Carbon Dioxide in the Atmosphere

Introduction

Carbon dioxide makes up only about four molecules in every ten thousand in the air, yet life on Earth depends on it. Every plant builds its body from carbon dioxide taken from the air, and every animal eats those plants. Carbon dioxide also helps to keep the planet warm. On this page you will see how much carbon dioxide there is, where it comes from, where it goes and why such a tiny fraction matters so much.

Core explanation

How much is there? Dry air contains about 0.04% carbon dioxide by volume. Because this is such a small number, scientists usually give it in parts per million (ppm) : 0.04% is the same as 400 ppm. In the mid-2020s the global average was a little over 420 ppm and rising by roughly 2 to 3 ppm each year. Before the Industrial Revolution it was about 280 ppm.

Properties. Carbon dioxide, CO₂, is a colourless, odourless gas about one and a half times as dense as air. It does not burn and does not support the burning of most substances. It dissolves slightly in water to form a weakly acidic solution, which is why rainwater and fizzy drinks are slightly acidic. It turns limewater (calcium hydroxide solution) milky, which is the standard test for it.

Processes that add carbon dioxide to air:

- Respiration by plants, animals and microorganisms: glucose + oxygen → carbon dioxide + water. - Combustion of fuels containing carbon, such as coal, oil, natural gas and wood: for example CH₄ + 2O₂ → CO₂ + 2H₂O. - Decay : decomposers such as bacteria and fungi respire as they break down dead material. - Volcanic eruptions release carbon dioxide from deep in the Earth.

Processes that remove carbon dioxide from air:

- Photosynthesis : carbon dioxide + water → glucose + oxygen, using light energy absorbed by chlorophyll. The balanced equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. - Dissolving in the oceans : seawater absorbs large amounts of carbon dioxide. Some is used by marine organisms to build shells of calcium carbonate, which may eventually become limestone.

For most of human history these additions and removals were roughly balanced, so the level of carbon dioxide stayed fairly steady.

Why a small amount matters. Carbon dioxide is a greenhouse gas . Sunlight passes through the atmosphere and warms the ground; the warm ground then gives off infrared radiation. Carbon dioxide molecules absorb some of this infrared radiation and re-emit it in all directions, including back towards the ground. This keeps Earth's average surface temperature at about 15 °C instead of well below freezing. Adding more carbon dioxide strengthens this effect, which is why the steady rise in its concentration matters.

Step-by-step reasoning

To decide whether a process adds or removes carbon dioxide:

1. Write its word equation. 2. If carbon dioxide appears among the products , the process adds it to air (respiration, combustion, decay). 3. If carbon dioxide appears among the reactants , the process removes it (photosynthesis). 4. Compare the rates of adding and removing to predict whether the level will rise or fall.

Visual explanation

Picture a bath with the tap running and the plug slightly open. The tap is respiration, combustion and volcanoes; the plughole is photosynthesis and the oceans. When the flow in equals the flow out, the water level (the carbon dioxide concentration) stays steady. Turn the tap up — by burning more fossil fuels — and the level slowly rises.

Real-world analogy

Carbon dioxide in air is like a pinch of salt in a large pan of soup. There is very little of it compared with the rest, yet it changes the whole dish. Doubling the pinch makes a noticeable difference even though the soup is still almost entirely not salt.

Real-world example

Commercial greenhouses often raise the carbon dioxide level inside to around 800 to 1000 ppm. Because carbon dioxide is a raw material for photosynthesis, the tomatoes, cucumbers and peppers grow faster and give bigger crops. Growers monitor the level carefully, since very high concentrations are harmful to the workers.

Why?

Why does the carbon dioxide level measured at an observatory rise and fall a little every year? In the northern hemisphere, which has most of the world's land plants, photosynthesis is fastest in spring and summer, so carbon dioxide is drawn down. In autumn and winter, leaves fall and decay while photosynthesis slows, so the level rises again.

Common misconception

"Carbon dioxide is just a poisonous waste gas." At normal levels it is harmless and essential: it is the raw material from which plants make all our food. It becomes dangerous only when it builds up to high concentrations in enclosed spaces, where it can displace the oxygen needed for breathing.

Worked example

Question: Convert 0.04% carbon dioxide into parts per million, and state how many CO₂ molecules there are in every million molecules of air.

Reasoning: Per cent means "per hundred". 0.04 per hundred = 0.04 × 10 000 per million = 400 per million.

Answer: 400 ppm, which means about 400 carbon dioxide molecules in every million molecules of air.

Quick check

1. Name one process that removes carbon dioxide from the air. Answer: Photosynthesis (dissolving in the oceans is also correct).

Exam focus

Know the figure of about 0.04% (400 ppm) and be able to list processes that add carbon dioxide (respiration, combustion, decay, volcanoes) and remove it (photosynthesis, dissolving in oceans). Write the photosynthesis and respiration equations, and remember that they are the reverse of each other.

Advanced insight

Measurements on Mauna Loa in Hawaii began in 1958 and form the famous "Keeling Curve", a steadily rising zig-zag line. Air bubbles trapped in Antarctic ice show that for the last 800 000 years carbon dioxide stayed between about 180 and 300 ppm, so today's value of over 420 ppm is far outside the natural range of that long period.

Summary

Carbon dioxide makes up about 0.04% (400 ppm) of air, and its concentration is rising. Respiration, combustion, decay and volcanoes add it; photosynthesis and the oceans remove it. Although the fraction is tiny, carbon dioxide is essential for photosynthesis and is a greenhouse gas that helps to control Earth's temperature.

Practice questions

1. Give the approximate percentage of carbon dioxide in air. Answer: About 0.04% (about 400 ppm). 2. Write the word equation for photosynthesis. Answer: Carbon dioxide + water → glucose + oxygen (using light energy). 3. State two human activities that add carbon dioxide to the atmosphere. Answer: Burning fossil fuels such as coal, oil and gas, and cutting down and burning forests (deforestation). 4. Explain how cutting down large areas of forest can increase the carbon dioxide level in air. Answer: Fewer trees means less photosynthesis to remove carbon dioxide, and burning or decay of the cleared wood releases more carbon dioxide.