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In 1922 British geologist Robert Sherlock put forth what is now considered to be the central argument for recognizing the Anthropocene (人类世) as a new geological era: the scale and character of human activities have become so great as to compete with natural forces. About one hundred years later, geologists have broadly accepted Sherlock's core idea, and the Anthropocene Working Group has proposed Crawford Lake in Canada as the official site for marking the Anthropocene.

The proposal attracted a great deal of press, much of it focused on a misguided controversy over how narrowly to define the Anthropocene. Amid this debate, observers may have been left to wonder why defining this chapter in Earth's history should matter to ordinary people at all.

Sherlock was not a lone wolf. He built on the work of others. One was an American scholar George Marsh, who had called attention to deforestation and the role of humans as "disturbing agents". In addition to revisiting deforestation, Sherlock described the changed courses of rivers through dams and canals; and the huge quantities of stuff people move while mining the raw materials of modern civilization. Human impacts were becoming so manifest, Sherlock argued, that the distinction between "natural" and "artificial" was becoming difficult to sustain. We needed a new term to study the effects of human activities on Earth. Scientists of later generations followed his footsteps. And in 2000 Eugene F. Stormer and Paul J. Crutzen formally proposed the word "Anthropocene" in a paper.

But science is conservative in nature - the burden of proof is always on those making a novel claim - and the social and economic consequences of recognizing the adverse effects of burning fossil fuels have led to enormous resistance beyond scientific community.

The definition of the Anthropocene matters for at least two reasons. First, it is a way for scientists to declare that the shifts going on around us are no small issue. Anthropogenic climate change is a profound change in the conditions of life on Earth. In countless ways, the past may no longer be a reliable guide to the future. We must rethink core assumptions about how we build our economies and our infrastructures, how we travel, and even how we eat.

Second, the definition of the Anthropocene extends the conversation beyond climate change. What geologists can now see in rocks - from the subtle to the gross - points to the widespread and lasting impact of human activities on Earth.

It is common for people to say that as climate change proceeds, we can "just adapt". Some wealthy people even think that, if necessary, they will move to higher ground or lower latitudes. No doubt some people will become climate refugees, either voluntarily or under force. But the definition of the Anthropocene reminds us that the challenge we face is geological in scale. It affects the whole Earth. It reminds us that as this new era unfolds, there won't be anywhere to hide.