Carbon Dioxide (CO2)

Carbon dioxide (CO2) is a colourless, odourless gas made up of one carbon atom joined to two oxygen atoms. It is in every breath you exhale, every fizzy drink you sip, every loaf of bread that rose in the oven and every cloud of car exhaust on the road. It is also the most important greenhouse gas driving modern climate change. In small amounts, CO2 is essential for life. In rising amounts, it is changing our planet.

  • FormulaCO21 carbon + 2 oxygen
  • StateGas at room temperatureSinks because heavier than air
  • In the air0.042 per cent420 parts per million in 2025
  • In your breath out4 per centAround 100x the air around you
  • Solid formDry ice (-78 C)Sublimes (skips liquid stage)
  • Made byBurning, breathing, fermentingPlants use it for food

What is CO2?

A CO2 molecule is a straight line: O=C=O. The carbon is in the middle with a double bond to each oxygen. The molecule is symmetrical, which means the slight pulls of each oxygen cancel out. CO2 is not a polar molecule, but it does absorb invisible infrared light very well, and that is why it traps heat in the atmosphere.

CO2 is heavier than air. A balloon full of CO2 sinks instead of rising. In old wine cellars and grain silos, CO2 from fermentation can quietly pool at the bottom and suffocate people who go in to investigate.

Where CO2 comes from

  • Breathing: every animal exhales CO2 as a waste product of using sugar for energy.
  • Burning: anything that contains carbon, when burned in oxygen, produces CO2. Wood, coal, petrol, gas, candles all release CO2.
  • Fermenting: yeasts turning sugar into alcohol release CO2 as a byproduct. This is what makes bread rise and beer fizz.
  • Volcanoes: erupting volcanoes pump huge amounts of CO2 into the air, though the total is small compared to human activity.
  • Rotting: dead plants and animals release CO2 as bacteria break them down.
  • Heating limestone: making cement involves heating calcium carbonate to release calcium oxide plus CO2. This single industry releases huge amounts of CO2.

Where CO2 goes

  • Plants use it: through photosynthesis, plants take in CO2 from the air, combine it with water, and make sugar. Without this, all life on land would starve.
  • Oceans absorb it: about a third of human-released CO2 dissolves in the ocean. This is making sea water more acidic, which is bad news for corals and shell-building animals.
  • Becomes rocks: over millions of years, CO2 in the ocean turns into calcium carbonate (limestone) as shellfish die and sink to the bottom.
Fact The CO2 in your fizzy drink is forced into the liquid under high pressure. When you open the bottle, the pressure drops and the CO2 escapes as bubbles. The drink tastes slightly sour because some of the CO2 dissolves to form weak carbonic acid (H2CO3). That sharp tingle on your tongue is partly the bubbles, partly the acid.

CO2 and the greenhouse effect

CO2 is a greenhouse gas. It lets sunlight pass through to warm the ground, but traps the heat (infrared radiation) that the ground gives back out. The trapped heat keeps the lower atmosphere warm.

This effect is natural and necessary. Without any greenhouse gases, Earths average temperature would be about minus 18 degrees Celsius (rather than the 15 degrees we enjoy). The trouble is that adding extra CO2 to the air traps more heat than is normal.

Before the Industrial Revolution, the air contained about 280 parts per million (ppm) of CO2. In 2025 it has risen to over 420 ppm: the highest in at least 800,000 years (we know from bubbles trapped in ice cores). The extra CO2 is mainly from burning fossil fuels and from deforestation. As a result, average global temperature has risen by about 1.2 degrees Celsius and is still climbing.

Useful CO2

Despite the climate worries, CO2 has many practical uses:

  • Fizzy drinks: dissolved CO2 gives them their tang and bubbles.
  • Fire extinguishers: CO2 is heavier than air and does not burn. It smothers fires by cutting off oxygen.
  • Dry ice: solid CO2 (frozen at minus 78 degrees Celsius). Sublimes directly into gas, used for cooling perishable food and creating dramatic stage fog.
  • Refrigeration: cleaner alternative to some other refrigerant gases.
  • Greenhouses: commercial greenhouses often pump in extra CO2 to boost plant growth.
  • Welding: CO2 blankets molten metal, keeping out oxygen that would cause defects.
  • Carbon capture: CO2 is now being captured from factory chimneys and sometimes pumped into used oil wells, where the pressure helps push more oil out and the CO2 stays trapped underground.
Did you know? A single mature tree absorbs about 25 kilograms of CO2 from the air each year. A typical petrol car releases about 4,500 kilograms of CO2 a year (5 tonnes). So one car needs about 180 trees just to absorb its yearly emissions. That is one of the reasons forests matter so much to the worlds climate.

Too much CO2 is dangerous

The fresh air around you is about 0.04 per cent CO2. Your body can cope with much more for short times. Indoor air with too little ventilation can rise to 2,500 ppm or higher; you might feel sleepy or have a headache. At 5 per cent CO2 (50,000 ppm), people get dizzy and confused. Above 10 per cent, breathing stops within minutes.

That is why fire extinguishers, dry ice and large CO2 tanks all need careful handling.

Try this Make CO2 in a balloon. Pour about 3 tablespoons of vinegar into an empty plastic bottle. Spoon 2 teaspoons of baking soda into the mouth of a balloon (you can use a small funnel). Stretch the balloon over the top of the bottle without letting the powder fall in. Then lift the balloon so the baking soda drops into the vinegar. The acid-base reaction releases lots of CO2 and the balloon inflates within seconds. Pinch it off, untie and tip a candle (with adult help): the heavy CO2 falling out of the balloon will smother the flame, showing why CO2 makes a good fire extinguisher.
Deeper dive: how scientists read past climates from ice bubbles

How do we know that CO2 levels in the air today are higher than they have been for hundreds of thousands of years? The answer lies in ice cores: long cylinders of ice drilled out of the great ice sheets of Antarctica and Greenland.

As snow falls onto the ice sheets year after year, it slowly compresses into ice, trapping tiny bubbles of the air that was there at the time. The deepest, oldest ice contains bubbles of air from hundreds of thousands of years ago. By drilling out the ice and carefully extracting the bubbles, scientists can measure exactly how much CO2 (and other gases) was in the atmosphere over Earths recent geological history.

The longest ice core, drilled at Dome C in Antarctica, goes back about 800,000 years. Even older ice has been recovered from blue-ice areas, going back 2 million years. Scientists are racing to drill cores that reach even deeper, perhaps to 1.5 or 2 million years, before any blue-ice surfaces.

The story the ice tells is clear. For 800,000 years, CO2 levels stayed between about 180 ppm (in ice ages) and 280 ppm (in warm periods between ice ages). Each peak and trough correlates closely with Earths temperature, going up and down with the ice ages.

Since the Industrial Revolution, CO2 has shot up to 420 ppm in less than 200 years: faster and higher than any change in the last million years. The matching temperature rise is also extremely fast by geological standards. Ice cores give us the most undeniable evidence that the current rapid CO2 rise is something genuinely new.

For more, see water and photosynthesis.