Scientists are a step closer to officially declaring a new geological time period that marks the start of humanity’s irreversible impact on the planet.
How to define the start of the Anthropocene epoch, which would need to be visible in layers of rock in millions of years’ time, has proved controversial.
Although humans began to have big impacts on the planet with the rise of widespread farming, and later the industrial revolution, neither happened in every part of the world at the same time.
Now an international team of experts has concluded that the Anthropocene is visible globally in the top sediment layer of the Earth’s surface, starting in the 1950s.
They say the period is marked by the appearance of plutonium, a radioactive element used in nuclear weapons, as well as other indicators of a surge in human activity, called “the great acceleration”.
Professor Colin Waters, chair of the Anthropocene Working Group (AWG), said the sudden changes are an indelible signature of human influence, a so-called “golden spike”.
He said: “The presence of the plutonium mark is a very useful tool to allow you to define that boundary.
“You [also] have all these other markers reflecting the big changes to the planet that happened in the great acceleration – increasing consumption of fossil fuels, the greater use of nitrogen fertilisers, the sort of increased trade globally that’s spreading species across the planet and homogenising the biota [plant and animal life] of the planet.
“All of these things change very rapidly at that point. That’s the critical thing about the Anthropocene.”
The scientists from the AWG have nominated Crawford Lake, near Toronto, Canada, as the official global monitoring site for the Anthropocene.
It was selected from a shortlist that included a peat bog in Poland and a coral reef in Australia.
Crawford Lake is 24m deep but has a small surface area, which means the muddy bottom is undisturbed.
Professor Francine McCarthy, who studies the lake and is a member of the AWG, said: “The bottom of the lake is completely isolated from the rest of the planet, except for what gently sinks to the bottom and accumulates in sediment.”
In effect the lake bottom is a time record of environmental changes to the planet over thousands of years.
Plutonium from nuclear weapons testing in the atmosphere suddenly appears in the sediment in the mid-20th century.
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At the same time there is evidence of microplastics, ash from coal-fired power stations and concentrations of heavy metals such as lead.
Officially we are living in the Holocene epoch, which began about 11,700 years ago when the climate became more stable.
The declaration of the new Anthropocene epoch, and the use of Crawford Lake as a monitoring site, still need to be signed off by three other scientific bodies that are recognised as the official guardians of the geological time record.
But some scientists have questioned the need for a new epoch defining the “human age”.
Dr Alexander Farnsworth, a researcher in geological sciences at the University of Bristol, said: “We are but a ripple in the river of gene flow through time.
“Is the purpose to detect the human impact on the natural earth system if we were to go extinct?
“If another advanced civilisation were to evolve in 100 million years, could they tell such a spike was due to a previous advanced civilisation or would they simply interpret it as an interesting natural excursion without any other evidence of our existence?”