Holocene carbon emissions as a result of anthropogenic land cover change

Kees Klein Goldewijk,Jed O. Kaplan,Kristen M. Krumhardt,Carsten Lemmen,William F. Ruddiman,Erle C. Ellis

doi:10.1177/0959683610386983

Abstract

Humans have altered the Earth’s land surface since the Paleolithic mainly by clearing woody vegetation first to improve hunting and gathering opportunities, and later to provide agricultural cropland. In the Holocene, agriculture was established on nearly all continents and led to widespread modification of terrestrial ecosystems. To quantify the role that humans played in the global carbon cycle over the Holocene, we developed a new, annually resolved inventory of anthropogenic land cover change from 8000 years ago to the beginning of large-scale industrialization (ad 1850). This inventory is based on a simple relationship between population and land use observed in several European countries over preindustrial time. Using this data set, and an alternative scenario based on the HYDE 3.1 land use data base, we forced the LPJ dynamic global vegetation model in a series of continuous simulations to evaluate the impacts of humans on terrestrial carbon storage during the preindustrial Holocene. Our model setup allowed us to quantify the importance of land degradation caused by repeated episodes of land use followed by abandonment. By 3 ka BP, cumulative carbon emissions caused by anthropogenic land cover change in our new scenario ranged between 84 and 102 Pg, translating to c. 7 ppm of atmospheric CO2. By ad 1850, emissions were 325–357 Pg in the new scenario, in contrast to 137–189 Pg when driven by HYDE. Regional events that resulted in local emissions or uptake of carbon were often balanced by contrasting patterns in other parts of the world. While we cannot close the carbon budget in the current study, simulated cumulative anthropogenic emissions over the preindustrial Holocene are consistent with the ice core record of atmospheric δ13CO2 and support the hypothesis that anthropogenic activities led to the stabilization of atmospheric CO2 concentrations at a level that made the world substantially warmer than it otherwise would be.

Highlights

T o quantify the role that humans played in the global carbon cycle over the Holocene, we developed a new, annually resolved inventory of anthropogenic land cover change from 8000 years ago to the beginning of large-scale industrialization
An alternative scenario based on the HYDE 3.1 land use data base, we forced the LPJ dynamic global vegetation model in a series of continuous simulations to evaluate the impacts of humans on terrestrial carbon storage during the preindustrial Holocene
In order to quantify anthropogenically induced land cover change (ALCC) emissions over the Holocene we (1) developed a new scenario of global ALCC from 8 ka bp to ad 1850 based on existing methodology, (2) assembled climate, soils and CO data used to drive a Dynamic Global Vegetation Model (DGVM), (3) modified the LPJ DGVM to handle ALCC and made several other small improvements to the model, and (4) ran the model in a number of experiments to characterize the range of possible emissions scenarios

Summary

Introduction

Several attempts have been made to reconstruct the history of anthropogenically induced land cover change (ALCC), and in some cases the resulting CO2 emissions, both in the industrial era and in preceding centuries ( Houghton, 2003; Houghton et al, 1999; Klein Goldewijk, 2001; Pongratz et al, 2008; Ramankutty and Foley, 1998, 1999; Strassmann et al, 2008). Because most countries lack reliable land use surveys prior to the middle of the twentieth century, these reconstructions rely on hindcasting techniques based on estimated historical populations and assumptions about how people used the land. Land use estimates based on this assumption inevitably show little human land use prior to the exponential population explosion that began near ad 1500 and accelerated through the present day (Figure 1). This finding has led to the conclusion that forest clearance by humans could not have played a significant role in the gradual rise of CO concentrations that began during the middle Holocene about 8 ka Atmospheric CO increased by ~22 ppm from the 8 ka minimum to the start of the industrial era (Ruddiman, 2007)

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