Global warming may have been accelerated by humans long before the industrial revolution, according to a controversial new study by American scientists.
The huge increase in population of the planet over the last 150 years and the burning of fossil fuels to power industry have been widely regarded as the main causes of global warming. Climate scientists regard populations thousands of years ago to be too small to affect the climate in any meaningful way.
However, researchers at the University of Virginia and the University of Maryland say that early humans had less efficient methods of farming and needed more land to grow food. This led to ‘slash and burn’ farming, where huge areas of forest were felled or burnt to clear space.
"They used more land for farming because they had little incentive to maximise yield from less land, and because there was plenty of forest to burn," said William Ruddiman, the study's lead author and a professor emeritus of environmental sciences at the University of Virginia. "They may have inadvertently altered the climate."
Ruddiman caused controversy five years ago with a paper claiming humans began affecting climate change thousands of years ago. He points out that as farming techniques improved, less land was needed to grow food. Forests in places such as Canada and Russia have since grown back, but any positive effects this may have had has been cancelled out by fossil fuels and the Industrial Revolution.
"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 modellers might consider how land use has changed over time, and how this may have affected the climate."
Ruddiman, and his co-author Erle Ellis, an ecologist at UMBC who specialises in land-use change, say climate models are not accounting for the possibly large effects on climate likely caused by early farming methods.
Ellis said: "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. 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."
The paper is based on studies by anthropologists, archaeologists and paleoecologists, who found older civilisations used huge amounts of land for a relatively small amount of food. The paper estimates the six billion people alive today use 90 percent less land per person for growing food than people thousands of years ago.
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