Asked by: Peter Thatcher, Lancaster


In one day, the average person breathes out around 500 litres of the greenhouse gas CO2 – which amounts to around 1kg in mass. This doesn’t sound much until you take into account the fact that the world’s population is around 6.8 billion, collectively breathing out around 2500 million tonnes of the stuff each year – which is around 7 per cent of the annual CO2 tonnage churned out by the burning of fossil fuel around the world.

So, on the face of it, we humans are a significant contributor to global warming. But, in reality, the CO2 we’re breathing out is part of a natural cycle, by which our bodies convert carbohydrates from CO2-absorbing plants into energy, plus water and CO2. As such, we’re not adding any extra CO2. In contrast, burning fossil fuels like coal releases CO2 which has been locked up for millions of years, producing a net contribution to global warming.

Read more:


Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun science facts.