Should we thank or blame the ancestors? Former farmers stop the ice age

Millennia ago, former farmers cleared land for wheat and corn, potatoes and squash. They flooded the fields to grow rice. They started raising livestock. And without knowing it, they may have fundamentally altered the climate of the Earth.

New evidence of ancient agricultural effects

A study published in the journal Scientific reports brings new evidence that past agricultural practices have led to an increase in atmospheric emissions of carbon dioxide and methane, an increase that has continued since then, contrary to the trend of the Earth's geological history.

It also shows that without this human influence, at the beginning of the industrial revolution, the planet would probably have known another ice age.

"Without early agriculture, the Earth's climate would be significantly cooler today," says lead author, Stephen Vavrus, principal investigator at the University of Western Ontario's Center for Climate Research. Wisconsin-Madison at the Nelson Institute of Environmental Studies. "The ancient roots of agriculture produced enough carbon dioxide and methane to influence the environment."

The climate of the Earth would be much colder today without the effects of ancient agriculture. (Image: © trattieritratti / Fotolia)

The results are based on a sophisticated climate model that compares our current geological period, called the Holocene, to a similar period 800,000 years ago. They show that the earlier period, called MIS19, was already 2.3 degrees Fahrenheit (1.3 C) colder than the Holocene equivalent time, around 1850. This effect would have been more pronounced in the Arctic, where the model shows that temperatures 9 to 11 degrees Fahrenheit colder.

Using ice core-based climate reconstructions, the model also showed that MIS19 and the Holocene had similar concentrations of carbon dioxide and methane, but that both greenhouse gases had decreased overall. 5000 years ago. of the two gases by 1850. The researchers deliberately cut the pattern at the beginning of the industrial revolution, when sources of greenhouse gas emissions became much more numerous.

Classical climate cycles

During most of the Earth's 4.5 billion-year history, its climate has been largely determined by a natural phenomenon called Milankovitch's cycle, which periodically changes the shape of its orbit around the sun. The earth wobbles and sinks on its axis.

Astronomers can calculate these cycles accurately and they can also be observed in geological and paleoecological records. Cycles affect the distribution of sunlight on the planet, resulting in glacial periods or ice ages as well as warmer interglacial periods. The last ice age ended about 12,000 years ago and the Earth has since been in the Holocene, an interglacial period. The Holocene and MIS19 share the same characteristics of the Milankovitch cycle.

All the other interglacial periods that scientists have studied, including MIS19, start with higher levels of carbon dioxide and methane, which are gradually decreasing over millennia, resulting in colder conditions on Earth. In the end, conditions cool down to a point where glaciation begins.

Ice-covered fiord on Baffin Island with Davis Strait in the background. Baffin Island is the largest island in the Canadian Arctic Archipelago and the fifth largest island in the world. Credit: The NASA / Michael Studinger

As a result of the gases

Fifteen years ago, co-author of the study, William Ruddiman, emeritus paleoclimatologist at the University of Virginia, was studying methane and carbon dioxide trapped in Antarctic ice for tens of thousands of years. # 39; years.

"I noticed that methane concentrations began to decrease about 10,000 years ago, and then reversed 5,000 years ago.I also noted that carbon dioxide had started to decrease there. about 10,000 years ago, about 7,000 years ago. " "This interglaciation has made me think of something strange … The only explanation I could find is early agriculture, which puts greenhouse gases in the atmosphere and that's the beginning of all."

Ruddiman called this the early anthropogenic hypothesis and a number of studies have recently emerged suggesting its plausibility. They document widespread deforestation in Europe for about 6,000 years, the emergence of large agricultural settlements in China 7,000 years ago, as well as the proliferation of rice paddies – robust sources of methane – in the region. Northeast Asia 5000 years ago.

The growth of rice cultivation 5000 years ago has contributed to the increase in the level of methane. (Picture: CC0)

Ruddiman and others have also worked to test the hypothesis. For many years, he collaborated with Vavrus, a climate modeling expert, and his most recent study used the community climate system model 4 to simulate what would have happened in the Holocene, if not for human agriculture . It offers a higher resolution than the climate models previously used by the team and provides new information on the physical processes underlying glaciation.

For example, during a simulation of MIS19, glaciation began with strong cooling in the Arctic and subsequently an expansion of pack ice and snow cover all year. The model showed this beginning in an area known as the Canadian Archipelago, which includes Baffin Island, where summer temperatures have dropped more than 5 degrees Fahrenheit.

"This is consistent with the geological evidence," says Vavrus.

Northeastern coast of Baffin Island north of the community of Clyde River, Nunavut, Canada. (Picture: CC BY-SA 2.0 )

Today, the Arctic is warming up. But before congratulating the former farmers for avoiding global cooling, Vavrus and Ruddiman warned that this fundamental change in our global climate cycle is an unknown territory.

"People say (our work) sends a wrong message, but science takes you where it leads you," says Vavrus. "Things are so far off the beaten track, the last 2000 years have been so far from the natural limits, we are so far beyond what is natural."

The reality is that we do not know what will happen next. And glaciers have long served as a source of fresh water for the Earth.

"The climate community agrees that we stopped the next glaciation in the near future and predictably, because even if we stopped putting carbon dioxide in the atmosphere, what we now have would remain ", says Ruddiman. "The phenomenal fact is that we may have stopped the Earth's major climate cycle and we are stuck in a warmer, warmer interglacial."

Top image: Labor with a team of oxen Source: © Александр Катаржин / Fotolia

The article, originally titled Former farmers have spared us from glaciers but have profoundly changed the climate of the Earth " was published for the first time on Science Daily.

Source: The University of Wisconsin-Madison. "The old farmers spared us glaciers but profoundly changed the climate of the Earth." ScienceDaily. ScienceDaily, September 6, 2018. < " rel="nofollow">www.sciencedaily.com/releases/2018/09/180906141507.htm>.