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Early agriculture drove global climate change, research shows

April 8, 2014

The industrial revolution is often cited as the trigger of human-caused climate change. However, preindustrial people may have influenced the global temperature about as much as their fossil-fuel burning successors. Before the first ounce of petroleum was ever burnt, deforestation and agriculture had raised the global temperature about 1.5 degrees Fahrenheit above expected levels.

Those are findings from a new study by Nelson Institute Center for Climatic Research scientists Feng He, Steve Vavrus and John Kutzbach, in collaboration with researchers at the Universities of Virginia and Geneva.

“It’s definitely a paradigm shift,” says He, lead author of the study recently published in the journal Geophysical Research Letters. “If people are not familiar with this hypothesis, they say, ‘You’re crazy to think the climate could warm that much before the industrial revolution.’”

But He says that that’s exactly what the evidence suggests.

UW-Madison climate researcher Feng He
Feng He

Normally greenhouse gas concentrations fall steadily between periods of glaciation. These inter-glacial periods have historically lasted about 10,000 years. However, greenhouse gas concentrations started rising about 5,000 years ago, halfway through the current interglacial period. That’s about the time that agriculture became widespread.

“We know that there was vast deforestation before the industrial revolution; that’s crystal clear,” says He. “With deforestation, there are two effects. The first is the carbon cycle effect; you increase the carbon dioxide in the atmosphere. The second is the albedo effect; you change surface reflection.”

An acre of forest can hold about 120 tons of carbon, which plants absorb from the air during photosynthesis. If that carbon is burned, each carbon atom bonds with two oxygen atoms, increasing its atomic weight by more than three times. So 120 tons of forest carbon can produce approximately 450 tons of carbon dioxide gas.

Six thousand years ago, when people burned millions of acres of forest to make room for wheat and barley, they were releasing a lot of carbon dioxide. Thus more heat was trapped than would be expected under “natural” conditions.

The loss of forest cover can also work the other way. As He explains, deforestation changes the reflectance of the Earth’s surface. In the far north, the removal of trees allows sunlight to hit more snow and bounce back toward space without heating the surface, thus having a cooling effect. So clearing forest in the north actually causes some loss of heat.

But with deforestation there’s also reduced transpiration – the release of water from plant leaves, which pulls more water up from the ground. Transpiration is powered by evaporation and has a cooling effect on the atmosphere. So loss of transpiration causes local atmospheric heating.

“It’s like you can’t sweat,” He says.

To determine how much influence each of these factors exerted on global temperature, the researchers employed a new climate model to simulate historical land cover changes up to the year 1850. They fed the model observational data, including the greenhouse effect of carbon dioxide and the net effect of removing trees from a plot of land, then incorporated archeological data such as the amount of land used for farming.

The model showed that clearing the land, without accounting for atmospheric carbon dioxide, would cause a slight global cooling of about 0.3 degrees Fahrenheit.

“It’s always a punch
line, that early agriculture
warmed the earth as much
as industrial times. But
the current trajectory of
man-made climate change
is much different from
the trajectory of natural
climate change."

When the team added natural levels of carbon dioxide fluctuation, based on observations from previous inter-glacial periods, the simulated global temperature warmed by roughly 1.6 degrees, meaning that the net effect of human land clearance was a bit more than 1.3 degrees of warming.

The climate of 1850 would have been much cooler if it weren’t for earlier agriculture. But climate change was probably a good thing for preindustrial people, according to He, because it stabilized a climate that otherwise would have been steadily cooling.

“All the agriculture, all the infrastructure, and all the human activity was centered around this climate state,” He says.

The researchers’ findings suggest that the idea of “natural” variability in preindustrial carbon dioxide levels needs to be reconsidered – an idea gaining traction in the scientific community. He, Vavrus and Kutzbach have worked closely with Bill Ruddiman, a paleoclimatologist with the University of Virginia and a pioneer in this area of research. The team is publishing an upcoming paper in The Anthropocene Review outlining the evidence that preindustrial people warmed the planet just as much as industrial people have.

That may be a paradigm shift, but the contrast in scale from modern day climate change is stark.

“It’s always a punch line, that early agriculture warmed the earth as much as industrial times,” He says. “But the current trajectory of man-made climate change is much different from the trajectory of natural climate change. Global warming critics should not use this as an example to say ‘You should not care about climate change.’”

It took more than 5,000 years of preindustrial deforestation to warm the climate by 1.3 degrees. Industrial societies matched that increase in only 150 years, and there’s no end in sight. 

Donald Radcliffe is a forest science and life sciences communication double major who is also pursuing the environmental studies certificate.