Research: Former farmers have spared us from glaciers but have profoundly changed the climate of the Earth –

A study published in the journal Scientific reports provides new evidence that past agricultural practices have led to an increase in atmospheric emissions of carbon dioxide and methane, heat-trapping gases, an increase that has continued since then, contrary to the trend in 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 much cooler these days," says lead author Stephen Vavrus, a researcher at the University of Wisconsin-Madison's Center for Climate Research at the Institute of Studies. environmental issues. "The ancient roots of agriculture produced enough carbon dioxide and methane to influence the environment."

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.

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.

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've also noticed that carbon dioxide began to decrease about 10,000 years ago, about 7,000 years ago, "says Ruddiman. "This interglaciation 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 everything.

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.

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.

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 bad message, but science takes you where it leads you," says Vavrus. "Things are so far off the beaten path, the last 2,000 years have been so out of reach that we are way beyond what is natural."

The reality is that we do not know what will happen next. And glaciers have long been the main source of fresh water on the Earth.

"The climate community agrees that we have stopped the next glaciation in the near future and predictably, because even if we stop putting carbon dioxide in the atmosphere, what we have now will persist." 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."