The isotope curve shows that the emissions of the greenhouse gas methane had several peaks in the last 2,100 years. 1: During Roman times, where a lot of wood was burned for heating and for the processing of metals. 2: During the warm Middle Ages, where forests caught on fire. 3: In the "Little Ice Age", which was a very cold and dry period. 4: The methane concentration has increased dramatically since approx. the year 1800, when industrialization took off and triggered energy and food production, for example, rice fields.