Frequently asked questions
The basic questions about CO₂, the greenhouse effect and forests, explained in plain words and with this site's own numbers.
What is CO₂?
CO₂, or carbon dioxide, is a natural gas in the atmosphere: invisible, odourless, produced when coal, oil and gas are burned, when we breathe, or when a volcano erupts. It is also the raw material of photosynthesis: plants absorb it to grow. The problem is not that it exists — life on Earth needs it — but how much of it there is: we are putting far more into the air than the planet can reabsorb.
Why is too much CO₂ in the air a problem?
CO₂ lets sunlight in but traps some of the heat the Earth radiates back towards space: that is the greenhouse effect, the same reason a greenhouse or a car in the sun heats up. Without it, the planet would be too cold for life as we know it. The problem is the amount: today the atmosphere holds 427 parts per million (as of 2025), against about 280 before the industrial revolution — 35% more. The more CO₂ there is, the more heat stays trapped, and the higher the average temperature climbs.
Where does the CO₂ we emit come from?
Mostly from fossil fuels: coal, oil and gas burned to produce energy, move around, heat buildings. Over the past ten years this source has accounted for about 88% of global CO₂ emissions; the rest, about 12%, comes from land-use change — mainly deforestation, when cut or burned trees release the carbon they built up over years of growth.
How is CO₂ absorbed? (by plants, oceans...)
About half the CO₂ emitted each year is reabsorbed: oceans dissolve part of it, plants and soils absorb another part through photosynthesis — as a tree grows, it pulls carbon out of the air and stores it in wood and roots. The world's forests alone absorb 14.4 billion tonnes of CO₂ a year on average (data to 2025). Whatever oceans and plants cannot absorb stays in the atmosphere, and that is the part that pushes the ppm figure up year after year.
What does "ppm" mean?
Ppm means parts per million: how many CO₂ molecules there are for every million molecules of air. A value of 427 ppm (as of 2025) means that out of a million air molecules, about 427 are CO₂ — a small share, but enough to change the climate, because CO₂ traps heat very effectively even in tiny amounts.
Why does CO₂ per person vary so much between countries?
It depends on how a country produces energy and how much of it is used: industrialised countries and major oil and gas producers tend to have the highest emissions per person, while lower-income countries, which use less energy, usually have far less. This site's map compares this indicator, and 18 others, for nearly 170 countries.
Does deforestation push up CO₂ in the air?
Yes, in two ways: when a tree is cut or burned, the carbon it stored for years returns to the atmosphere; and that tree stops absorbing new carbon in future. It is a double effect — release plus lost absorption. This site's correlation lab shows the real relationship between forests lost and CO₂ growth, year by year.
Is the correlation between CO₂ and temperature just a statistical coincidence?
No: it is a physical mechanism measured in the lab for over a century, not just a statistical observation. CO₂ absorbs and re-emits heat at specific wavelengths — basic chemistry, known since the 1800s. The correlation in the data — very high, because both series grow over time — confirms what physics already predicts; it is not the main evidence for it.
Where can I find the data cited on this site?
All the data is open, downloadable as CSV, with the original sources listed one by one, on the Open data and method page.