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The ozone layer is a part of Earth’s atmosphere that captures some of the Sun’s ultraviolet (UV) light emissions. When the ozone layer is compromised, its ability to absorb radiation weakens, allowing more UV rays than usual to permeate the atmosphere’s innermost layers. This is known to contribute to climate change.
The Ozone Layer: One Piece of an Important Puzzle
Earth’s atmosphere comprises multiple layers, each with its own role. Above the troposphere, Earth’s innermost layer (which contains Earth’s oxygen and houses what we call weather), is the stratosphere. This is a comparatively dry atmospheric layer associated with weather balloons, high-altitude planes, and polar stratospheric clouds—as well as the ozone layer.
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The ozone layer sits toward the bottom of the stratosphere, roughly 20 to 40 kilometers (66,000 and 131,000 feet) above the ground. It gets its name from its high concentration of ozone, a molecule born from UV light’s contact with the stratosphere. When UV rays strike oxygen molecules (O2) in this layer, those molecules break apart, resulting in two highly reactive individual oxygen atoms known as atomic oxygen. Atomic oxygen joins regular oxygen molecules to create ozone (O3). This cycle, called the ozone-oxygen cycle, creates an ozone concentration of roughly 2 to 8 parts per million to form the ozone layer.
While oxygen molecules don’t absorb UV radiation, ozone molecules do. Specifically, they absorb UVB, a short-wavelength form of UV light that would otherwise penetrate the atmosphere. Not only is UVB known to damage skin and warm the Earth’s lower atmosphere, but it’s thought to disrupt the cycling of elements like nitrogen. These elements are responsible for absorbing other types of UV radiation (UVA and UVC) that are also thought to exacerbate climate change. This makes ozone a particularly vital component of a larger Earth-saving puzzle.
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The Ozone Hole
Ozone’s importance alone isn’t why it’s in the headlines all the time. If you’ve glanced at a newspaper or browsed the internet over the last few decades, you’ve probably heard of the “ozone hole.” This concentrated area of ozone depletion allows UVB to penetrate the stratosphere, resulting in the health and environmental effects we touched on above.
Scientists have known about Earth’s ozone layer for over a century, but it wasn’t until the 1970s that they grew concerned about its integrity. Early in the decade, two chemists named Paul Crutzen and Harold Johnston independently studied the environmental risks posed by planned supersonic aircraft. Crutzen and Johnston found that nitrogen oxides, which those aircraft would release into the lower stratosphere, contributed to ozone decay. Though the proposed aircraft were never brought to fruition, the chemists’ efforts are widely believed to be the first academic research to suggest that human behavior could significantly impact global atmospheric ozone.
Credit: Pressestelle BFK Urfahr-Umgebung/Wikimedia Commons
Another pair of chemists quickly followed their lead. In 1974, Mario Molina and Sherwood Rowland published a paper in Nature detailing the effects of chlorofluorocarbon (CFC) gasses on atmospheric ozone. The paper pointed out that CFCs, which were widely used in aerosols, firefighting foams, and refrigerants at the time, accumulated in the stratosphere. Once they reached the ozone layer, CFCs were converted into their constituents by UV radiation.
A particular CFC’s chemical makeup depends on its specific purpose, but all CFC molecules contain carbon and chlorine. When UV radiation hits a CFC molecule, that precarious bundle of elements breaks apart, releasing ultra-reactive chlorine. Left to its own devices, chlorine goes around “stealing” oxygen atoms from ozone molecules, turning them back into O2 and reducing the amount of ozone.
Molina and Rowland suggested that if CFCs weren’t tightly regulated, they’d gradually whittle away the ozone layer over the coming decades. Sure enough, a trio of scientists from the British Antarctic Survey (BAS) found in 1985 that the stratosphere over the Antarctic region had suffered large ozone losses. Their research quickly led to the discovery and naming of the ozone hole, which was 22.4 million square kilometers (13.9 million square miles) at the time.
The ozone hole over Antarctica in 1985.
Credit: NASA
Looking back, humanity’s intergovernmental response was relatively quick. On Sept. 16, 1987, 197 countries universally ratified the Montreal Protocol on Substances that Deplete the Ozone Layer. The treaty symbolized federal governments’ agreement to phase out and regulate the use of ozone-depleting substances (ODS), which included CFCs, carbon tetrachloride, methyl chloroform, methyl bromide, and halons.
Did the Montreal Protocol Help? (Spoiler Alert: Yes, It Did)
Today, the Montreal Protocol is considered one of the most effective environmental treaties ever signed. Almost all (99%) banned ODS have been successfully phased out, according to the United Nations Environmental Programme (UNEP). This has allowed the ozone hole to shrink gradually. In 2019, the ozone hole reached its smallest size on record.
In looking at the ozone hole’s history, it might be alarming to notice that it reached peak size in 2006. After all, if banning ODS is supposed to help “heal” the ozone hole, why would the hole get bigger nearly three decades after the phase-out began? According to NASA, a few different factors were at play. The ozone hole naturally fluctuates in size based on temperatures and polar vortices over Antarctica (which is why it was so big in October 2023). It’s also not immune to leftover ODS from previous years, meaning many ODS phase-out efforts will yield delayed results. Colder-than-average temperatures also play a part—and 2006 saw many of those. Overall, though, the ozone hole has become smaller year over year.
What’s in the Ozone Layer’s Future?
Again, it’s normal to see some fluctuation in the ozone hole’s size, especially during periods of unusual meteorological activity. But overall, the UNEP estimates that the ozone hole over the Antarctic is on track to return to 1980’s size by 2066 if current ODS-regulating policies remain in place. (Manufacturers will need to follow those policies, too.) Ozone thinning in other areas is expected to return to 1980 levels by 2040.