Ozone depletion

Ozone depletion is the thinning of a special layer of air called the ozone layer high above the Earth. Scientists noticed this problem starting in the late 1970s. It happens mostly around the poles during spring and is called the ozone hole.

The main reason for this thinning are special chemicals made by people, such as refrigerants, solvents, and propellants. These chemicals, called ozone-depleting substances, float up into the air and break down the ozone. This layer helps protect us by blocking harmful ultraviolet light from the sun.

Because of the worry over health problems like skin cancer and sunburn, countries around the world agreed to the Montreal Protocol in 1987. This agreement stopped the making of many of these harmful chemicals. Since then, the ozone layer has started to heal. Scientists believe it will fully recover by 2045. In 2019, NASA reported the smallest ozone hole ever measured.

Ozone cycle overview

Three forms of oxygen are part of the ozone-oxygen cycle: single oxygen atoms (O), oxygen gas (O₂), and ozone gas (O₃). Ozone forms when oxygen gas molecules break apart after absorbing energy from sunlight, creating oxygen atoms. These atoms then combine with oxygen gas to form ozone. Ozone helps protect Earth by absorbing harmful sunlight.

Ozone can be broken down by very reactive particles, called radicals. Human activities have increased the amount of chlorine and bromine in the upper atmosphere. These elements come from stable compounds, such as chlorofluorocarbons, that can reach the upper atmosphere. Once there, they release chlorine and bromine atoms that break down ozone. A single chlorine atom can break down many ozone molecules before it is removed from the cycle. This process has been a big concern for the health of our planet's protective ozone layer.

Observations on ozone layer depletion

The ozone hole is measured by how much ozone is above a point on Earth, usually using a unit called Dobson units (DU). The biggest drops in ozone happen in the lower part of the upper atmosphere. Scientists have seen big drops in ozone over Antarctica in the spring and early summer since the 1970s.

These drops can be as much as 70 percent. Even in 2010, ozone levels in Antarctica were still 40–50 percent lower than before the ozone hole was noticed. There was some good news in 2016 that the ozone layer seemed to be getting a little better. In 2017, the ozone hole was the smallest it had been since 1988 because the air up there was warmer. Scientists think the ozone layer will get back to normal around the year 2070.

Ozone loss changes a bit every year in the Arctic, but the biggest drops happen in the winter and spring when it gets very cold up there. Special clouds in the very cold air over the poles help make ozone loss worse, which is why the ozone hole is biggest over Antarctica.

Compounds in the atmosphere

CFCs and related compounds

Chlorofluorocarbons (CFCs) and other chemicals that hurt ozone are mostly to blame for the loss of ozone. These chemicals were invented in the 1930s and were used in cooling machines, sprays, and cleaning things. They don’t come from nature—they are made by people. When they get high up, they break apart and release chemicals that can destroy many ozone molecules.

One chemical called CFC-113a is still increasing in the air, and scientists aren’t sure where it’s coming from. Another chemical, CFC-11, has been released a lot from north-eastern China since 2013, which could make the ozone hole take longer to heal.

Aluminum oxide

When satellites burn up when they fall back to Earth, they leave behind tiny pieces of aluminum oxide. More satellites could make this problem worse.

Very short-lived substances (VSLS)

"Very short-lived substances" are chemicals that break down quickly. Most of them come from nature, like from seaweed, but some are made by people. They can still hurt the ozone layer a little.

Computer modeling

Scientists use computer models and real-world data to understand how chemicals like CFCs are hurting the ozone layer. These models help them see how these chemicals move and react in the air.

Ozone hole and its causes

The Antarctic ozone hole is a place in the sky above Antarctica where ozone levels drop very low each year. It happens in the Antarctic spring, from September to early December. Strong winds circle the continent, creating a sort of container in the air. Inside this container, over half of the ozone in a certain layer of the sky gets destroyed.

The main reason for this loss of ozone is certain chemicals released into the air, especially CFCs. These chemicals break apart when they meet sunlight, releasing particles that quickly destroy ozone. This process is made much worse by special clouds that form in the very cold Antarctic winter. These clouds help change the chemicals into forms that destroy ozone when the sunlight returns in the spring.

Effects

Since the ozone layer blocks a type of sunlight called UVB, when the ozone layer gets thinner, more of this harmful light reaches the Earth. This can harm people by increasing the chance of skin problems. It can also hurt plants, animals, and water creatures.

When the ozone layer is weaker, more UVB light reaches us. This type of light can damage the DNA in our skin cells, which can lead to skin conditions. Even though we don’t fully understand all the ways this affects health, scientists know that stronger UVB light can cause problems. The ozone layer also helps control winds and weather patterns, so changes in ozone can affect our climate.

Prospects of ozone depletion

Since we started using the Montreal Protocol to cut down on certain chemicals, the amount of these harmful substances in the air has been going down. By 2008, the level of these chemicals had dropped by about 10 percent since 1994. This helps protect our ozone layer.

Experts say that the ozone layer has been getting better since it shrank a lot in the 1970s and 1980s, but it still hasn’t gone back to how it was before the 1980s. It is expected to keep getting better in the next few decades if we keep following the rules to stop using harmful chemicals. The big drop in ozone over Antarctica, called the ozone hole, is expected to last for many years. By around 2060–2075, it should go back to levels seen before 1980. This is a bit later than we thought before because of changes in how much of these chemicals are still being used in some places. In 2023, the ozone hole was one of the biggest ever seen, possibly because of a big volcanic eruption in 2022. If rules stay the same, the ozone layer should return to 1980 levels by about 2066 over Antarctica, by 2045 over the Arctic, and by 2040 for the rest of the world.

Research history

The basic processes that create the ozone layer were discovered in 1930. Sunlight splits oxygen molecules, and these pieces can form ozone. Ozone helps protect us from harmful sunlight.

In the 1970s, scientists realized that human activities could harm this layer. Certain chemicals released from the ground could reach the upper atmosphere and break down ozone. This led to international efforts to reduce these harmful chemicals.

The discovery of a large ozone loss over Antarctica surprised scientists. They found that special clouds in very cold Antarctic air helped these chemicals destroy ozone more effectively. This led to the term "ozone hole."

Since then, the world has worked to reduce the use of these chemicals. The ozone layer is slowly recovering, but it will take many more years to return to its earlier state.

Ozone depletion and global warming

Robert Watson helped bring together different scientific reports about ozone depletion and global warming before the 1980s. Because of this work, groups like the EU, NASA, and the UN started to agree on their findings. This helped create a clear report for leaders to use.

Ozone depletion and global warming are connected in several ways. The same gases that cause Earth to warm can make the upper atmosphere cooler. This cooling can lead to more ozone loss, especially around the poles. On the other hand, losing ozone also affects the climate. Less ozone means the upper atmosphere absorbs less sunlight, which cools that layer but can warm the lower atmosphere a little. Overall, the cooling effect is stronger. Scientists have measured this effect and found it to be about −0.15 ± 0.10 watts per square meter.

Some chemicals that damage ozone also trap heat in the atmosphere. These chemicals add about 0.34 ± 0.03 watts per square meter to the total heating effect from greenhouse gases. Studying these changes takes many years, and scientists are still learning more about how ozone is destroyed and how it might recover.

Misconceptions

CFC weight

People often think that CFC molecules are too heavy to reach high up in the sky. But winds mix all gases well, so heavier CFCs can still get to the stratosphere. Some heavier CFCs aren’t spread out evenly.

Percentage of human-made chlorine

Some believe natural chlorine sources are much bigger than human ones. But natural chlorine from ocean spray gets washed away by rain before it can reach high up in the sky. CFCs don’t get washed away and can reach high up. In lower air, human-made chlorine from CFCs and similar substances is more common than natural chlorine. In high air, human-made sources are the main cause of chlorine, making up about 80 percent, while natural sources make up only about 20 percent.

First observation

When measurements of ozone over Halley Bay in Antarctica were first taken in 1956, scientists found lower levels than expected. This wasn’t an ozone hole yet, but a normal yearly high amount of ozone. Antarctic ozone drops sharply each spring, unlike in the Arctic.

Location of hole

Some thought the ozone hole should be above places where CFCs are used. But CFCs mix around the world. The ozone hole forms above Antarctica because cold temperatures help create special clouds that affect ozone. There have also been smaller ozone dips observed above other areas, like Central Asia.

Awareness campaigns

Many people misunderstand issues like ozone depletion. Early on, some groups thought it was too complicated to talk about. Later, campaigns helped explain the problem better. People worried about health effects like skin problems and damage to plants. Many changed their habits, like stopping aerosol sprays, before laws were made. The discovery of a big ozone dip over Antarctica in 1985 got a lot of attention. While the Antarctic ozone dip doesn’t greatly affect global ozone levels, it worried many because they feared similar dips might appear elsewhere. If conditions get worse, global ozone could drop faster. When the Antarctic ozone dip ends each year, it can affect nearby areas, with some places seeing lower ozone and more harmful sunlight.

World Ozone Day

In 1994, the United Nations General Assembly chose September 16th as the International Day for the Preservation of the Ozone Layer, also called "World Ozone Day." This date marks when the Montreal Protocol was signed in 1987, an important agreement to protect our planet's ozone layer.