The vegetation covering the continents has a decisive influence on the climate. Through the uptake of CO2 from the atmosphere, plants play a central role in the global carbon cycle. Furthermore, they influence the exchange of energy, water, and momentum between the atmosphere and the land surface. Via land use, humans are altering these processes. The study presented here investigates the expansion of agriculture over the last millennium. It shows that humans had a strong influence on climate, especially on the regional scale, already before industrialization began.