How does carbon dioxide increase Earth's temperature?

Carbon dioxide (CO₂) is a greenhouse gas. This means that it causes an effect like the glass in a greenhouse, trapping heat and warming up the inside. This effect is important: without the CO₂ that naturally exists in the atmosphere, Earth might be too cold to support human life. However, the atmosphere is very sensitive to changing levels of CO₂. Even though this gas makes up less than 0.1% of the atmosphere, it can have a huge effect on how much heat the planet's surface retains.

When energy from the Sun reaches the top of our atmosphere, most of it passes through to Earth's surface, where it is absorbed. Some of this energy is re-emitted, heading back towards space. At this stage, it interacts with molecules of CO₂ in a way that prevents some of it from escaping Earth's atmosphere. The trapped heat energy leads to increased average global surface air temperatures.

One reason carbon dioxide has such a big impact on global temperatures is that hotter air can hold more water vapour. Water vapour is itself a greenhouse gas, which further enhances the greenhouse effect.

While the presence of carbon dioxide in Earth's atmosphere is natural, the rising levels since the Industrial Revolution in the 1800s are due to human activities, primarily the burning of fossil fuels such as coal and oil.