Cooler Climate, Not Population Loss, Led To Less Fires In The Past
July 30, 2012

Cooler Climate, Not Population Loss, Led To Less Fires In The Past

Lawrence LeBlond for - Your Universe Online

After Columbus sailed to the New World, the burning of forests and fields subsided, a phenomenon that has been attributed to loss of native population by the onset of European diseases carried over by explorers and settlers.

But new research led by the University of Utah suggests that global cooling and not population decline resulted in fewer fires because both preceded Columbus in many regions around the world.

The team of researchers could not link a population collapse to reasons why fewer fires had been seen after about 1500 A.D. “We're saying no, there is enough independent evidence that the drop in fire was caused by cooling climate,” said lead author Mitchell Power, 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,” said Power.

To be published in the August issue of the journal Holocene, the study analyzed charcoal samples from around the world from the past 2,000 years. The study also deals with the Little Ice Age, a period that is thought to have begun between the 1200s and 1500s and extended into the 1800s. Possible causes of this cooling period include a combination of dust from volcanic eruptions, decreased solar activity, and changes in the ocean and atmospheric circulation.

The cooling climate was a major factor in the decline in fires over a very large scale, which began “prior to the population collapse, and climate alone is sufficient to explain large scale changes in burning,” said Power. “In a cooler atmosphere, you tend to get reduced convection, so you get reduced thunderstorms and ignition from lightning. Cooler climate also tends to maintain high levels of fuel moisture and soil moisture.”

European explorers brought diseases such as small pox to the New World, which “decimated populations in the Americas — 10 million to 100 million dead, with most estimates in the 60 million range,” said Power.

Mayans, Incans, the Aztecs -- all of these civilizations were affected, he added. As a result, agriculture was significantly 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 acknowledged that population collapse may have led to reduced biomass burning in some local regions of the Americas. But Power´s study indicates that the reduction in fire was global and began well before Columbus sailed to the New World, suggesting the Little Ice Age triggered most of the reduction in burning. However, he noted there was still some room for debate because the Little Ice Age varied in time and space, and didn´t affect all parts of the world equally.

The study samples of burnt wood and other biomass were found in sediment cores from lake bottoms and bogs from nearly 600 sites around the world, nearly half from the Americas alone, and dated as far back as 2,000 years. Power manages the Global Charcoal Database that compiles data from all the existing studies that date charcoal samples and describe where they came from.

“Greater than 80 percent of biomass burning records show a decline post-1500 in the Americas,” he said. “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."

They found that biomass burning in the Americas declined between 1500 and 1650 and stayed at a minimum until around 1700, the peak of the Little Ice Age. That period also had the lowest level of burning in the past 6,000 years.

In tropical Middle America climate cooling began around 1350, when burning also began to decline. Population collapse didn´t begin there until around 1500. The same occurred for tropical South America.

In southern South America, ice-core and tree-ring growth studies show cooling began there about 1450, well before the abrupt decline in burning in 1550. That may have supported the theory that population collapse reduced burning, except for the fact that there was little population there to begin with, certainly not enough for any decline to trigger a reduction in burning.

Today, warming climate and drought have been tied to increasing fires in the US 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 concluded.

The study was funded by the National Science Foundation and the Natural History Museum of Utah.

An international team of 20 researchers from the US, UK, Germany, Switzerland, Chile, France and Canada, among others, partook in the study.