The ozone layer and its depletion since the late 1970s and early ‘80s have been hot topics at the forefront of the discussion regarding human-induced climate change. However, in recent years, many climate scientists have asserted that the Antarctic Ozone Hole is shrinking in size and the ozone layer is on its path to recovery.
History
The ozone layer was one of the most critical developments in the history of life on Earth. Like many other crucial events that have occurred on Earth in the past 4 billion years, the driving force behind its creation was a change in atmospheric oxygen levels.
Around 600 million years ago, during the late Proterozoic geological era, large blue-green algae blooms caused levels of atmospheric oxygen gas (O2) to spike drastically.
Photo Credit: Users LordToran and WolfmanSF/Wikipedia
A graph of the atmospheric oxygen levels across the last billion years
In a process known as photodissociation, O2 molecules absorbed photons from sunlight and split into single oxygen atoms. These atoms then reacted with intact O2 molecules, forming ozone (O3). This ozone accumulated in the upper atmosphere, forming a thin protective layer that helped to facilitate the transition of aquatic life to terrestrial environments. While most organisms were initially only able to survive in deeper areas of water, where harmful UV rays could not penetrate, the formation of the ozone layer and high levels of atmospheric oxygen allowed organisms to complexify in an event known as the Cambrian Explosion.
Photo Credit: John Sibbick/Natural History Museum
One of the Earth's first apex predators, the Anomalocaris, which appeared during the Cambrian Explosion
These diversified organisms moved into shallower areas of water and were eventually able to move to and live on land once the ozone became thick enough. A wide array of evolutionary opportunities opened up to many organisms, and life progressed at ever-increasing rates, in large part due to the protection offered by the ozone layer.
However, even though the ozone played such an important role in sustaining life on Earth, humans were unaware of the layer’s existence until 1913, when the French scientists Charles Fabry and Henri Buisson noticed that radiation from the sun did not reach the Earth if it had a wavelength shorter than 310 nanometers (nm). Fabry and Buisson then theorized that this radiation was being absorbed by something in the Earth’s atmosphere, and they realized that the absorption spectrum was consistent with that of ozone. Using these observations, they put forth the idea that there was an ozonosphere that absorbed the vast majority of the high energy radiation emitted by the sun before it could get to Earth.
This ozonosphere that Fabry and Buisson discovered is located between 15 and 35 km high in Earth’s atmosphere. While 90% of the atmosphere’s ozone is found in and around this range in the stratosphere, ozone that forms at lower elevations in the troposphere can be harmful to life. This type of ozone forms when nitrogen oxides emitted by machinery and factories react with gaseous organic compounds such as paint thinners. The resultant ozone that these pollutants yield may form in dangerously high concentrations in centers of population with warm, stagnant air. They have the potential to cause throat and lung issues, and may exacerbate breathing problems that certain people already struggle with. While human activity causes the creation of harmful ozone in the lower troposphere, it also results in the destruction of beneficial stratospheric ozone.
Significance and Scientific Explanation
Because the stratospheric ozone plays such an important role in blocking harmful radiation from entering Earth’s atmosphere, it is easy to understand the detrimental impacts of a hole in the ozonosphere. However, many people lack an understanding of the science that allows ozone to block out radiation.
Like all molecules, ozone has an absorption spectrum. Ozone is most effective at absorbing radiation with a wavelength of 250 nm, and it becomes less effective as the waves grow in size. 250 nm waves are at the shorter end of the UV spectrum, meaning that they are the type of UV wave with the most energy and potential to harm life on Earth. These are referred to as UVC waves, and are, for the most part, entirely absorbed by the ozone layer. UVB waves, which have wavelengths between 280 and 315 nm, are mostly absorbed by the ozone, but some still penetrate the atmosphere, and only half of UVA waves, with wavelengths between 315 and 400 nm, are blocked by the ozone. These penetrating UVB and UVA waves are the reason that using sunscreen and wearing sunglasses is necessary to prevent skin and eye damage.
Photo Credit: UVFAB
The spectrum of UV light
When ozone absorbs higher energy, shortwave radiation, it then re-emits this radiation at longer wavelengths in all directions. As a result, lethal levels of radiation are prevented from entering the earth’s atmosphere and harming life forms. Specifically to humans, UV radiation is detrimental not only because it can cause skin cancer, but also because it can destroy the protein folate in the bloodstream, which leads to birth defects and a higher infant mortality rate. This is why people who live in lower latitudes, and therefore experience sunlight at more direct angles of insulation, have evolved darker skin. The eumelanin pigment in dark skin blocks out 99.9% of UV rays, which subsequently maintains intact folate levels. Evidently, the ozone layer plays an indispensable role in the wellbeing of humankind.
Read about the ozone layer’s depletion and recent recovery here: https://www.cleanwestchester.org/articles/ozonelayerstillunderthreat
Sources:
https://www.cbsnews.com/news/arctic-ozone-hole-largest-closed/
https://www.britannica.com/story/is-the-ozone-layer-finally-healing-itself
https://ozonewatch.gsfc.nasa.gov/monthly/monthly_2020-09_SH.html
https://ozonewatch.gsfc.nasa.gov/facts/history_SH.html
https://www.britannica.com/print/article/437240
https://www.uvfab.com/uv-spectrum-and-applications/
https://commons.wikimedia.org/wiki/File:Sauerstoffgehalt-1000mj2.png
https://eartharchives.org/articles/life-on-land-made-possible-by-ozone-layer/index.html
https://ozonewatch.gsfc.nasa.gov/facts/SH.html
https://globalnews.ca/news/8970778/ozone-layer-hole-tropics/ Accessed 12 Mar. 2022.