Imagine you’re at the petrol station filling up your car, when a splash of petrol hits the ground.
The last time these atoms saw the Sun was 300 million years ago, when the world crawled with millipedes over 8 feet in length and dragonflies flew with 30 inch wingspans.
Petrol is ancient, but the carbon dioxide it turns into when burned is the chemical that’s shaping our 21st Century.
At first glance it doesn’t look like much. Like oxygen, carbon dioxide is a colourless, odourless gas that is a natural part of our atmosphere.
But the two are invisible partners in an immense dance between all life on the planet that’s been going on for hundreds of millions of years, older than even the 8 foot millipedes.
Animals breathe out carbon dioxide, plants absorb it. Plants emit oxygen, and animals breathe it in.
You could say that the cycle starts within plants, where they use carbon dioxide in ‘photosynthesis’.
They pull it out of the air and mix it with water from the soil.
Using the energy of the Sun that hits their leaves they break the molecules in half and recombine them, making oxygen and a new chemical called ‘glucose’.
The equation for photosynthesis. Source: BBC
This may be surprising to hear, but oxygen is one of the most destructive chemical elements that exists. It can damage plant cells, and the plant hurriedly pushes it back into the atmosphere as waste.
Plants releasing bubbles of oxygen into the atmosphere.
Glucose, which now carries some of the Sun’s energy, is moved to young branches where it’s used as a LEGO block to build new plant cells.
When animals eat plants (or other animals), they redirect glucose and its energy into their own growing cells.
Image source: BBC
They break it apart by inhaling the caustic oxygen in the air, which attacks the glucose molecule.
This is called ‘respiration’. It’s a reversal of photosynthesis, and it allows animals to access the energy that plants originally got from the Sun.
Just like burning a lump of wood, the reaction produces heat plus carbon dioxide as waste, and the cycle is completed.
Plants and animals have evolved in this way to sustainably reuse each other’s waste for (almost) their entire history.
Image source: Photosynthesis Education
While the number of oxygen and carbon atoms on Earth stays more or less unchanged, the portion of them in that’s in the air can change.
300 million years ago, the Earth was living through a geological period called the ‘Carboniferous’.
Back then, the portion of oxygen and carbon dioxide in the air was different – there was much more of both.
The higher oxygen levels meant that insects could grow to enormous sizes. Millipedes reached eight feet in length, and species similar to dragonflies grew to have wingspans of over 30 inches.
The animals that would become dinosaurs were large, six-foot long amphibians.
The higher carbon dioxide levels in the air trapped sunlight and heated up the Earth, turning almost the entire planet into a sprawling swamp that was the habitat of these giant creatures.
The Carboniferous. Image source: The Economist
But the biggest difference between our world and Carboniferous was actually something tiny.
Plants had only just evolved bark, in order to grow tall and beat other plants into the sunlight. It’s the first time on Earth that there were forests of plants (although in the past there has been forests of fungi).
But their invention of bark was still so new that no bacteria, fungus, or animal had evolved to digest it and access the glucose hidden inside.
So when the trees died and fell into their swamps, they stayed there.
They never rotted, because there was no bacteria to rot them.
Image source: Mary Parrish at the Smithsonian Institution
Millions of years later, bacteria and fungus worked it out. Nowadays a fallen tree rots through in a few years, and it’s energy is returned to its ecosystem.
But this evolutionary lag between bark and digesting it was a small but crucially important thing in the history of the Earth’s, like the proverbial butterfly’s wing that causes a hurricane.
300 million years later, it allowed a species of bipedal apes to invent the Industrial Revolution.
Layers of dead trees piled up in the swamps, and sunk under the silt until they were beneath the ground.
After millions of years they were compacted by the internal pressures of the Earth and changed forever. Glucose fused together to create a mix of hydrocarbon chains, also known as coal, crude oil, and natural gas.
The energy that the plants once absorbed from the Sun was still in there. It was now held in a ‘fossil fuel’, the most efficient and energy-dense fuel source ever discovered. To release the energy, all you had to do was burn it.
It sat in the ground more or less untouched until around 1760, when we invented the coal powered steam engine. What followed was the greatest period of economic change in human history, as we used the energy of fossil fuels to make machines work for us.
But just like with respiration, burning fossil fuels reintroduces carbon dioxide into the air.
Except that this carbon dioxide has been trapped in a time capsule.
300 million years ago it was in the Carboniferous air, where there was so much carbon dioxide in the air that the Earth was a global swamp.
Carbon dioxide is an atmospheric blanket that lets in sunlight, but traps it once it gets through. The hot, global swamps of the Carboniferous were caused by a carbon dioxide blanket that was way too thick.
But since the Industrial Revolution, burning fossil fuels has spread from a few European countries to the entire world and simultaneously, our population has exploded. The the amount of carbon dioxide in the atmosphere has increased by a whopping 45%.
The effect on global temperatures has been dramatic, and even now has permanently changed our climate. We are already seeing melting ice caps and rising water levels, as well as changes in rainfall patterns and the intensity of storms.
Our actions now and in the following decades will determine how severe these effects will be.
The best visualisation I’ve come across is embedded below. If it doesn’t work, click here to go through to XKCD and see it.