Recovery of forests following the collapse of human populations in the Americas after the arrival of Europeans may have driven the period of global cooling from 1500-1750 known as the Little Ice Age, report researchers speaking at the annual meeting of the American Geophysical Union in San Francisco.
By some estimates, diseases introduced by Europeans may have killed more than 90 percent of population on the New World within a century of first contact. The rapid depopulation led to large-scale abandonment, and subsequent reforestation, of agricultural lands in the Americas. Analyzing charcoal found in soils and lake sediments at sites across the Americas, Richard Nevle and Dennis Bird found evidence to suggest that this forest regeneration sequestered enough carbon to trigger global cooling.
“We estimate that the amount of carbon sequestered in the growing forests was about 10 to 50 percent of the total carbon that would have needed to come out of the atmosphere and oceans at that time to account for the observed changes in carbon dioxide concentrations,” said Nevle, a visiting scholar in the Department of Geological and Environmental Sciences at Stanford.
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The researchers focused specifically on changes in charcoal deposits at their study sites. Charcoal deposits were on the rise, indicating increasing conversion of forest for cropland, until 500 years ago when they showed an abrupt drop, coinciding with the human population crash in the Americas. The researchers compared the timing with the climate record and found a close correlation.
“We looked at ice cores and tropical sponge records, which give us reliable proxies for the carbon isotope composition of atmospheric carbon dioxide. And it jumped out at us right away,” Nevle said. “We saw a conspicuous increase in the isotope ratio of heavy carbon to light carbon. That gave us a sense that maybe we were looking at the right thing, because that is exactly what you would expect from reforestation.”
Due to the tendency of plants to use the lighter form of carbon during photosynthesis, the change in the ratio of heavy carbon to light carbon in the atmosphere reflects a massive increase in global plant growth.
The researchers say their data lines up better with the historic climate record than other theories proposed to explain the Little Ice Age, including changes in sunlight and an increase in volcanic activity in the late 16th century.
“The timing of these events doesn’t fit with the observed onset of the carbon dioxide drop,” wrote Louis Bergeron in a statement released by Stanford. “These events don’t begin until at least a century after carbon dioxide in the atmosphere began to decline and the ratio of heavy to light carbon isotopes in atmospheric carbon dioxide begins to increase.”
Nevle says that while these factors may have influenced climate, “reforestation is certainly a first-order contributor.”
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