SALT LAKE CITY, July 30, 2012 – In the years after Columbus' voyage, burning of New World forests and fields diminished significantly – a phenomenon some have attributed to decimation of native populations by European diseases. But a new University of Utah-led study suggests global cooling resulted in fewer fires because both preceded Columbus in many regions worldwide.
"The drop in fire [after about A.D. 1500] has been linked previously to the population collapse. We're saying no, there is enough independent evidence that the drop in fire was caused by cooling climate," says the study's principal author, Mitchell Power, an assistant professor of geography at the University of Utah.
"The implication is that climate is a large-scale driver of fire. That's a key finding. Climate is driving fire on global and continental scales," says Power, who also is curator of the Garrett Herbarium at the Natural History Museum of Utah, which is part of the University of Utah.
The new study analyzed worldwide charcoal samples spanning 2,000 years. It will be published online during August in the journal The Holocene, which is the name of the geological epoch covering roughly the last 11,500 years of Earth's history. It was funded by the National Science Foundation and the Natural History Museum of Utah.
The study deals with the Little Ice Age, a period when Earth's climate cooled, causing New York Harbor to freeze over in 1780, among other effects. Estimates of when the Little Ice Age started range from the 1200s to the 1500s. It ended in the early 1800s. Possible causes include some combination of increased dust from volcanic eruptions, decreased solar activity, and changes in circulation of the ocean and atmosphere.
"The decrease in fire on a very large scale – globally and in the Americas – was controlled by this cooling climate, which began prior to the population collapse, and climate alone is sufficient to explain large scale changes in burning," says Power.
"In a cooler atmosphere, you tend to get reduced convection, so you get reduced thunderstorms and ignition from lightning," he says. "Cooler climate also tends to maintain high levels of fuel moisture and soil moisture."
Today, warming climate and drought have been tied to increasing fires in the U.S. West and elsewhere. "In a world where climate is rapidly changing we need to pay more attention to this relationship between climate and fire," Power says
Power conducted the study with 19 other scientists, including paleoecologist Frank Mayle at the University of Edinburgh, U.K., and climatologist Patrick Bartlein at the University of Oregon. Other coauthors – who provided charcoal data or samples – are from University of Wisconsin, Madison and Oshkosh; Northern Arizona University; University of Gottingen, Germany; Canadian Forest Service; University of Montpellier, France; University of Bern, Switzerland; University of Calgary, Canada; University of Tennessee; Virginia Tech; University of North Carolina; University of Chile; Laval University, Quebec; Fordham College, New York; and Central Washington University.
Cooling Climate or Population Collapse?
After Columbus reached the New World in 1492, explorers brought European diseases such as smallpox that "decimated populations in the Americas – 10 million to 100 million dead, with most estimates in the 60 million range," Power says.
"All these people died abruptly – Mayans, Incas, Aztecs and down in Patagonia – they were all affected," he adds. "Agriculture was sharply reduced. Landscapes that had been cleared for agriculture started a process of plants growing back and infilling those abandoned fields. In terms of greenhouse gases, when you change from maintained cropland to woodlands, plants take up more carbon dioxide and there is less in the atmosphere. This has been pointed to as one mechanism for causing the Little Ice Age."
Power agrees population collapse may have led to reduced biomass burning in some local regions of the Americas. But the new study indicates the reduction in fire was actually global and began before Columbus in most areas, suggesting the Little Ice Age triggered most of the reduction in burning – not the other way around, Power says.
"If you look at independent climate records, cooling from the Little Ice Age was happening about 200 years before the population collapse," or about A.D. 1300, he says.
Power notes there is room for debate because the Little Ice Age varied in time and space, and didn't affect all parts of the world equally, although most places cooled.
A Record of Fire Left in Charcoal
The study used existing records and-or new samples of charcoal – burnt wood or other biomass – found in sediment cores from lake bottoms and bogs from some 600 sites around the world, about half in the Americas, and dated within the past 2,000 years.
"Whatever was burning, we see a record of that fire in lake sediments, from either aerial transport or erosion" of burned material, Power says.
Power manages the Global Charcoal Database that compiles data from all the existing studies that date charcoal samples and describe where they came from. The new study included 498 existing charcoal records and 93 new samples.
"We have gone back in and calculated the ages of all these charcoal samples," except for some dated independently in other recent studies, and then used recent radiocarbon dating calibrations to make sure all data are consistent, Power says.
"Greater than 80 percent of biomass burning records show a decline post-1500 in the Americas, he says. The other 20 percent may be from areas that were still fire-prone despite cooling or that simply had burning declines for which there are inadequate charcoal samples, he adds.
The study compared the charcoal records with previously published ancient climate records and population reconstructions. It found:
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