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Carbon dioxide is the chemical that is shaping the 21st Century.

At first glance it doesn’t look like much. Like oxygen, it’s a colourless odourless gas that is a natural part of our atmosphere.

But the two are part of an immense dance between all life on the planet. Animals breathe out carbon dioxide, plants absorb it. Plants emit oxygen, and animals breathe it in.

The cycle starts within plants, where they use carbon dioxide for photosynthesis. They bring carbon dioxide and water together in their cells, and when exposed to sunlight they rearrange its carbon atoms to form a long chain called ‘glucose’. It leaves behind oxygen, which gets pushed back into the atmosphere as waste.

The glucose is transported to where the plant wants to grow, like young leaves and branches, where it is repurposed into other molecules like ‘cellulose’, which is the tough outer coating of plant cells.

Glucose is sent to buds on the plant to provide materials and energy for growth

An animal might interrupt this process by pulling off a leaf and eating it. They digest the plant material, releasing and redirecting the glucose into their own growing cells.

To break glucose apart, an animal cell will recruit oxygen, one of the most destructive elements. It breathes oxygen from the air, which attacks the glucose and their reaction produces carbon dioxide, water, and heat – allowing the animal to access the energy the plant originally got from sunlight.

This is called ‘respiration’, and it’s a reversal of photosynthesis. It uses the oxygen in the air that came from plants, while returning carbon dioxide to complete the cycle. Plants and animals have evolved together as pieces of a larger system for their entire history.

But this isn’t why carbon dioxide is going to shape the 21st Century.

To find out why we have to go back 300 million years ago, when the Earth was living through a geological period called the ‘Carboniferous’.

The balance of oxygen and carbon dioxide was completely different in this period – there was much more of both.

Plants had only recently discovered how to make bark, and ferns utilizing the innovation to climb into the sky were the plants that dominated the Earth. The higher oxygen in the air meant that insects grew enormous. 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 amphibians. The higher carbon dioxide levels in the air heated the Earth, turning much of into sprawling swampland 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.

When the fern-trees died and fell into their swamps, they stayed there. Their invention of bark was still so new that no bacteria had evolved to digest it.

Layers of dead trees piled up in the swamps, and as silt built up around them they slowly sunk deep underneath the ground. After millions of years they became compacted and were changed forever by the internal pressures of the Earth.

The pressure of being underground for so long turned their glucose into hydrocarbons.

Hundreds of millions of years later, we discovered that despite the chemical changes, the energy that the plants once absorbed from the Sun was still there. It had been turned into crude oil, or ‘fossil fuels’, the most efficient and energy-dense fuel source ever discovered.

A coal seam in Antarctica. Carboniferous ferns, compacted and compressed for eons. This would burn if you held a flame to it.

To release the energy, all you had to do was burn it.

Fossil fuels powered the greatest period of economic change in human history, as we built machines to replace many types of work done by livestock or human hands.

But just like respiration, burning these chemicals reintroduces carbon dioxide into the air.

The carbon inside these old plants has been trapped in a time capsule. Unfortunately, it’s as much as a greenhouse gas now as it was in the Carboniferous.

This means that it acts as a chemical blanket in the atmosphere. The global swamplands of the Carboniferous were caused by a blanket that was way too thick.

But burning fossil fuels has increased the amount of carbon dioxide in the atmosphere by 45% since the Industrial Revolution.

The effect on global temperatures has been dramatic. 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.

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Ben McCarthy

Ben McCarthy

Ben is the Founder of Discover Earth and the author of the Big Ideas Network.