Are we in the ‘Anthropocene’, the human age? No, say scientists.

The Triassic was the beginning of the dinosaurs. The Paleogene saw the rise of mammals. The Pleistocene included the last ice ages.

Is it time to mark humanity’s transformation of the planet with its own chapter in Earth’s history, the ‘Anthropocene’, or the human era?

Not yet, scientists have decided, after a debate that lasted almost fifteen years. Or the blink of an eye, depending on how you look at it.

A committee of about two dozen scientists has overwhelmingly rejected a proposal to declare the start of the Anthropocene, a newly created epoch of geological time, according to an internal announcement of the voting results seen by The New York Times.

According to geologists’ current timeline of Earth’s 4.6 billion year history, our world is now in the Holocene, which began 11,700 years ago with the most recent retreat of the great glaciers. Changing the chronology to say that we have moved into the Anthropocene would be an acknowledgment that recent human-induced changes in geological conditions had been profound enough to bring the Holocene to an end.

The statement would shape terminology in textbooks, research articles and museums around the world. It could guide scientists in their understanding of our still-unfolding present for generations, perhaps even millennia, to come.

But ultimately, the committee members who voted on the Anthropocene last month weren’t just thinking about the consequences this period had had on the planet. They also had to figure out exactly when it started.

According to the definition that an earlier panel of experts spent nearly a decade and a half debating and tinkering with, the Anthropocene began in the mid-20th century, when nuclear bomb tests spread radioactive fallout across our world. For several members of the scientific committee who have been considering the panel’s proposal in recent weeks, this definition was too narrow and too clumsy recent to be an appropriate guidepost to the reshaping of planet Earth by Homo sapiens.

“It limits, limits, limits the entire importance of the Anthropocene,” says Jan A. Piotrowski, a committee member and geologist at Aarhus University in Denmark. “What happened during the beginning of agriculture? What about the Industrial Revolution? What about the colonization of America, of Australia?”

“The human impact goes much deeper into geological time,” said another committee member, Mike Walker, an earth scientist and emeritus professor at Trinity Saint David, University of Wales. “If we ignore that, we ignore the real impact, the real impact, that people have on our planet.”

Hours after the voting results were circulated within the committee early Tuesday, some members said they were surprised by the margin of votes against the Anthropocene proposal compared to those in favor: 12 to four, with two abstentions.

Still, it was unclear Tuesday morning whether the results constituted a final rejection or whether they could still be appealed or appealed. In an email to The Times, the committee’s chairman, Jan A. Zalasiewicz, said there were “some procedural issues to consider” but declined to discuss them further. Dr. Zalasiewicz, a geologist at the University of Leicester, has expressed support for the canonization of the Anthropocene.

This question of how to situate our time in the storyline of Earth’s history has thrust the rarefied world of geological timekeepers into an unfamiliar spotlight.

The grand chapters of our planet’s history are governed by a group of scientists, the International Union of Geological Sciences. The organization uses strict criteria to decide when each chapter began and what characteristics defined it. The goal is to maintain common global standards for expressing the planet’s history.

Geoscientists do not deny that our era stands out within that long history. Radionuclides from nuclear tests. Plastics and industrial ash. Concrete and metal contaminants. Rapid greenhouse warming. Significantly increased species extinction. These and other products of modern civilization leave unmistakable remnants in the mineral records, especially since the mid-20th century.

Yet to qualify for its own entry onto the geological time scale, the Anthropocene would have to be defined in a very specific way, one that would meet the needs of geologists and not necessarily those of anthropologists, artists, and others who already use the condition.

That is why several experts who have expressed skepticism about enshrining the Anthropocene emphasized that the vote against it should not be read as a referendum among scientists on the general state of the Earth. “For the most part, this has been a narrow, technical issue for geologists,” said one of those skeptics, Erle C. Ellis, an environmental scientist at the University of Maryland, Baltimore County. “This has nothing to do with the evidence that humans are changing the planet,” said Dr. Ellis. “The evidence continues to grow.”

Francine MG McCarthy, a micropaleontologist at Brock University in St. Catharines, Ontario, is the opposite of a skeptic: She helped lead some of the research supporting the ratification of the New Era.

“We are in the Anthropocene, regardless of any line on the time scale,” said Dr. McCarthy. “And behaving accordingly is our only way forward.”

The Anthropocene proposal started in 2009, when a working group was convened to investigate whether recent planetary changes merited a place on the geological timeline. After years of discussion, the group, which also included Dr. McCarthy, Dr. Ellis and about three dozen others were among those who did so. The group also decided that the best starting date for the new period was around 1950.

The group then had to choose a physical location that would most clearly demonstrate a definitive break between the Holocene and the Anthropocene. They settled on Crawford Lake, Ontario, where the deep waters have preserved detailed records of geochemical changes in the sediments at the bottom.

Last fall, the working group submitted its Anthropocene proposal to the first of three governing committees under the International Union for Geological Sciences. Sixty percent of each committee must approve the proposal to move on to the next.

The members of the first, the Subcommittee on Quaternary Stratigraphy, cast their votes in early February. (Stratigraphy is the branch of geology that deals with rock layers and how they relate to time. The Quaternary is the continuous geological period that began 2.6 million years ago.)

According to the rules of stratigraphy, every interval of Earth time needs a clear, objective starting point, a starting point that applies worldwide. The Anthropocene Working Group suggested the mid-20th century because it bracketed the postwar explosion of economic growth, globalization, urbanization, and energy consumption. But several subcommittee members said humanity’s overthrow of the Earth was a much bigger story, one that may not even have a single start date for every part of the planet.

This is why Dr. Walker, Dr. Piotrowski and others prefer to describe the Anthropocene as an ‘event’, and not as an ‘age’. In the language of geology, events are a looser term. They do not appear on the official timeline and no committee has to approve their start dates.

Yet many of the most important events on our planet are called events, including mass extinctions, rapid expansions of biodiversity and the filling of the sky with oxygen 2.1 to 2.4 billion years ago.

Even if the subcommittee’s vote is upheld and the Anthropocene proposal is rejected, the new era could still be added to the timeline at a later date. However, it would have to go through the entire process of discussion and voting again.

Time will move on. Evidence of the effects of our civilization on Earth will continue to accumulate in the rocks. The task of interpreting what it all means, and how it fits into the grand scheme of history, could fall to the future heirs of our world.

“Our impact will continue and will be recognizable in the geological record in the future – there is absolutely no doubt about that,” said Dr. Piotrowski. “It will be up to the people who come after us to decide how they will rank it.”