Forest razing by ancient Maya worsened droughts, says study

Forest razing by ancient Maya worsened droughts, says study
The Maya rain god, Chaac, is a common motif in Maya architecture. Chaac appears on this frieze at El Castillo. Credit: Kevin Anchukaitis/Lamont-Doherty Earth Observatory.
( -- For six centuries, the ancient Maya flourished, with more than a hundred city-states scattered across what is now southern Mexico and northern Central America. Then, in A.D. 695, the collapse of several cities in present day Guatemala marked the start of the Classic Maya's slow decline. Prolonged drought is thought to have played a role, but a study published this week in the journal Geophysical Research Letters adds a new twist: The Maya may have made the droughts worse by clearing away forests for cities and crops, making a naturally drying climate drier.

"We're not saying deforestation explains the entire drought, but it does explain a substantial portion of the overall drying that is thought to have occurred," said the study's lead author Benjamin Cook, a climate modeler at Columbia University's Lamont-Doherty Earth Observatory and the NASA Goddard Institute for Space Studies.

More than 19 million people were scattered across the Maya empire at its height, between A.D. 250 and A.D. 900. Using population records and other data, the study authors reconstructed the progressive loss of rainforest across their territory as the civilization grew. The researchers ran to see how lands newly dominated by crops would have affected climate. In the heavily logged Yucatan peninsula, they found that rainfall would have declined by as much as 15 percent while in other Maya lands, such as southern Mexico, it would have fallen by 5 percent. Overall, the researchers attributed 60 percent of the drying estimated at the time of the Maya's peak to deforestation.

As crops like corn replace a forest's dark canopy, more sunlight bounces back into space, said Cook. With the ground absorbing less , less water evaporates from the surface, releasing less moisture into the air to form rain-making clouds. "You basically slow things down—the ability to form clouds and precipitation," he said.

Forest razing by ancient Maya worsened droughts, says study
The Maya cleared away forests to grow crops and build their cities and temples. El Castillo at Xunantunich was an ancient Maya ceremonial site in western Belize. (Ian Mackenzie/wikicommons)

The idea that the Maya changed the climate by clearing away jungle, partly causing their demise, was popularized by historian Jared Diamond in his 2005 book Collapse. In the first study to test the hypothesis, climate modeler Robert Oglesby and his colleagues ran a computer simulation of what total deforestation of Maya lands would do to climate. Their results, published in 2010 in the Journal of Geophysical Research, showed that wet season rainfall could fall 15 to 30 percent if all Maya lands were completely cleared of trees. Oglesby, who was not involved in the Cook study, said that Cook's estimate of a 5 to 15 percent reduction in rainfall, though lower than his own, makes sense since Cook's simulation used a realistic tree-clearing scenario.

Archeologists attribute a variety of factors to the collapse of the Classic Maya, whose descendants are still living today in parts of Mexico, Guatemala, Belize, El Salvador and Honduras. In addition to a drying climate in several regions, the city-states struggled with overpopulation, changing trade routes, war and peasant revolts.

The Maya cleared the forests to grow corn and other crops, but they also needed the trees for cooking large amounts of lime plaster used in constructing their elaborate cities. Thomas Sever, an archeologist at the University of Alabama, Huntsville, and a co-author of the 2010 deforestation study, said that it would have taken 20 trees to produce a single square meter of cityscape. "When you look at these cities and see all the lime and lime plaster, you understand why they needed to cut down the trees to keep their society going," he said.

The Maya also lacked the technology to tap the groundwater several hundred feet beneath them. Their reservoirs and canals were able to store and distribute water when rain plentiful, but when the rain failed, they had nowhere to turn. "By the time of the collapse, every square mile of soil had been turned over," said Sever.

Scientists know from studying climate records held in cave formations and lake sediments that the Maya suffered through a series of droughts yet they continue to debate their severity. In a paper earlier this year in Science, researchers Martín Medina-Elizalde and Eelco Rohling of Mexico's Yucatan Center for Scientific Research found that annual rainfall may have fallen as little as 25 percent during the Maya's decline, from about A.D. 800 to A.D. 950. Most of the reduction in rainfall, however, may have occurred during the summer growing season when rain would have been most needed for cultivation and replenishing freshwater storage systems, they added.

Today, many of the Maya's abandoned cities are overgrown with jungle, especially on the Yucatan peninsula. Satellite images, however, show that deforestation is happening rapidly elsewhere, including in other regions the Maya once occupied. The study may offer a warning about the consequences: "There's a tremendous amount of change going on in Guatemala," said Oglesby. "They may be that much more vulnerable to a severe drought."

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Study shows Maya civilization collapse related to modest rainfall reductions

Journal information: Geophysical Research Letters

Citation: Forest razing by ancient Maya worsened droughts, says study (2012, August 21) retrieved 26 June 2019 from
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Aug 21, 2012
Well gee, we have chopped down most of our forests too !

Aug 21, 2012
Well gee, we have chopped down most of our forests too !

Don't be daft. More land area under forestation now than at any time in the last 150 years (North America).

Liberalism is a philosophy of consolation for Western Civilization as it commits suicide.

Aug 21, 2012
Thanks 'rubberman'. I'd also like to see that.
Hopefully, 'shootist' can provide your request but I won't hold my breath. :-)
What I have seen, from the air, is miniscule reforestation both in area and depth. Particularly in depth of forest canopy.

Aug 21, 2012
"March 12, 2012 U.S. Forest Service scientists today released an assessment that shows forest land has expanded in northern states during the past century despite a 130-percent population jump and relentless environmental threats. At the same time, Forest Service researchers caution that threats to forests in the coming decades could undermine these gains.

According to the Forests of the Northern United States report, forest coverage in the United States has increased by 28 percent across the region that includes Minnesota, Iowa, Missouri, Wisconsin, Illinois, Indiana, Michigan, Ohio, West Virginia, Pennsylvania, Maryland, Delaware, New Jersey, New York, Connecticut, Rhode Island, Massachusetts, Vermont, New Hampshire and Maine.

Forested land currently accounts for 42 percent of the northern land area. Population in the region rose from 52 to 124 million people during the past 100 years, while northern forest coverage expanded from 134 to 172 million acres."

Aug 22, 2012
Aye. With current agricultural technology we use ~ 1/10 of the area /person that earlier wasteful cultures did.

To baseline AGW they did a survey of ancient forests by way of fossil seeds et cetera. Turns out the first agriculturalists killed off a large part of the continental forests that grew after the ice age ~ 8000 years ago.

Much of it is regrowth, and it seems we continue the trend.

Aug 22, 2012
In Oregon, which has basically been raped by "foresters" and who's decimation I have watched personally over the last 50 years, there are thousands of acres of 10 to 30 meter high trees covering vast areas of previously forested lands as well as previous farm lands. They are densely grown Poplar trees for use as toilet paper. The Forestry Service is an ass. I would no more trust the US Forestry Service to define forest than I would the toilet paper industry. The Forestry Service IS the Timber Industry!

Aug 22, 2012
Tree monoculture is NOT good for biodiversity.
Also, the varieties chosen for planting often provide unpleasant surprises, when it turns out they are less tolerant to drought, parasites or hard winters than the natural growth they replaced.
Here in Sweden we have large areas planted with the fast-growing Canadian pinus contorta. Two decades into the program it turned out they had high mortality.

Aug 22, 2012
@ Vendicar Decarian:

"Which is good because we are 100 thousand times larger in number than earlier cultures. "With current agricultural technology we use ~ 1/10 of the area /person that earlier wasteful cultures did." - FlatchFlitch"

Are you responding to me? I don't recognize the reference. If it is supposed to be denigrating, I suppose I shouldn't reply.

Until that is cleared out, I'll respond to the analysis. Our population density, which is mostly in cities, are not that much different from them seen over areal use. Also, we are stationary, not using up as much forest as the slash&burn cultures. I was just pointing out one factor, areal use for food, that predicts the observed regrowth, which isn't in doubt.

@ Birger:

Except that our forests are more or less monocultures as part of the norther forest belt, it is the natural growth. (Modula forest fires - which according to the above makes the landscape drier.)

Your example would argue that heteroculture is worse.

Aug 22, 2012
What about the more primitive southern states?

Primitive? Isn't it primitive thinking that results in tourette-like vileness from you based on utter ignorance?

Aug 22, 2012
I think all the land under cultivation for corn ethanol should be returned to its natural state.

Aug 22, 2012
U.S. Forest Facts and Historical Trends U.S. DEPARTMENT OF AGRICULTURE FOREST SERVICE 2000 "It is estimated that ... in 1630 the area of forest land that would become the United States was 423 million hectares or about 46 percent of the total land area. By 1907, the area of forest land had declined to an estimated 307 million hectares or 34 percent of the total land area. Forest area has been relatively stable since 1907. In 1997, 302 million hectares - or 33 percent of the total land area of the United States - was in forest land. Today?s forest land area amounts to about 70 percent of the area that was forested in 1630.......Stand Age and Average Annual Harvest Area - After intensive logging in the late 19th century and early to mid 20th century, 55 percent of the forests on the Nation's timber land is less than 50 years old. Six percent of the Nation's timber land is more than 175 years old."

Aug 22, 2012
By 1907, the area of forest land had declined to an estimated 307 million hectares or 34 percent of the total land area.

"Since 1630, about 120 million hectares of forest land
have been converted to other uses ? mainly agricultural.

More than 75 percent of the net conversion to other uses
occurred in the 19th century."

Aug 22, 2012
Any way you slice it, this article is about research that is clearly conjectural in its conclusions. And those conclusions themselves are partly contradicted by the author himself?!?!

The Maya lands comprise the Yucatan Peninsula and southern Mexico, with some additional contiguous portions of other modern central american nations.

This researcher has, apparently, ignored the glaringly obvious fact that the entire area lies between two vast bodies of water that are the main sources of nearly ALL THE RAINFALL in the area.

Aug 23, 2012

This researcher has, apparently, ignored the glaringly obvious fact that the entire area lies between two vast bodies of water that are the main sources of nearly ALL THE RAINFALL in the area.

"The 760 AD drought signalled the end of a 200 year 'wet' period in the Yucatan, during this time the cities prospered, but populations grew to such great numbers that agricultural production became over stretched.

Much of the Maya civilisation and its agriculture was fatally sited on a vast area of porous, karst limestone; here rain often simply disappears into deep underground rivers flowing though caverns to the sea.

In normal years many regions have a long dry spell lasting 4 – 6 months; much of the Maya lowlands is a seasonal desert. "

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