid-latitude ozone is projected to occur around mid-century, while the Antarctic ozone hole is expected to fully close, with springtime total column ozone returning to 1980 values, in the 2060s. Continued monitoring by organizations like NASA and NOAA, using satellite data and ground-based instruments, is important to track this ongoing recovery.
Influences on Ongoing Recovery
While the Montreal Protocol has been effective, other factors can influence the ongoing recovery. Climate change, driven by increasing greenhouse gas concentrations such as carbon dioxide, methane, and nitrous oxide, plays a complex role. These gases can cool the stratosphere, which can impact ozone chemistry; lower stratospheric temperatures, for example, can extend the presence of polar stratospheric clouds, potentially increasing winter polar ozone depletion.
Natural events like large wildfires also pose a challenge. Smoke particles from intense wildfires can reach the stratosphere, providing surfaces for chemical reactions that release ozone-depleting substances like chlorine and bromine. The 2020 Australian bushfires, for instance, led to a measurable increase in stratospheric chlorine levels.
Emissions of very short-lived substances (VSLS) containing chlorine or bromine, which are not directly controlled by the Montreal Protocol, also contribute to uncertainty in recovery rates. Nitrous oxide (N₂O) is a potent ozone-depleting substance not fully regulated, and its increasing atmospheric concentrations can perturb stratospheric ozone. These factors underscore the need for continued vigilance and research to support the ozone layer’s recovery.