Ozone Depletion, Smog and the Anthropogenic Greenhouse Effect: Three Different Problems

Let's start with a mix-up

Ask around and you'll find many people who think climate change is caused by "the hole in the ozone layer", or that smog and the greenhouse effect are the same thing. They're not. They are three different problems with different causes, different effects and different solutions, although they're connected in interesting ways.

By the end of this lesson you'll be able to untangle them, and you'll see one of the biggest success stories in environmental science: how the world fixed the ozone hole.

Problem 1: The anthropogenic greenhouse effect (a quick summary)

You've studied this in Lessons 7 to 11. In general terms:

Causes: human activities add greenhouse gases: CO₂ from burning fossil fuels, cement making and deforestation; methane from livestock, landfills and oil and gas; nitrous oxide from fertilizers; halocarbons from refrigeration. More greenhouse gas means less infrared escapes to space, so Earth gains energy.

Effects: global average warming of about 1.3 °C since the late 1800s (and about twice that in Canada as a whole); more frequent heat waves; changes in precipitation, with heavier downpours in many places and more drought in others; shrinking glaciers, sea ice and snow cover; thawing permafrost; rising sea level; and shifts in ecosystems. Extra CO₂ also makes the ocean more acidic (a direct chemical effect, separate from warming).

Problem 2: Depletion of stratospheric ozone

How the ozone layer forms: high in the stratosphere, strong UV light splits oxygen molecules (O₂) into single oxygen atoms. Each atom joins another O₂ to make ozone (O₃). Ozone absorbs most of the Sun's dangerous UV-B and all UV-C radiation, protecting living things from DNA damage, skin cancer, cataracts and harm to crops and plankton.

What went wrong: CFCs were invented in the 1920s and 1930s and seemed perfect: non-toxic, non-flammable and long-lasting. They were used in refrigerators, air conditioners, foam and aerosol sprays. But because they're so stable, they drift up into the stratosphere. There, UV light breaks them apart, releasing chlorine atoms, which destroy ozone in a cycle. Bromine compounds (from halons in some fire extinguishers) do the same.

The ozone hole: in 1985, British Antarctic Survey scientists reported a dramatic thinning of ozone over Antarctica each spring (September to November). Extremely cold winter temperatures there form polar stratospheric clouds, on which chemical reactions free up chlorine; when sunlight returns in spring, ozone is destroyed rapidly. Ozone has also thinned over the Arctic and, less dramatically, around the world, including over Canada.

The fix: the Montreal Protocol

In 1987, countries signed the Montreal Protocol on Substances that Deplete the Ozone Layer, in Montreal. It has been strengthened several times and has been ratified by every member country of the United Nations. It phased out CFCs, halons and most other ozone-depleting substances.

The result: levels of ozone-depleting chlorine in the stratosphere are slowly falling, and the ozone layer is recovering. Recent international scientific assessments project that ozone over Antarctica should return to 1980 levels around the 2060s, and sooner elsewhere. The protocol also avoided a lot of warming, because CFCs are powerful greenhouse gases.

One catch: many CFCs were replaced with HFCs, which don't harm ozone but are strong greenhouse gases. The Kigali Amendment (2016) to the Montreal Protocol now phases down HFCs, which is expected to avoid roughly 0.3 to 0.5 °C of warming by 2100. Canada has ratified it.

A Canadian connection: scientists at what is now Environment and Climate Change Canada developed the Brewer spectrophotometer, an instrument for measuring ozone that is used at monitoring stations around the world. Canada also began issuing a public UV Index in 1992 to help people protect themselves from UV.

Problem 3: Ground-level ozone and smog

Near the ground, the problem is the opposite: there is too much ozone, not too little. Ground-level (tropospheric) ozone has increased since pre-industrial times.

Ground-level ozone isn't released directly. It forms when nitrogen oxides (NOx) from vehicles, power plants and industry react with volatile organic compounds (VOCs) from gasoline vapour, solvents, paints and even trees, in sunlight and heat. That's why ozone is usually highest on hot, sunny summer afternoons, often downwind of cities. Some of the ozone and particles in southern Ontario's air blow in from the United States.

Smog is mainly ground-level ozone plus fine particulate matter (PM2.5) from vehicle exhaust, industry, and wood and wildfire smoke. A temperature inversion (Lesson 2) can trap smog near the ground.

Effects: ozone and PM2.5 irritate airways, trigger asthma attacks and worsen heart and lung disease. Ozone also damages leaves and reduces crop yields. Ground-level ozone is also a greenhouse gas, and climate change can make smog worse, because hotter days speed up ozone formation and longer fire seasons bring more smoke.

Untangling the three problems

The three problems are connected, but they are not the same.

A success story to learn from: after a clear scientific warning, the world agreed on a treaty, industry developed alternatives, and the ozone layer is healing. In Ontario, shutting down coal-fired power plants (the last one stopped burning coal in 2014), together with cleaner vehicles and pollution cuts in the U.S., made smog advisories much rarer than they were in the early 2000s. Climate change is harder to solve, because CO₂ comes from so many sources at the heart of the economy, but these successes show that coordinated action can work.