Effects of human land-use on the global carbon cycle during the last 6,000 years

Humanity has become a major player within the Earth system, particularly by transforming large parts of the land surface and by altering the gaseous composition of the atmosphere. Deforestation for agricultural purposes started thousands of years ago and might have resulted in a detectable human influence on climate much earlier than the industrial revolution. This study presents a first attempt to estimate the impact of human land-use on the global carbon cycle over the last 6,000 years. A global gridded data set for the spread of permanent and non-permanent agriculture over this time period was developed and integrated within the Lund-Potsdam-Jena Dynamic Global Vegetation Model (LPJ-DGVM). The model was run with and without human land-use, and the difference in terrestrial carbon storage was calculated as an estimate of anthropogenic carbon release to the atmosphere. The modelled total carbon release during the industrial period (A.D. 1850-1990) was 148 gigatons of carbon (GtC), of which 33 GtC originated from non-permanent agriculture. For pre-industrial times (4000 B.C.-A.D. 1850), the net carbon release was 79 GtC from permanent agriculture with an additional 35 GtC from non-permanent agriculture. The modelled pre-industrial carbon release was considerably lower than would be required for a substantial influence on the climate system.