The beginning of the Anthropocene geologic era -- during which humans have increasingly have shaped the planet -- should be dated to July 16, 1945, the date of the Trinity test of the world's first nuclear weapon, an international science group suggests.

Science has traditionally divided Earth's history into time units such as epochs and periods determined by biological or geological signals.

"Defining the Anthropocene as a new geological epoch would imply that humans are a geological force every bit as powerful as the 'natural' ones that caused such things as the onset of ice ages and major extinction events in Earth's past," says paleontologist Anthony Barnosky of the University of California, Berkeley, a co-author of a paper published in the Quaternary International journal.

The paper, and its recommendation of the Trinity start data, was written by members of the Anthropocene Working Group, an panel of international researchers assembling evidence to decide if the Anthropocene should receive formal designation as a new geological epoch and what to mark as its beginning.

Nobel Prize-winning atmospheric chemist Paul Crutzen and the late University of Michigan biologist Eugene Stoermer at the University of Michigan coined the term "Anthropocene" in 2002, applying it to a period of Earth time when human activities began to forever alter geologically significant processes and conditions.

Because the Trinity test in New Mexico can be surveyed easily as the start point of the worldwide spread of artificial radioactivity from nuclear bomb tests, and because it approximately coincides with the spread of major human-caused effects leaving permanent evidence in the global geological record, the scientists argue it is a logical start point for the Anthropocene era.

"Like any geological boundary, it is not a perfect marker -- levels of global radiation really rose in the early 1950s, as salvos of bomb tests took place," says Jan Zalasiewicz, chair of the working group. "But it may be the optimal way to resolve the multiple lines of evidence on human-driven planetary change."

The Trinity explosion, which sent a mushroom cloud 40,000 feet into the air, was the first test of the nuclear weapons developed by the Manhattan Project during World War II.

Radioactive particles from the explosion eventually settled to earth from the poles to the equator, leaving a detectable signal in modern geological strata around the globe.

It is a strong candidate marking the point in the mid-20th century when humans went from leaving traces of their presence on Earth to physically altering Earth systems through an acceleration of population, carbon emission, species invasions and physically altering the face of the Earth through construction of cities and infrastructure, the working group says.

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