evidence: Scientists know that the source of CO₂ in the atmosphere is the result of human activity (e.g. burning fossil fuels to produce electricity, transport, and industrial processes). This is because they can analyse the different isotopes of carbon in atmospheric CO₂ to understand their sources. [...] On the other hand, animals – including humans – breathe out CO₂ in the process of respiration. When plants and animals die, their stored carbon is also released as CO₂. Natural processes such as respiration and decay, forest fires and volcanic eruptions add an additional 190.2 billion tonnes of CO₂ to the atmosphere per year. [...] Humans generate CO₂ when burning fossil fuels such as gas, petrol, oil, and coal. This adds an additional 9.1 billion tonnes of CO₂ to the atmosphere each year.