22 Nov 2010, 12:01pm
Cultural Landscapes Fire History
by admin

The Columbian Encounter and the Little Ice Age: Abrupt Land Use Change, Fire, and Greenhouse Forcing

Robert A. Dull, Richard J. Nevle, William I. Woods, Dennis K. Bird, Shiri Avnery, and William M. Denevan. 2010. The Columbian Encounter and the Little Ice Age: Abrupt Land Use Change, Fire, and Greenhouse Forcing. Annals of the Association of American Geographers, 100(4) 2010, pp. 1–17.

Full text [here]

Selected excerpts:


Pre-Columbian farmers of the Neotropical lowlands numbered an estimated 25 million by 1492, with at least 80 percent living within forest biomes. It is now well established that significant areas of Neotropical forests were cleared and burned to facilitate agricultural activities before the arrival of Europeans. Paleoecological and archaeological evidence shows that demographic pressure on forest resources—facilitated by anthropogenic burning—increased steadily throughout the Late Holocene, peaking when Europeans arrived in the late fifteenth century. The introduction of Old World diseases led to recurrent epidemics and resulted in an unprecedented population crash throughout the Neotropics. The rapid demographic collapse was mostly complete by 1650, by which time it is estimated that about 95 percent of all indigenous inhabitants of the region had perished. We review fire history records from throughout the Neotropical lowlands and report new high-resolution charcoal records and demographic estimates that together support the idea that the Neotropical lowlands went from being a net source of CO2 to the atmosphere before Columbus to a net carbon sink for several centuries following the Columbian encounter. We argue that the regrowth of Neotropical forests following the Columbian encounter led to terrestrial biospheric carbon sequestration on the order of 2 to 5 Pg C, thereby contributing to the well-documented decrease in atmospheric CO2 recorded in Antarctic ice cores from about 1500 through 1750, a trend previously attributed exclusively to decreases in solar irradiance and an increase in global volcanic activity. We conclude that the post-Columbian carbon sequestration event was a significant forcing mechanism.


… Widespread biomass burning and agricultural forest clearance predate the Industrial Revolution by several millennia in the Americas, Africa, Asia, Europe, and Australia, as well as on many oceanic islands (Sauer 1958; Crutzen and Andreae 1990; Goldhammer 1991; Chew 2001; Saarnak 2001; Williams 2003). The idea that preindustrial Holocene land use could have produced quantities of atmospheric CO2 and CH4 sufficient to impact the climate system was first outlined by Ruddiman’s (2003) seminal paper, “The Anthropogenic Greenhouse Era Began Thousands of Years Ago.” Although evidence mounts for a preindustrial Anthropocene (Ruddiman 2003, 2005, 2007; Faust et al. 2006; van Hoof et al. 2006; Nevle and Bird 2008; van Hoof et al. 2008; Vavrus, Ruddiman, and Kutzbach 2008), some critics maintain that human impacts in terms of climate forcing were negligible until the nineteenth century (Broecker and Stocker 2006; Olofsson and Hickler 2008; Elsig et al. 2009; Stocker, Strassmann, and Joos 2010).

If pre-industrial farmers did contribute measurably to the greenhouse effect via increased emissions of CO2 and methane, only a massive and catastrophic collapse of agricultural populations could have led to significant decreases in anthropogenic emissions at any time. The post-Columbian encounter epidemics and pandemics were certainly the most rapid, thorough, and widespread to have occurred during the late Holocene (Crosby 1972; Lovell 1992), resulting in a loss of approximately 90 to 95 percent of the agricultural population throughout the Neotropics (Dobyns 1966; Lovell and Lutz 1995). The sixteenth- and seventeenth-century epidemics resulted in the abrupt abandonment of agricultural clearings in otherwise forested landscapes together with an unprecedented reduction in human fire ignitions, thus providing an ideal scenario for backcasting anthropogenic climate forcing before European contact. The widespread forest recovery that followed the native population crash after the Columbian encounter resulted in elevated biospheric sequestration of atmospheric CO2 in plant biomass because (1) forests rapidly reoccupied abandoned cultivated landscapes via secondary succession, and (2) existing forests became more carbon dense due to a reduction in wildfires related to anthropogenic fire ignitions, both intentional and accidental.

In this article we review the evidence for prehistoric anthropogenic biomass burning in the Neotropics and provide new data supporting the thesis that the aggregate carbon footprint of Neotropical farmers was sufficient to raise global temperatures via greenhouse forcing before the Columbian encounter. Furthermore, we argue that Little Ice Age (LIA) cooling and the attendant atmospheric CO2 decrease can be explained in part by biospheric carbon sequestration following the native population collapse. The LIA is identified in surface temperature reconstructions of the past millennium as a global thermal anomaly of about -0.1 deg C in which cooling was most pronounced from 1550 to 1750 AD in the Northern Hemisphere, particularly in northern Europe (Esper, Cook, and Schweingruber 2002; Jones and Mann 2004; Moberg et al. 2005). Atmospheric CO2 concentration decreased by ~7 ppm during the same period (Meure et al. 2006). Previously, the LIA thermal anomaly and concomitant decrease in atmospheric CO2 concentrations were attributed to solar–volcanic forcing (Joos et al. 1999; Hunt and Elliott 2002; Von Storch et al. 2004). Recent analyses, however, suggest that variations in solar luminosity are insufficient to drive significant climate variations on centennial to millennial timescales (Foukal et al. 2006), and were likely a negligible climate forcing factor at the onset of the LIA (Ammann et al. 2007). We conclude that the fifteenth- and sixteenth-century arrivals of Europeans in the Americas set into motion an unprecedented anthropogenic carbon sequestration event and contributed to the LIA climate anomaly. This event represents perhaps the best example of anthropogenic influence on Earth’s climate system during the pre-industrial period. …

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