There have been a few times in the history of mankind when we nearly died out as a species. Anthropologists call these events “bottlenecks”, times when the population of humans shrank – perhaps to as few as 2,000 people over 50,000 years ago. At those levels, we would be categorised as an endangered species on the IUCN Red List, existing in even fewer numbers than wild tigers do today.
We survived, and in fact we’ve thrived, mainly because we adapted our environment to suit our needs. But to what degree has the survival and triumph of our species changed our planet?
The best people to answer this could well be those who have the grandest perspective. Geologists can take a 4.5-billion-year step back and look at the human impact on our planet in the context of its long history – they can identify changes in the rock record within layers of deposited sediments that build up and are compressed over time.
There have been plenty of dramatic changes to our planet in the past: this spinning lump of rock has flipped from a “Snowball” Earth to searing temperatures devoid of ice; there have been times when life has flourished, and times when it has been beset by mass extinctions. Big changes in planetary states show up in the rock record as stripes that reveal the sudden disappearance or appearance of certain fossil species, or climatic changes that reveal changes in past sea level. We can even identify the concentration of certain gases in the atmosphere or oceans by looking at the types of minerals deposited, because some elements, such as carbon, nitrogen and oxygen exist in fractionally heavier or lighter types – isotopes – and the ratio of these isotopes can vary according to the element's source. Geologists can also identify when a huge meteorite struck Earth 65 million years ago and led to the extinction of much of life on Earth, including the dinosaurs, because they see a sudden arrival of the metal iridium in the rock record, which was carried to our planet on the asteroid.
Many geologists think that the changes humans are making to our planet now are so significant in scale and lastingness, that they rank up there with past changes made by asteroid impacts and supervolcanoes – and as a result many are calling this period the Anthropocene. But would these changes be identifiable as human-caused, would they be significant enough to show up as a clear boundary in the rock record, and would they be preserved in the same way as the changes made 65 million years ago by the comet, for instance? In other words, is the term Anthropocene just a handy reference for current scientists, sociologists and politicians, or does it have a more significant geological meaning?
For geologists, this is no easy decision.
But as the painstaking process of deciding whether to formalise the new time period into official geological nomenclature begins, I've been talking to geologists, ecologists, biodiversity experts and ice scientists about how humans have changed the planet, and which changes they think would be readable in the rocks millions and tens of millions of years from now.
At a farm in Maryland, ecologist Erle Ellis told me how the global-scale conversion of forests to farmland –-Ellis calculates that 75% of Earth's land surface has been modified by humans –-would show up in changes to pollen fossils. There would be an obvious increase in the presence of certain grain species, such as wheat, and a decrease in the wild pre-human flora diversity. Our discovery of how to make nitrogen fertilisers would also be clear. Humans have doubled the amount of reactive nitrogen – the type of nitrogen our bodies can use – through fertiliser use and from fossil fuel pollution. And much of the nitrogen newly entering the planet's cycle is of a different isotope (weight) to that naturally formed by bacterial processes, so it would be identifiable in the rock record using a machine called a mass spectrometer.
We've also been rapidly altering our atmosphere, changing the climate and ocean chemistry in ways reminiscent to previous planetary state-changes the Earth has experienced. Like those ancient changes, our recent warming will be visible in the rocks millions of years from now. Polar scientists described ice melt that would lead to sea level rise, which would alter sediment patterns. Melting sea ice in the Arctic would increase the amount of plankton living – and dying – there, and hence lead to an accumulation of organic matter in the seabed. And the acidifying oceans, from the greater amount of carbon dioxide being dissolved, would lead to a dying off of corals and shelled creatures dying off, which would leave a mark in ocean sediments.
We are shifting species around the world, spreading some animals and plants everywhere, such as rats, and eliminating others altogether. In California, biologists Anthony Barnosky and Elizabeth Hadly described a mass extinction we are causing, on a par with the one that wiped out the dinosaurs. Like the meteor-caused extinction, our human-led one would be clearly visible in the rocks, they say, as fossilised species disappear in higher layers. And the evolutionary legacy of lost lineages would also be apparent in the descendants of survivors.
Our impact is perhaps most obvious in our cities, roads and infrastructure, and in all the materials we have mined, created or synthesised and spread around. This is where our human mark will be most clearly ingrained into the rocks, according to geologist Jan Zalasiewicz. Outside Leicester, he showed me where a railway cutting had exposed a clear line in the rocks made 180 million years ago, marking the so-called Toarcian extinction event in the early Jurassic, when dinosaurs were beginning their domination of the world. A similarly clear mark in the rocks would reveal our human geological age, he said. The rocks of the Anthropocene would show an accumulation of novel chemicals, like artificial PCBs, and aluminium and steel, which have to be manufactured.
The shapes of our buildings, our manufactured products like phones and drinks bottles, and our underground cables would leave their mark as synthetic fossils, akin to the imprint of a leaf from the Cretaceous period, he thinks.
None of the people I spoke to had any doubt that humans were leaving a profound and lasting mark on our planet's geology. But whether the Anthropocene will be a long-lasting period in Earth's history, or a geologically short episode depends on us – that is, how long humans survive as a species and how much further we modify our rocky home. The Earth is in it for the long haul, it’s up to us, the new planet-changers, as to what legacy we choose to be etched into its rocks.
You can hear my conversations with these scientists and others, discussing our impacts on the planet's geology, in a new four-part series called The Age We Made, starting on Monday 22 October on BBC World Service.