 carbon dioxide. CO2 is everywhere. We breathe out CO2. Trees breathe in CO2. CO2 is in our air. We have really since the Industrial Revolution been emitting more and more CO2 into our atmosphere as we drive our cars. When we use gasoline, when we use petrol, as we operate our factories, our power plants. CO2 is also released when we make things. When we make things like iron and steel. When we make things like cements, chemicals. When we run our power plants with coal and natural gas. And we burn that coal, we burn that natural gas. Out the other end comes CO2. Carbon dioxide is absorbed by different living things on earth by plants, for example by oceans. When it is in the atmosphere, it acts as a warming agent. It's actually a very important aspect of sustaining life on earth. Without this greenhouse gas we will be very cold. So it all works perfectly in sort of equilibrium. The issue is when we start having too much carbon dioxide and basically we release CO2 at a rate which is much greater than the rate at which we absorb CO2 in all these natural things. Forest, land, oceans. And so we're getting to a point where we are warming our atmosphere much faster than we should. And this is putting upward pressure and contributing to global warming which is causing climate change.