THE ANTHROPOCENE | See Global warming, climate change

‘Dawn of a new epoch?, The Economist 03rd Sept 2016, p.69

In the 1500s Nicolaus Copernicus kicked Earth from its perch at the centre of the universe. Later, Charles Darwin showed that humans are just another species of animal. In the 20th century geologists found that all human history amounts to less than an eye-blink in the span of a planet that they discovered is 4.6 billion years old.

On August 29th Colin Waters, the secretary of the Anthropocene Working Group (AWG), an ad hoc collection of geologists, addressed the International Geological Congress in Cape Town. He told his colleagues that there was a good case for ringing down the curtain on the Holocene—the present geological epoch, which has lasted for 12,000 years—and recognising that Earth has entered a new one, the Anthropocene.

Dr Crutzen is an atmospheric chemist, and the growth of carbon-dioxide concentrations in the atmosphere is perhaps the most familiar symptom of the Anthropocene. For instance, acidity in oceans caused by extra CO2 affects the make-up of creatures whose shells will form Earth’s future limestones. Nitrogen, too, is affected. The process by which this vital element cycles through the air, the soil and living organisms has been turbocharged by human use of artificial fertilisers. One consequence is the expansion of food production. In 1750 about 5% of the Earth’s surface was farmed. That figure is now around 50%, and the transformation from wilderness to agricultural land leaves lasting changes in the nature of the soil. On top of this, dams hold back billions of tonnes of silt. As a result, river deltas everywhere are shrinking.

Markers of the Anthropocene will surely be visible in the fossil record. On present trends, numerous species will vanish from that record—exterminated by human activity. Meanwhile, “technofossils” will appear. A favourite for long-term preservation, for example, is the porcelain water closet. New types of mineral may come into existence as a result of things like the deposition of elemental aluminium in the soil (the stuff is unknown in nature) and the settling to the sea bed of zillions of plastic scraps now littering the ocean. Beds of fly ash from power stations may get consolidated into novel rocks. And who knows what refuse tips will look like when buried, compressed and metamorphosed?

The AWG, then, believes that the Anthropocene is real. The next question is how to define it. The traditions of geology demand a clear and sudden change, visible in the rocks. There are several contenders, including the appearance of plastics in the 1950s and the exchange of species between the New and Old Worlds in the 1600s. But most of the AWG’s members plumped for the high point of nuclear-weapons testing, in 1964. Fallout from those tests scattered plutonium, an element vanishingly rare in nature, far and wide across the planet. Future geologists, depending on precisely how much time has passed and therefore how much radioactive decay has occurred, will be able to see a layer of plutonium, or of uranium, or (eventually) of lead in the rocks. At the congress, the AWG’s members voted for this “bomb spike” to be the marker. That makes the Anthropocene more than half a century old already.

The next step will be to point to a single piece of the geological record (an ice core, perhaps, or samples from lake sediments) that can serve as the officially accepted reference point. Then the proposal must make its way through several strata of geological bureaucracy, any of which could scupper it. The last step will be a vote at a meeting of the International Union of Geological Sciences. If that passes, then geological time, whose passage is famously slow, will have ticked perceptibly on.