Posted by: Johan Normark | October 15, 2011

Depopulation of the Americas is blamed for the Little Ice Age

Correlating climate change with local, regional, and global changes in human history is not an easy task and should in my view be kept at a minimum. A good example of this problem can be seen in a recent article that reports on research by Richard Nevle, a geochemist at Stanford University. He proposes an interesting explanation for the emergence of the Little Ice Age but I do not believe he is correct. Most explanations for this period of global cooling propose either a decrease in solar activity, increased volcanic activity, or colder oceans that absorbed more carbon dioxide.

Nevle suggests that the European colonization of the Americas initially led to the depopulation of the indigenous population (true) and reforestation (partly true). Since up to 90% of the indigenous population died within a century after the conquest, through pandemics and other causes, their untended lands are believed to have been filled by trees. The new forests are then believed to have absorbed billions of tons of carbon dioxide from the atmosphere and this process lowered the temperature on a global scale since the heat-trapping capacity diminished. This global cooling is argued to have initiated the Little Ice Age.

It is suggested that the only global anthropogenic events that could explain the sudden drop in atmospheric carbon dioxide during the 16th and 17th centuries is the initial colonization of the Americas. In this article it is argued that many indigenous farmers “burned trees to make room for crops, leaving behind charcoal deposits that have been found in the soils of Mexico, Nicaragua and other countries.”

There are several problems in this model. The first, and perhaps most problematic, is that we do not know when the Little Ice Age began. Some researchers trace it back to 1250 and for the Maya region Hodell and others have suggested that the Little Ice Age caused droughts and other events in the early to mid 15th century, about a century before the arrival of the Spaniards. In conclusion, the Little Ice Age began before Columbus.

Nevle appears not to be familiar with differences in land use before and after the conquest. True, Maya relied partly on swidden agriculture, but they also practiced horticulture that did not involve large scale clearings. Most of the land in the Maya area was not deforested at the time of the conquest. In comparison the population was much lower than at the time of the Terminal Classic collapse. Now, the 9th-11th century depopulation and forest regrowth coincided with the beginning of the Medieval Warm Period. Sure, this was a smaller region than that affected by the early European colonization, but the reverse trend in climate after the Maya collapse casts doubt on Nevle’s generalized model. Further, the Spaniards did not let the forests regrow where they settled. They introduced cattle that kept the forests at bay. A century after the conquest there were more cattle in the Valley of Central Mexico than there were people.

In the comment section of the article some people also bring up the increased need for wood for ships that most likely was more devastating than previous indigenous agricultural practices. The North American colonies exported lumber and the export of tobacco and cotton also needed deforestation. The largest tracts of forest in the world, the Amazon, had supported some large settlements during the Prehispanic period but these had transformed the infertile lands into fertile black soils. These settlements appear to have been abandoned long before the conquest and due to the later demographic collapse after the Spaniards arrived people became far more mobile. This mobility did not allow fertile black soils to be created. Since the soil was poor larger tracts of forests needed to be cleared in relation to population size when people became mobile.

My own research from the Maya area also indicates that the Spaniards attempted to control the mobility of the Maya population. Mobile farmers needed to burn down greater areas to support their livelihood. Hence, even though depopulation partly led to reforestation, the greater mobility among the survivors led to changes in land use which demanded that forests were cut down to a greater degree per capita.

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Responses

  1. Agree with you there. I dont think it has anything to do with deforestation (or the reverse). Since it started before any europeans arrived, there is no correlation there.

    And If you closely examine the TREND of the curve, the fall actually starts already at the peak of the medieval warm period in the 9th and 10th century… Theres a slow upbuild and a slow downfall that spans almost 2000 years. Theres probably a natural cycle going on there correlating with milancovitch cycles (and peaked with the know increase in solar spots in the 16th-18th century).

    But theres a lot we dont know about this stuff. So many types of problems where the m cycles dont match the known climate exists still.

  2. Some of the commentators in the article (probably climate sceptics) argued that this was rather a political statement attempting to project anthropogenic causes for climate change into the past as well.

  3. Theres actually a massive downfall in agriculture, and rise in forests from 1300-1600 because of the plauge, and as far as I know, that doesnt show.

    • I am suspicious of any attempt to correlate large scale human events with climate changes in the past. The are simply too many unknown factors that may counteract the effects of climate change.


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