February 15, 2010
Julia Pongratz, Christian Reick (Max Planck Institute for Meteorology)
The study of church registers is an activity that is generally associated with theologians and genealogists rather than with natural scientists. Indeed, most people would find it very hard to imagine that such registers also contain important information for climate researchers. But these records, which go back many centuries, contain important information about population development and thus also about how much land area was used for agriculture. However, transforming natural vegetation into agricultural lands has consequences for the climate. We climate researchers are thus very fortunate that demographers have already done the work in recent decades and derived data on global population development from historical documents. Based on this data, we can deduce information about the human influence on climate many centuries into the past.
The pre-industrial era is particularly suitable for analyzing the consequences of land use on climate. Prior to 1850, the global expansion of agriculture was the only “manmade” disturbance to the global climate system. Since the expansion of agriculture often required the clearing of forests, the carbon stored in the wood eventually ended up in the atmosphere as part of carbon dioxide, a greenhouse gas. It was not until the mid-20th century that the amount of carbon dioxide that humans released into the atmosphere by burning fossil fuels significantly exceeded that caused by agricultural expansion. Since then, the global climate change that is now being observed has been caused largely by emissions arising from the combustion of coal, oil, and gas.