Humans have been messing with the climate for thousands of years
By clearing forests and raising animals, early farmers cranked up the global thermostat, possibly preventing another ice age.
Thousands of years ago, ancient farmers grew oats, corn and wheat, just as they do today. They also cultivated rice and raised livestock. But a millennia ago, they cleared much more land than modern day farmers do, despite having fewer people to feed. That’s because farming was far less efficient. Mechanized harvesters didn’t exist, and growers had yet to develop crops that could be planted in tightly packed rows, yielding more food from less space. All those years of agricultural inefficiency likely had a lasting impact on the planet. Early farming practices unleashed a potent combination of greenhouse gases — carbon dioxide from deforestation, methane from the rice paddies and livestock — that may have profoundly changed the Earth’s climate and kept the planet from moving into another ice age. In short, according to new research, if not for these earliest agronomists, Earth might be significantly cooler today than it is. That doesn’t mean that, were it not for early farmers, we wouldn’t be struggling with the ravages of climate change, nor does it let modern societies off the hook for pouring carbon pollution into the atmosphere. Even if prehistoric growers had used less land, the Earth still would be heating up rapidly, but the warming trend would have started from a cooler baseline.
“There is a huge difference between the very gradual and accidental warming trend that early farmers probably caused, versus the much more rapid climate changes that our modern industrial world is effecting knowingly,” said Stephen Vavrus, a senior scientist in the University of Wisconsin-Madison Center for Climatic Research who conducted the study, which recently appeared in the journal Scientific Reports.
His climate modeling suggests that the amount of warming caused by farming prior to the Industrial Revolution is similar to the amount of warming caused by industrialization over the last two centuries, he said. “So if you negate the contribution from early farmers, then the earth would still be warming up but from a cooler starting point,” he said. “In that case, the world would probably still be grappling with the same sense of concern about climate change, but Earth’s temperature would be lower than it currently is.”
Conventional thinking holds that early humans were too sparse and too technologically primitive to affect the global climate, “but this hypothesis argues otherwise,” Vavrus said. However, he noted, “A sobering corollary is that if the relatively tiny populations of the past could still generate significant global warming, then the massive number of people in the world today with our amplified carbon emissions must be having a huge climatic effect. From this perspective, our research findings underscore the reality and seriousness of contemporary climate change.”
The scientists used a computerized climate model to simulate the climate nearly 777,000 years ago. The climate back then looked more or less what the climate today would look like if not for the warming caused by carbon pollution from ancient farming and modern industrialization, he said. This climate model offered higher resolution than previous models used by the team.
“The inputs to run the climate model include the concentration of heat-trapping greenhouse gases such as carbon dioxide and methane, as well as parameters that describe Earth’s ‘orbital geometry,’ meaning how much Earth tilts toward the sun, how elliptical Earth’s orbit is around the sun, and how close Earth is to the sun in the different seasons,” he explained. “These parameters vary on timescales of tens of thousands of years. For our climate simulation of 777,000 years ago, Earth’s orbital geometry happened to be very similar to conditions at present, so the only major difference was that the concentration of greenhouse gases was much less then — as measured from air bubbles trapped in ice cores — presumably because modern humans weren’t around to artificially boost them.”
This fact offers some insight into the impact of early farmers. “This simulated ancient climate was even colder than the Earth was at the advent of the Industrial Revolution, at around the year 1850, when the only artificial warming trend had been from early agriculture,” Vavrus said. “This result leads us to conclude that early agriculture had a surprisingly large warming impact on Earth’s climate.”
William Ruddiman, emeritus paleoclimatologist at the University of Virginia, and a co-author on the paper, first posited this theory 15 years ago while studying methane and carbon dioxide trapped in Antarctic ice going back tens of thousands of years. He discovered something unusual, and he has been working with Vavrus over the years to test it.
“I noticed that methane concentrations started decreasing about 10,000 years ago and then reversed direction 5,000 years ago, and I also noted that carbon dioxide also started decreasing around 10,000 years ago and then reversed direction about 7,000 years ago,” Ruddiman said. “It alerted me that there was something strange about this inter-glaciation. The only explanation I could come up with is early agriculture, which put greenhouse gases into the atmosphere — and that was the start of it all.”
The latest paper “demonstrates how much cooler Earth’s climate ought to be today in the absence of humans, based on an actual climate that existed in the past,” Vavrus said. “If the Early Anthropogenic Hypothesis is correct, then it represents a huge change in how we think of past human impact on the environment.”
Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.