Monday, 26 September 2011

Stone axes and the Little Ice Age (LIA)
What do stone axes have to do with the LIA ?

In his famous paper entitled “The anthropogenic greenhouse era began thousands of years ago” Ruddiman [2003] put forward a fascinating idea: “CO2 oscillations of 10 ppm in the last 1000 years are too large to be explained by external (solar-volcanic) forcing, but they can be explained by outbreaks of bubonic plague that caused historically documented farm abandonment in western Eurasia. Forest regrowth on abandoned farms sequestered enough carbon to account for the observed CO2 decreases. Plague-driven CO2 changes were also a significant causal factor in temperature changes during the Little Ice Age (1300–1900 AD)”. There has been a lot of controversy surrounding Ruddiman’s paper.
More recently, the idea that plagues caused farmland abandonment and were followed by re-forestation has been applied to the Amazon Basin and the LIA. Several scholars have proposed that the depopulation caused by the diseases that Europeans brought to the Americas after 1492 induced a large scale re-forestation which, in turn, decreased the amount of atmospheric CO2 and contributed to the LIA [Dull et al., 2010; Faust et al., 2006; Nevle and Bird, 2008]. In order to assess the likelihood of this hypothesis we need to know i) population size in pre-Columbian America and ii) the kind of agriculture pre-Columbians practiced.
Citing Denevan, Nevle and Bird [2008] write that “Evidence for the habitation and modification of American landscapes by tens of millions of Pre-Columbian agriculturalists [Denevan, 1992] exists in the widespread distribution of anthropogenic Amazonian Dark Earth soils, raised fields, irrigated terrace zones, roads, aqueducts, and numerous large-scale earthworks distributed throughout Amazonia, the Andes, Central America, and parts of North America”.
Many of the papers addressing this topic cite Denevan with regards to pre-Columbian population densities and agriculture. So, what are Denevan’s views on the matter? I will focus on Amazonia, as it is the largest forested area in the world and most of the work on pre-Columbian population density and agriculture that is cited to support this hypothesis have been done in Amazonia (for example the works of Denevan himself, Erickson and Heckenberger).
How many people lived in Amazonia in 1491?
The first estimate was given by Betty Meggers who said that population density in pre-Columbian Amazonia was 0.3 people Km-2. She didn’t do any distinction between floodplains (varzea) and uplands (terra firme) because varzea’s fertility was offset by unexpected and destructive floods, which made varzea as unsuitable for people as terra firme.
Denevan then proposed a model in which people settled on the rivers’ bluffs. They were able to take advantage of the varzea but avoided the danger of the floods. According to [Denevan, 1992] population density was 14.6 people km-2 in the varzea and 0.2 people km-2 in the terra firme forests. It is interesting that Denevan’s estimate for terra firme is lower than Meggers’ estimate. This is important as terra firme represents 98% of the Amazonian rain forest.
In 2003, Denevan changed idea and wrote: “For varzea population density would be 10.4 per square kilometer […] For terra firme forests it is impossible to estimate an average population density and a total population […] Estimating average population densities for the savannas with any confidence is impossible.” Then, he concluded: “...consequently I now reject the habitat-density method I used in the past to estimate a Greater Amazonia population in 1492 of from 5.1 to 6.8 million. I nevertheless still believe that a total of at least 5 to 6 million is reasonable” [Denevan, 2003].
The stone axes
Although Denevan has rejected his own estimate of 0.2 people km-2 for terra firme, it is still important to highlight how he justified that his estimate was smaller than Meggers’. Denevan defends that pre-Columbians did not practice slash and burn agriculture because they did not have metal tools and cutting the forest with stone axes would have been too much work. Hence, they preferred to live in savannahs, where they developed raised field agriculture, or on the river bluffs, where Amazonian Dark Earth (ADE) sites are actually found. In Denevan’s view, raised fields and ADE developed in order to minimize the need of clearing the forest: pre-Columbians preferred to build raised fields and ADE because such type of agricultural intensification required less work than cutting the forest with stone axes.
The very same archaeological evidence that Nevle and Bird [2008] use to infer high rates of pre-Columbian deforestation are used by Denevan to infer that pre-Columbians actually did not cut the forest!
The questions I have should now be clear: 1) could have such a small population of 0.2 people km-2 significantly modified the Amazon forests? 2) How did they have such an impact if they had to cut the forest with stone axes? 3) Do raised field agriculture and ADE suggest high levels of deforestation? Or is it the other way round?
I don’t want to be misinterpreted here; I am not saying that pre-Columbian population was small or that they did not have an important impact on atmospheric CO2 content. I am just saying that we do not know how many people lived in Amazonia in 500 AD or in 1491AD and what impact they had on the Amazon forests. We need far more field and lab work before reliable estimates can be put forward. 

Denevan, W. M. (1992), The Native Population of the Americas in 1492, The University of Wisconsin Press, Madison.
Denevan, W. M. (2003), The native population of Amazonia in 1492 reconsidered, Revista de Indias, 63(227), 175-188.
Dull, R. A., R. J. Nevle, W. I. Woods, D. K. Bird, S. Avnery, and W. 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), 755-771.
Faust, F. X., C. Gnecco, H. Mannstein, and J. Stamm (2006), Evidence for the Postconquest Demographic Collapse of the Americas in Historical CO2 Levels, Earth Interactions, 10(11), 1-14.
Nevle, R. J., and D. K. Bird (2008), Effects of syn-pandemic fire reduction and reforestation in the tropical Americas on atmospheric CO2 during European conquest, Palaeogeography, Palaeoclimatology, Palaeoecology, 264(1-2), 25-38.
Ruddiman, W. (2003). The Anthropogenic Greenhouse Era Began Thousands of Years Ago Climatic Change, 61 (3), 261-293 DOI: 10.1023/B:CLIM.0000004577.17928.fa