Humans have had a profound impact on the planet, often unintentionally and sometimes with destructive consequences, and now our traces are starting to show up in the Earth's uppermost mineral layer, implying we may be living in a new geological epoch.

Not only have we caused the extinction of many plant and animal species that now only remain as fossils. We've also polluted ponds, rivers and oceans with micro-plastic particles.

Plastics are now being found by geologists in sediments, along with other manufactured materials including concrete and elemental aluminium. In nature, that metal is never found free, but always in compounds with other elements.

A group of scientists says humanity's impact on the Earth has been so great that the current geological epoch needs a name of its own: the "Anthropocene" - from the Greek anthropo, for "man," and cene, for "new."

In a study published recently in the US-based journal Science, they present evidence in support of their position.

Debate in scientific circles over justification for a new epoch has heated up since 2000, when the term "Anthropocene" was popularized by the Dutch atmospheric chemist and Nobel laureate Paul Crutzen.

He argued that the epoch hitherto, the Holocene ("entirely recent"), was over. It began about 11,700 years ago after the last major Ice Age and has been characterized - until around the mid-19th or -20th century - by relatively constant environmental conditions.

Atmospheric concentrations of carbon dioxide and methane were fairly steady right through the Holocene. 

The international team behind the paper in Science includes Reinhold Leinfelder, a geologist at the Free University of Berlin, who in Germany has taken the lead in introducing the Anthropocene concept to laypeople. 

"We've compiled everything there is - all of the criteria showing that the Anthropocene differs from the Holocene," Leinfelder said. 

The indicators include the appearance in sediments of manufactured materials along with artificial radioactive isotopes from nuclear weapons testing and particulates from fossil fuel combustion, accelerating species extinction, increasing greenhouse gas concentrations as well as glacier retreat and sea-level rise due to climate warming, among other lasting impacts.

They show that the Anthropocene concept is based on fact, said study co-author Jan Zalasiewicz, a geologist at Leicester University in Britain and chairman of the Anthropocene Working Group of the International Commission on Stratigraphy (ICS).

Another co-author, Colin Waters, principal geologist at the British Geological Survey, told the Guardian newspaper: "What this paper does is to say the changes are as big as those that happened at the end of the last Ice Age. This is a big deal." 

Leinfelder sees the study as a good basis for a decision on whether to formally declare a new epoch, which could happen this year. The ICS, part of the International Union of Geological Sciences and the body responsible for defining geologic time spans, is expected to meet in late August.

But agreement must be reached on a starting date for the Anthropocene. Proposals include an "early Anthropocene" beginning about 8,000 years ago with the spread of agriculture and deforestation, the exchange of Old and New World species after the discovery of America by Columbus in the late 15th century, the Industrial Revolution around 1800, and what the authors of the Science study call the "great acceleration" of population growth in the mid-20th century.

The authors themselves point to a Holocene-Anthropocene boundary sometime between 1945, when the first atomic device was detonated in the US state of New Mexico, and the so-called "bomb spike" in nuclear weapons testing that peaked in 1964 and left radioactive fallout detectable in sediments and glacial ice.

A more general question is: What's the point in formally recognizing an Anthropocene epoch?

Geologist Manfred Menning from the German Research Centre for Geosciences, who until recently was chairman of the German Stratigraphic Commission, objects that the body's repeated Anthropocene debates always ended with the consensus that "the term is absolutely useless for geological work and therefore dispensable in geology." 

Other critics say that contemporary scientists are much too close to the matter to have a clear perspective.

Leinfelder said, however, that formal recognition of a new geological epoch could make people more aware of their impact on the planet, helping the Anthropocene go down in history not only as a time of environmental destruction.

"There won't be just one correct path," he remarked. "But if we have this geological power, as we now see we do, we should be able to wield it positively with today's knowledge."

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