University of Utah researcher Mitchell Power with refrigerated sediment samples in a lab at the Natural History Museum of Utah, where he is a curator. Charcoal in such sediments was the key evidence used in a new study by Powers and colleagues suggesting that the Little Ice Age—a period of global cooling—explains why there were fewer fires after about 1500. Credit: Lee J. Siegel, University of Utah |
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.
Mitchell Power pulls a sediment core out of Spring Lake near Delta, Utah. The sediment core showed a reduction in charcoal—and thus fire—in the area during the Little Ice Age, a time of global cooling that began sometime between that A.D. 1200s and 1500s and ended in the early 1800s. A new study led by Powers suggests the Little Ice Age led to a worldwide reduction in fires after 1500, and that reduction was not caused by decimation of New Word populations by European diseases in the wake of Columbus. Credit: University of Utah |
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:
- Clumping all the charcoal data in two groups—from the Americas or the
Eastern Hemisphere—shows that in the Americas, biomass burning declined
between 1500 and 1650 and stayed at a minimum until 1700, the same time
as the peak of the Little Ice Age. That period was the lowest level of
burning in the past 6,000 years. - In
the Eastern Hemisphere, there was a prominent decline in burning that
began about 1400—well before the population collapse in the Americas.
Power says cooling also started about a century earlier in the Eastern
Hemisphere than in the Americas—more evidence cooling caused reduced
burning. There was no parallel population collapse large enough to
explain the reduction in burning, although a small downward blip in
burning is noted in Europe around the time of the bubonic plague or
Black Death. - In
tropical Middle America—the Caribbean Basin, Mexico and Central
America—climate cooling starting around 1350, when burning also begins
to decline. Population collapse didn’t begin until around 1500. - In
tropical South America, climate changed around 1350 to 1400. There is
debate whether it warmed or cooled. The population collapsed after 1500.
Power says neither climate nor population strongly influenced
post-Columbian biomass burning in that region, which declined only
subtly and not until 1700. It also is possible the population that
collapsed didn’t use fire very much in agriculture—something a recent
study coauthored by Power found in French Guiana. - In
southern South America, ice-core and tree-ring growth studies show
cooling began about 1450, well before an abrupt decline in burning in
1550. That would seem to support the theory that population collapse
reduced burning—except that the region had little population, certainly
not enough for any decline to trigger a reduction in burning. - Ice
cores from Greenland show cooling started about 1450, and fire started
to decline about 1500, according to charcoal for boreal Canada and the
western United States. Cooling and reduced burning stopped about 1800.
Despite the 50-year lag, Power says that is more evidence tying climate
cooling to reduced biomass burning, particularly since the region had
relatively few people at the time.
