In the same way as many activities emit greenhouse gases, others have the potential to absorb them. For instance, trees feed on carbon dioxide, so forests absorb large quantities of carbon. This absorption capacity can be quantified in tons, the same way emissions are calculated. Each ton of carbon absorbed from the atmosphere constitutes a carbon credit, or carbon offset. Thanks to this accounting system, people and organizations can purchase carbon credits to make up for emissions they cannot avoid: this is called carbon offsetting.