The Earth hasn’t always had ozone — or even oxygen — in its atmosphere.
Our planet spent more than 2.4 billion years with an oxygen concentration of less than 1 part per million, in fact. While the element is plentiful throughout the universe, what little Earth was born with quickly evaporated away into space.
It wasn’t until the arrival of plants and photosynthesis, which consumes abundant carbon dioxide from the atmosphere and leaves oxygen as a byproduct, that the concentration of oxygen reached a measly few percent.
Around 500 million years ago, those levels shot up again — this time to the roughly 20 percent concentrations that we have today. And with that abundant oxygen came ozone.
Most of the oxygen in the atmosphere comes in pairs of two atoms. Ultraviolet radiation from the sun breaks apart these molecules, enabling free oxygen atoms to combine with an existing pair and create a “triplet” called ozone.
Once it had formed, the Earth’s ozone layer settled in the stratosphere, between 10 and 20 miles above the surface. The most important job of this layer is to continuously protect life down below. Ozone absorbs most wavelengths of ultraviolet radiation from the sun, preventing its most damaging effects from reaching us.
But it also plays a role in regulating our climate, as a pair of researchers at the University of Victoria in Canada recently discovered. They altered the amount of ozone present in simulated climate models and published the effects in a paper on the preprint server arXiv.
The greatest effect the ozone layer has on our climate, it turns out, is keeping us warmer than we would otherwise be with it. When the researchers completely eliminated ozone, the average global surface temperature dropped by 3.5 Kelvin.
That may not seem like a lot; but consider that governments around the world are currently racing to prevent the catastrophic effects of a 1 Kelvin increase due to global warming. A world this much colder would be disastrous.
The researchers additionally found that the culprit behind the cooling is the complex interaction between UV radiation and ozone. The latter currently keeps the stratosphere warm through UV absorption.
Without it, the upper atmosphere becomes a lot cooler and (because colder air can’t hold on to as much moisture) drier. This creates a self-reinforcing cycle: With less water vapor in the upper atmosphere, it can’t trap as much heat, and the Earth cools off even more.
The researchers also found that the ozone layer acts as a stabilizer for the upper atmosphere. Without ozone, the upper troposphere becomes more unstable, preventing the formation of clouds at that layer. Only clouds at lower and higher altitudes can then form, changing the balance of solar energy reflected into space.
Lastly, the colder stratosphere impacts the giant air circulation patterns that cover the globe.
The researchers found that the Hadley circulation, the cycle of air from the equator through the tropics, becomes stronger without an ozone layer. The same happens to the familiar jet streams that cruise through the troposphere at mid-latitudes. And in response, the stratospheric polar jets become much weaker.
Local weather patterns rely on the patterns set by these giant cycles.
For example, Europe is much warmer than it should be, given its high latitude, due to warm air continually coming in from the equator. The West Coast of the U.S. experiences regular seasonal rainfall, making it one of the most productive agricultural regions in the entire world, due to cycles that bring in cool, wet air from the north.
Disrupting these global rhythms, even by a small amount, would devastate local communities and endanger our food supplies.
Thankfully, past efforts to reduce ozone-destroying pollutants have been successful — and we can continue to enjoy the protecting, stabilizing and warming effects of the ozone for years to come.