Agricultural methods of early civilizations may have altered global climate, study suggests

Aug 17, 2009

Massive burning of forests for agriculture thousands of years ago may have increased atmospheric carbon dioxide enough to alter global climate and usher in a warming trend that continues today, according to a new study that appears online Aug. 17 in the journal Quaternary Science Reviews.

Researchers at the University of Virginia and the University of Maryland-Baltimore County say that today's 6 billion people use about 90 percent less land per person for growing food than was used by far smaller populations early in the development of civilization. Those early societies likely relied on slash-and-burn techniques to clear large tracts of land for relatively small levels of food production.

"They used more land for farming because they had little incentive to maximize yield from less land, and because there was plenty of forest to burn," said William Ruddiman, the lead author and a professor emeritus of environmental sciences at the University of Virginia. "They may have inadvertently altered the climate."

Ruddiman is a climate scientist who specializes in investigating ocean-sediment and ice-core records. In recent years he has searched across scientific disciplines - anthropology, archaeology, , climatology - to gain insight into how humans may have affected climate over the millennia.

He said that early populations likely used a land-clearing method that involved burning forests, then planting among the dead stumps in the enriched soil. They would use a large plot until the yield began to decline, and then would burn off another area of forest for planting.

They would continue this form of rotation farming, ever expanding the cleared areas as their populations grew. They possibly cleared five or more times more land than they actually farmed at any given time. It was only as populations grew much larger, and less land was available for farming or for laying fallow, that societies adopted more intensive farming techniques and slowly gained more food yield from less land.

Ruddiman notes that with the highly efficient and intensive farming of today, growing populations are using less land per capita for agriculture. Forests are returning in many parts of the world, including the northeastern United States, Europe, Canada, Russia and even parts of China.

The positive environmental effects of this reforestation, however, are being canceled out by the large-scale burning of fossil fuels since the advent of the Industrial Revolution, which began about 150 years ago. Humans continue to add excessive levels of carbon dioxide to the atmosphere, contributing to a global warming trend, Ruddiman said.

Five years ago, Ruddiman made headlines with a hypothesis that humans began altering thousands of years ago, not just since the Industrial Revolution. That theory has since been criticized by some climate scientists who believe that early populations were too small to create enough carbon dioxide to alter climate.

According to projections from some models of past land use, large-scale land clearing and resulting carbon emissions have only occurred during the industrial era, as a result of huge increases in population.

But Ruddiman, and his co-author Erle Ellis, an ecologist at UMBC who specializes in land-use change, say these models are not accounting for the possibly large effects on climate likely caused by early farming methods.

"Many climate models assume that land use in the past was similar to land use today; and that the great population explosion of the past 150 years has increased land use proportionally," Ellis said. "We are proposing that much smaller earlier populations used much more land per person, and may have more greatly affected climate than current models reflect."

Ruddiman and Ellis based their finding on several studies by anthropologists, archaeologists and paleoecologists indicating that early civilizations used a great amount of land to grow relatively small amounts of food. The researchers compared what they found with the way most land-use models are designed, and found a disconnect between modeling and field-based studies.

"It was only as our populations grew larger over thousands of years, and needed more food, that we improved farming technologies enough to begin using less land for more yield," Ruddiman said. "We suggest in this paper that climate modelers might consider how land use has changed over time, and how this may have affected the ."

Source: University of Virginia (news : web)

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Arkaleus
3 / 5 (4) Aug 17, 2009
So what? Why is this speculation useful? How do the investigators differentialte between human deforestation and the constant wildfires that burned unchecked before human settlements disrupted them?

I think it is completely useless speculation to try to quantify ancient environmental damage based on estimates of carbon dioxide output from burning forests. First of all, there is no real proof that carbon dioxide leads tempurature change, and the causality of climate change is far from fully understood.

Another bothersome habit of climate researchers is separating human activity from nature. We are nature. We are as much a part of the natural world as plants and animals. It's not right to separate humans from the rest of the environment and call our activities unnatural.
Doggonit
3.3 / 5 (4) Aug 17, 2009
Arkaleus, gee I don't know maybe this speculation is useful because it might help us understand the impact past human societies have had on the environment. Unfortunately, your prerequisite of "real proof" could never be met. Does that mean we should not study human environmental impact at all?

And, just in case you didn't know scientist usually try to isolate their variables as much as they can in order to study their interrelationships. It is kind of hard to do otherwise.
Arkaleus
2.6 / 5 (5) Aug 17, 2009
Doggnonit:

It seems futile to claim "human-caused climate change" based on the activities of neolithic and bronze age civilizations. The population of the ancient world was significantly lower than today. Their impact on world climate would have to be considered a *very* minor part of a *much* greater system.

It would be like trying to convince me that you are able to predict the motions of clouds based upon the wing activity of bee colonies.

Of course, I could calculate the total force generated by the threshing wings of billions of bees, and insert this factor into a climate model simulation, and then show how the beatings of billions of tiny wings "changes" the climate of the planet. Is this the kind of reasoning you advocate?

How do you trace the impact of humans burning forests vs natural fires? That was my question.

We all know that wildfires are a constant and regular occurance, and in ancient times we should expect there to be even larger numbers of unchecked wildfires.

I think it's bogus for anyone to claim they can quantify the climate change caused by pre-historic human-caused forest fires. How can they tell the difference between the consequences of natural fires and human caused fires? This reasearch is certainly speculative, assumes a great number of things to be given, and relies on computer modeling with a large number of "guess factors" on the part of initial conditions and variables.

This, like all other computer climate model fiddle faddle, is nothing but speculation and hype for AGW. I think if the investigators behind this research can somehow bolster the notion that climate change is human-caused (and therefore human-preventable) then they have accomplished their mission.

I can arrange a model to give any sort of result I want. Computer climate models have been shown to be highly speculative, often wrong, and for people to accept them as evidence is premature and naive.
Doggonit
not rated yet Aug 18, 2009
Arkaleus, you can use that line of argument against any field of science. While I think a healthy does of skepticism is always healthy in scientific discussions outright dismissal is not.

You might think that computer modeling, although imperfect, is a waist of time but I am very happy, living in Florida, that we have such systems especially during hurricane season. Weather modeling just like climate modeling has shown to be highly speculative and often wrong. However, we know much more about weather now because of its use and lives have been saved. It is ludicrous to think it is not a tool that can help us understand climate as well.

Yes you can arrange a model or a medical trial or any scientific study to return whatever result you want. That is why studies are published, peer reviewed and many times replicated.
GrayMouser
not rated yet Aug 18, 2009
You might think that computer modeling, although imperfect, is a waist of time but I am very happy, living in Florida, that we have such systems especially during hurricane season. Weather modeling just like climate modeling has shown to be highly speculative and often wrong. However, we know much more about weather now because of its use and lives have been saved. It is ludicrous to think it is not a tool that can help us understand climate as well.

I suspect that the improvements in forecasting are more directly related to the better observation of actual weather than it is to computer modeling. Even now, when forecasters give you a percentage chance of something happening it is a percentage based on what has been to occur previously given the same conditions.
Doggonit
not rated yet Aug 19, 2009
GrayMouser, you are right that probability statistics makes a large contribution to weather forecasting. But when it comes to hurricane prediction there is a heavy reliance on computer models like the GFS (Global Forecast System model), UKMET (United Kingdom Meteorological model), GFDL (NWS/Geophysical Fluid Dynamics Laboratory model) and SHIPS (Statistical Hurricane Intensity Prediction Scheme).

You can see a comparison of some different models predicting the path of our latest hurricane Bill at:

http://www.wunder...del.html

Computer modeling is a fascinating subject and has helped us better understand weather and climate. It is unfortunate that people with political agendas have seized on the degree of uncertainty inherent in this type a analysis to try to undermine the credibility of the discipline. But, this is a legitimate form of science and should be pursued.
Arkaleus
5 / 5 (1) Aug 19, 2009
I think computer modelling is a powerful tool, but it needs to be used properly. Computer models are best used to calculate highly precise outputs where the subject being modelled is well understood and all of its features are known and described mathematically in the model. We don't have a full understanding of the climate system, its causes, its relationships, or even a full history of weather.

I have no emotional prejudice against computer modeling in research, but there needs to be more self control in the conclusions they make with them. Their work is being twisted into a political scheme. If I threw a bunch of money at a bunch of poor university postgrads to "research something that validates AGW" I would expect research remarkably similar to this.
GrayMouser
not rated yet Aug 24, 2009
GrayMouser, you are right that probability statistics makes a large contribution to weather forecasting. But when it comes to hurricane prediction there is a heavy reliance on computer models like the GFS (Global Forecast System model), UKMET (United Kingdom Meteorological model), GFDL (NWS/Geophysical Fluid Dynamics Laboratory model) and SHIPS (Statistical Hurricane Intensity Prediction Scheme).



You can see a comparison of some different models predicting the path of our latest hurricane Bill at:



http://www.wunder...del.html



Computer modeling is a fascinating subject and has helped us better understand weather and climate. It is unfortunate that people with political agendas have seized on the degree of uncertainty inherent in this type a analysis to try to undermine the credibility of the discipline. But, this is a legitimate form of science and should be pursued.


I suspect that computer models appear to let us understand weather and climate but only in a course and imprecise fashion. None of the weather models are any good even 3 days out. The climate models fair no better.

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