These figures prove that a net volume of around 28 gigatons of carbon was released into the atmosphere as a result of agricultural development in the pre-industrial period of the last millennium. These emissions remained very small for hundreds of years, and it was only during the period between the 16th and 18th centuries that they affected the concentration of atmospheric carbon dioxide beyond a level that could be explained by natural climate variations alone. As a result, it would appear that humans did not increase the carbon dioxide concentration in the atmosphere until a relatively late point in time – albeit still prior to the advent of industrialization. However, this increase in carbon dioxide was too small to perceptibly alter the temperature at the global level.
At the regional level, in contrast, humans already influenced the climate prior to industrialization. Simulations show that, due to the changes in the albedo of the land surface through land use, mankind altered the energy balance in some regions as early as a thousand years ago. In Europe, India and China, in particular, the amount of absorbed solar radiation decreased by around two watts per square meter. A change of this magnitude at the regional level is just as large as the current global greenhouse effect; however, it has the opposite impact, as it causes cooling rather than warming.
Even historical events can leave their traces on the climate through such biogeophysical effects. For example, there was a clear reversal of the increasing human influence on Europe’s energy balance in the 14th century. This change was brought about by the bubonic plague, which claimed the lives of around one third of the population and in the wake of which large expanses of agricultural land were temporarily abandoned. The Mongol invasion of China in the 13th century and the diseases spread among the high cultures of the Americas by the invasion of the Europeans had similar consequences.