Fig. 1. Number of documents published (Y-axis) on environmental footprints from 1996 to 2018 (X-axis) in Science Citation Index Expanded (SCI-EXPANDED) or Social Sciences Citation Index (SSCI). Footprints are depicted in different panels due to the different magnitude of the number of documents: (a) the three most published footprints; (b) other footprints with less publications and (c) umbrella terms “environmental footprint” and “footprint family”. Publications using terms close to “footprint”, such as “embedded resource” or “virtual resource”, are omitted.