New start date for the Anthropocene proposed – when humans first changed global methane levels

Humans have been reshaping the environment for at least 10,000 years. But the Anthropocene is the name given to the specific period of Earth history during which humans have had a global effect on the planet’s climate and ecosystems. Despite formal rejection as a geological epoch, it’s widely understood within academic research as useful shorthand for the age of human interference in the Earth system.

Various dates have been proposed for when the anthropocene effectively began, from the early 17th century to the mid-20th century, when the first atomic weapons were detonated. My new research into atmospheric methane concentration supports the idea of an early date, when European arrival in the Americas first had a notable impact on the atmosphere, but slightly before previous estimates.

Ice cores – cylinders of ice drilled from glaciers and ice sheets – provide important evidence of historical changes in the global atmospheric composition. It is from these records that a date for the Anthropocene’s pre-industrial beginnings was first proposed in 2015 by two Earth systems scientists at the University College London, Simon Lewis and Mark Maslin.

They suggested that an unprecedented drop in the level of CO₂ in the atmosphere that was recorded in ice cores – known as the “Orbis spike” – dates back to 1610. This unusually low level reflects additional atmospheric CO₂ absorption into trees from forest regrowth in the Americas following European arrival in the late 1400s.

From European arrival in 1492 and colonisation in the 1500s, the introduction of disease, mostly smallpox, resulted in demographic collapse of around 50 million people across the Americas. Lewis and Maslin proposed that, as millions of hectares of farmland went untended, forests could regrow and this increased CO₂ removal from the atmosphere.

This happened in sufficient quantities to be recorded in glacial ice. And that change became a global marker for the start of the so-called Anthropocene.

My own research into changing methane concentrations indicates that the Anthropocene began slightly earlier than that, in 1592. Ice core records show a minimum atmospheric methane concentration exactly 100 years after explorer Christopher Columbus first set foot in the Americas. This, I believe, strengthens support for the hypothesis put forward by Lewis and Maslin a decade ago.

In a paper published in Nature Reviews, Earth and Environment, I consider the effects of global fluctuations in how trees and forests exchange methane. Methane is a greenhouse gas that is around 80 times more powerful than carbon dioxide over a 20-year period. Crucially, methane has a short lifetime of just under ten years, so any ice core record will be far more responsive to changes to the methane cycle than that of longer-lived CO₂.

Trees are a methane sink

So what’s the link to trees? Trees and their woody bark surfaces, despite their biologically inert appearance when compared to leaves, are important interfaces of methane exchange. In swamps and forested floodplains like the Amazon, they are exit points of methane to the atmosphere from the saturated soils where the methane is formed by anaerobic soil microbes.

However, last year, my team uncovered how the more extensive areas of forest growing on free-draining soils interact with atmospheric methane. The trees host microbes that directly remove methane from the atmosphere.

This is one of two mechanisms that, together, might explain an unprecedented drop in atmospheric methane concentrations recorded in Antarctic ice cores in the first century following European arrival in the Americas. This would support Lewis and Maslin’s idea that regrowing forests in that period had global effects.

With more trees growing on abandoned farmland, there was more woody tree surface area in contact with the atmosphere. This meant more methane being taken up by the microbes they host.

The second mechanism relates to how trees intercept incoming rainfall. Some rainfall is re-evaporated before reaching the soil. Any rain reaching the soil may then be taken up by tree roots and released back to the atmosphere. The rest moves into the soil or washes off into rivers and wetlands.

It is possible that the spike in forest regrowth led to more evaporation and transpiration. So more water was released by the trees back to the atmosphere and less washed off over the soil surface.

This limited water flowing into wetlands. Those wetlands are a major methane source. So a small shrinkage in wetland area, combined with more trees absorbing atmospheric methane, could have reduced the atmospheric methane concentration and explain the minimum methane levels observed in 1592.

When exactly the Anthropocene began may be an argument that has been overtaken by the decision to not label it a new epoch. Indeed, it’s possible that forest clearance for early agriculture by humans around 5,000-8,000 years ago in the mid-Holocene, (a period of relative climate stability in the Neolithic period) contributed to the atmospheric methane increase observed in Antarctic ice from that time.

As well as an ancient trace of human influence over our forests, the ice core methane records provide a chance to evaluate newly discovered processes operating in the world’s forests. This is something I’m now investigating with my colleague Peter Hopcroft, a palaeoclimate modeller at the University of Birmingham.

Whether through forest clearances for early agriculture or through the effects on forests of massive depopulation of Indigenous peoples following European contact, these traces of our past influence point to something significant: that there has always been an intimate and evolving connection between humanity and the natural world. A connection so fundamental that, for the vast span of our existence as a species, we have been inseparable from nature itself.

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