Carbon dioxide has a pretty rough reputation. You probably know that it’s a gas, and that it’s causing climate change. But you might not know that it’s not all bad, and that we actually do want some of it in the atmosphere.

Like a lot of things, the problem we have with carbon dioxide is one of balance. We’ve pushed it so far to one side that it’s like we’ve woken some ancient eldritch Lovecraftian god that’s indifferent to our suffering and has started summoning the seas to submerge our civilisation.

I can’t help but think that it could be a pretty cool testament to how powerful humans have become, if if wasn’t us that’s about to get completely f*cked. But let’s jump in.

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 through a vast global swampland.

This was the geological period known as the Carboniferous, which was long before mammals or even dinosaurs had evolved.

Our ancestors were alive back then too, and were probably medium-sized reptiles. Just by itself that’s such an insane idea that I have to dwell on it for a moment. Palaeontologists have called the most likely candidate for our ancestor at this time Archaeothyris, who was about 50 cm long (20 inches). It may have survived because it could open its jaws wider and close them with greater strength than other reptiles, which presumably meant that you didn’t want to mess with it. Probably a good idea in a world with 8-foot millipedes.

The reason why the world was a giant swampland at the time was because Earth had a different atmosphere, with more carbon dioxide and more oxygen than we’re used to today. It was the extra oxygen that allowed insects to grow to monstrous proportions.

Today, most of these extra gasses have long been pulled out of the atmosphere by natural processes. They now exist trapped in rocks, the ocean, in soil, and in fossil fuels like petrol and coal.

Burning fossil fuels releases one of the reservoirs of these gasses, returns them to the atmosphere, and as a consequence these ancient atoms are now changing our atmosphere and shaping our 21st Century.

What is CO²?

At first glance it doesn’t look like much. Just like oxygen, it’s a colourless and odourless gas. But the two are invisible partners in an immense dance between all life on the planet, and it’s been going on for hundreds of millions of years.

The basic pattern is this: Animals breathe out carbon dioxide, plants absorb it. Plants emit oxygen, and animals breathe it in.

Plants use the carbon dioxide in ‘photosynthesis’, where 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 into their components and recombine them in a different way, making a new chemical called ‘glucose’.

Glucose is moved to young branches and leaves where it’s used as a molecular LEGO block to build new plant cells.

Oxygen is left over, which the plant hurriedly pushes back into the atmosphere as waste. It does this because oxygen is one of the most destructive chemical elements that exists, and it can damage glucose and plant cells.

When animals eat plants (or other animals who ate plants), they capture glucose and redirect its energy into their own growing cells.

Animals don’t use glucose in the same way that plants do. Whereas they use it as a building block, we want to break it apart to access the energy inside. So every time we inhale, we reverse photosynthesis to release the energy stored in the captured glucose.

The oxygen we inhale attacks the glucose molecules in our blood and tissues, which is called ‘respiration’. Most of the animals that we’re familiar with do this through their lungs, but insects are a bit different. They’re still animals, and they breathe oxygen like we do, but they don’t have lungs. Instead they have deep pores on the outside of their bodies which fundamentally do the same thing. It’s way less efficient, but it works.

Respiration produces heat plus carbon dioxide as waste, and the cycle is completed. Plants emit oxygen, animals breathe it in. Animals breathe out carbon dioxide, and plants absorb it. We have evolved in this way to sustainably reuse each other’s waste for (almost) our entire history.

But over millions of years, the the portions of oxygen and carbon dioxide in that’s in the air can change.

The Carboniferous, the time of the giant millipedes, was about 300 million years ago. As we said, back then there was more oxygen and more carbon dioxide in the air.

We now know that insect respiration is less efficient than it is for other animals. This means that their body size is limited by the amount of oxygen that they can breathe in. But when there was more oxygen in the air, they could become enormous, and they grew to 8 feet.

Meanwhile, the higher carbon dioxide levels in the air acted as a thick chemical blanket that trapped sunlight in the atmosphere. This heated up the Earth, turning almost the entire planet into a sprawling swamp that was the habitat of these giant creatures.

But the greatest legacy of the Carboniferous came from something tiny, and this is where the story connects to filling up your car at the petrol station.

Plants had only just evolved bark, and they used its strength to grow tall and beat other plants into the sunlight. It was the first time on Earth that there were forests of plants (although in the past there have been forests of fungi).

But the 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 the swamps, they stayed there.

They never rotted, because there was nothing that could rot them.

Millions of years later, bacteria and fungus evolved and worked it out. Nowadays a fallen tree rots through in a few years, and its 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, like the flapping of a butterfly’s wing that causes a hurricane.

When layers of dead trees piled up in the swamps, they sunk under the mud and silt until they were beneath the ground.

After millions of years they were compacted by the internal pressures of the Earth and changed. Under the pressure of the Earth, 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. But it was now changed into a ‘fossil fuel’, an extremely energy-dense fuel source. 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.

Since 1760, burning fossil fuels has spread from a few European countries to the entire world and simultaneously, our population has exploded. This has meant that the amount of carbon dioxide in the atmosphere has increased by a whopping 45%.

Now if we had no carbon dioxide in the atmosphere, global temperatures would average about -18°C (0°F); not great. Pre-industrial revolution, we averaged about 14°C (57°F). But we’re on track to bump that up to 17°C (63°F). It doesn’t sound like much, but we’re not even there yet and we’re already seeing melting ice caps and rising sea levels, changing rainfall patterns, and more intense storms and weather as a result of more heat being in the atmosphere.

It all comes from burning this carbon reservoir, reviving a relic of the deep history of our planet. Like an angry Cthulhu, it’s risen from its ancient slumber to find a world covered in some puny and insignificant thing called ‘civilisation’. If it gets in his way, civilisation is going to come out rather worse off. Every time we burn a fossil fuel, we grant Cthulhu more power.

So our actions now and in the following decades will determine how severe these effects will be.

The best visualisation that I’ve come across of the dramatic change in atmospheric carbon dioxide is embedded below. If it doesn’t work, click here to go through to XKCD and see it.