Anthropogenic Deforestation and its Effect on the Carbon Cycle of Europe Over the Past Three Millennia

Over the past three millennia, both climate and anthropogenic land use and land cover change (LULUC) have substantially affected the European landscape. Anthropogenic deforestation for agriculture and pasture has been the most significant of these land cover changes, though climate variability itself may have had an impact on European ecosystems. In this study we attempt to quantify the influence of both LULUC and climate change on the carbon cycle of Europe during preindustrial time, and speculate on the ramifications for global atmospheric composition and biogeochemical feedbacks to the climate system. To quantify the effect of millennial-scale climate change and LULUC on the carbon cycle over the past three millennia, we assembled spatially explicit datasets of these quantities and ran a dynamic global vegetation model (LPJ-DGVM) in a number of experiments and sensitivity tests on a high-resolution grid for Europe. Climate data needed to run LPJ were synthesized from gridded datasets of mean monthly temperature and precipitation based on multiproxy climate reconstructions. Though it is certain that many European countries were substantially deforested before 1850, no coherent data set of the progression of deforestation that occurred during preindustrial time was available to us. We have therefore created a 10km, annually resolved gridded time series of European LULUC for the past three millennia by digitizing and synthesizing a database of population history for Europe and finding a relationship between population density, land quality for agricultural and pastoral activities, and anthropogenic deforestation. With these input data, we ran a series of experiments and sensitivity tests with LPJ to simulate the effect that changes in climate, LULUC and length- of-run (starting the run at 1700, 1850 or 1900) have on European carbon storage and its trajectory at year 2000. Climate variability in Europe over the past three millennia years caused modest reductions in carbon stored in living biomass, which outweigh increases in soil carbon, leading to a net loss of ca. 10 Pg of carbon over the most recent 500 years. In contrast, the time-history of increasing LULUC intensity over the past three millennia years leads to substantial reductions in both living biomass and soil carbon, including ca. 145 Pg over the most recent 500 years. Combining the effects of climate and land cover change results in a smaller total reduction in terrestrial carbon storage compared to LULUC only. Thus, climate change appears to ameliorate the amount of carbon lost after anthropogenic deforestation, as cooler temperatures suppress microbial respiration of soil organic matter. Length-of-run experiments indicate that the terrestrial biosphere is sensitive to the time history of both climate and LULUC.