Nuclear bombs transformed the planet and set off the Age of Humans, geologists say

Around 1950, as atomic bombs fell on empty deserts, crowded cities and island atolls; as highways carved concrete paths across the planet; as populations exploded; as consumption skyrocketed; and as the average global temperature began its dangerous upward creep in earnest, Earth entered the epoch of humans.

Or at least, that’s the consensus of the Working Group on the Anthropocene, the team of 35 scientists tasked with figuring out whether humans have left enough of a mark on this planet to qualify for our own unit of geological time.

In January, the working group published a paper in the journal Science arguing that the Anthropocene is a “functionally and stratigraphically distinct” unit of geologic time. In other words, the planet has been altered so thoroughly by human presence that the changes are permanently inscribed in the rock record, much as scientists can still see evidence of the asteroid impact that killed the dinosaurs.

Monday, members of the group presented their findings at the annual International Geological Congress in Cape Town, South Africa. The presentation was not a formal recommendation — the working group will only take that step after they can identify a clear “golden spike,” a physical reference point in the rock record that marks the start of a new era. Then it will be up to the International Commission on Stratigraphy (ICS), the group charged with carving up time into units that can be delineated in the geologic record, to make a final call.

That decision is years away — possibly even decades, based on the pace at which the geological community moves (only slightly speedier than glaciers). Designating a new unit of geologic time is a big decision, and if it is approved, the Anthropocene epoch will be shorter than any other time frame in geology history. Most other epochs are at least a million years long. The Anthropocene, so far, is barely six decades old.

Still, all but five of the working group’s 35 members believe it’s worth formalizing the Anthropocene, and of those, 28 agree that it should start around 1950, when plutonium fallout and increased radioactive carbon from nuclear weapons testing becomes evident in the rock record.

“Now we have to start the hunt for the golden spike,” said Jan Zalasiewicz, a geologist at the University of Leicester and the chair of the working group.

That involves an extensive search for signatures of pronounced global change. He and his colleagues will likely go looking for signs of the Anthropocene in polar ice, in the layers of sediments in at the bottoms of lakes and oceans, in the branching arms of corals and the rings of trees.

“It’s a case of trying to assess all of those in general and looking for the best examples of the ones we think will give the best picture,” he said. “We’ll probably try and have a one global marker, and there will also be a few auxiliary stratotypes,” or secondary signatures that future geologists can look for.

There still isn’t consensus within the group that the mid-20th century is the best start date for the Anthropocene, or even that the Anthropocene merits its own epoch. Some of the working group members favored a starting point of about 3,000 years ago, or a “diachronous” designation that would acknowledge that the epoch’s onset varied from place to place. Three members opposed any formalization of the epoch (even though most agreed the Anthropocene is stratigraphically real), arguing that the move is premature. If the ICS rejects the proposal, both the idea of the Anthropocene and the geology community at large could be discredited.

“I feel like a lighthouse with a huge tsunami wave coming at it,” Stan Finney, a geologist at California State University at Long Beach and the chair of the ICS, told Science last week.

The geology community has very strict standards for determining new epochs, and many members are resentful of the influence that outside communities — politicians, climate activists, etc. — have had on the Anthropocene discussion. Zalasiewicz is acutely aware that he’s got to convince a skeptical audience.

“The science has got to be very carefully done,” he said of the search for the golden spike. But, knowing how skeptical members of his own working group have already been, “if we’re happy with the formal recommendation we hope it will also have a chance of convincing our colleagues.”

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