What Is Carbon Dioxide? Definition, Sources & Why It Matters for Emissions Tracking

Carbon dioxide (CO₂) is a colourless, odourless gas made up of one carbon atom and two oxygen atoms. It exists naturally in Earth’s atmosphere and plays an essential role in the planet’s life systems.

Plants absorb CO₂ during photosynthesis to produce energy, making it critical for growth and food production. Natural processes such as animal respiration, volcanic eruptions and the breakdown of organic material all release carbon dioxide into the air.

Where Does Carbon Dioxide Come From?

Carbon dioxide is released into the atmosphere through both natural processes and human activity. These sources are part of the global carbon cycle, but human emissions have tipped the balance, leading to more CO₂ in the air than the planet can absorb.

Natural sources of carbon dioxide

Natural sources of carbon dioxide include processes that have occurred for millions of years, contributing to Earth’s climate system without causing imbalance:

• The decomposition of dead plants and animals, which releases carbon stored in organic matter
• Wildfires, which release carbon dioxide and other gases as trees and vegetation burn
• The world’s oceans, which exchange CO₂ with the atmosphere depending on water temperature and movement
• Volcanic eruptions, which emit carbon dioxide stored in the Earth’s mantle

Human sources of carbon dioxide

Human sources, in contrast, have increased significantly in the past two centuries and are the primary cause of rising atmospheric CO₂ levels:

• Burning fossil fuels such as coal, oil and natural gas for transport, electricity generation and heating
• Industrial processes, especially the production of cement and steel, which release CO₂ through both energy use and chemical reactions
• Land use changes, including deforestation and land degradation, which reduce the number of trees and plants available to absorb CO₂ from the air

How Much CO₂ Is in the Atmosphere?

As of 2024, the amount of carbon dioxide in the atmosphere is around 420 parts per million (ppm). While this may sound small, it marks a dramatic increase from pre-industrial levels of roughly 280 ppm. Even small shifts in atmospheric CO₂ can cause major changes in global temperature because of the gas’s heat-trapping properties.

Current levels of CO₂ are the highest in millions of years. Historical records from ice cores show that such concentrations are closely linked with changes in global climate.

Why Does Carbon Dioxide Matter for Climate Change?

Carbon dioxide is a greenhouse gas. It traps heat in Earth’s atmosphere, preventing it from escaping into space. This process, known as the greenhouse effect, helps keep the planet warm enough to support life. However, when there is too much CO₂, this balance is disrupted. Excess heat is retained, leading to rising temperatures, shifting weather patterns and environmental instability.

What makes CO₂ particularly challenging is that it remains in the atmosphere for hundreds to thousands of years. This long lifespan means that today’s emissions will continue to affect the climate far into the future.

Reducing emissions is a central part of addressing this challenge. To understand how this works, read more about what carbon reduction means and how we can build a low carbon future.

How Is CO₂ Measured and Tracked?

Carbon dioxide is measured and tracked through a process known as carbon accounting. This involves calculating emissions from various sources and converting them into standard units to compare and manage. Because different greenhouse gases have different warming effects, many systems use CO₂ equivalents (CO₂e) to express their combined impact.

At the national level, governments track emissions to set targets and report progress, such as through the UK’s legally binding carbon budgets. Find the updated local UK emissions on our website.

What Can Be Done to Reduce CO₂ Levels?

Reducing carbon dioxide in the atmosphere requires action across multiple areas:

• Switching to renewable energy such as wind, solar and tidal power
• Enhancing energy efficiency in buildings, transport and industry
• Restoring natural carbon sinks like forests, peatlands and healthy soils
• Using carbon capture and storage technologies to remove and store CO₂ from power stations or the air

FAQs

What is carbon dioxide and what does it do?

Carbon dioxide is a natural gas that helps regulate Earth’s temperature. It is essential for processes like photosynthesis, but in excess it traps too much heat, contributing to climate change.

Where does carbon dioxide come from?

Carbon dioxide comes from natural sources such as decomposition, ocean release and volcanoes. Human activities like burning fossil fuels, industrial production and deforestation have greatly increased CO₂ levels.

How much carbon dioxide is in the atmosphere today?

As of 2024, there are approximately 420 parts per million of carbon dioxide in the atmosphere, which is far above historical averages.

Why is carbon dioxide a problem for the climate?

Carbon dioxide is a problem because it traps heat in the atmosphere and remains there for centuries. This leads to global warming, more extreme weather and rising sea levels.

How is carbon dioxide measured and what can we do about it?

Carbon dioxide is measured through carbon accounting and expressed in CO₂ equivalents. We can reduce emissions by using renewable energy, improving efficiency, protecting nature and investing in carbon capture.

Sources:

• https://climate.nasa.gov/vital-signs/carbon-dioxide/?intent=121
• https://climate.mit.edu/ask-mit/how-can-such-small-amount-carbon-dioxide-atmosphere-only-around-420-parts-million-cause-so

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