London: 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 2000 people, some 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.
What our planet would look like now, if humans had gone extinct thousands of years ago? Earth would likely still be largely forested and roamed by large creatures, like mammoths and fearsome sabre-toothed tigers, as the planet headed slowly for the next ice age.
Perhaps a better question is: to what degree has the survival and triumph of our species changed our planet? From my desk, I see an entirely human world built of fashioned materials from glass to bricks. But we are not the only species to modify our environment – termites build towering castles in the sand and beavers deforest and divert rivers with their dams, for example. Tiny microorganisms such as bacteria and algae, arguably have the most profound effect on the planet’s environment, through their production of oxygen. So how do humans compare?
The best people to answer this could well be those with 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 Earth’s long history. And they can also look forward to predict what our current times might look like to their future colleagues. Like a time travelling sleuth, I sought out these geologists for a new radio series I’m presenting, which begins next week on BBC World Service.
There have been plenty of dramatic changes to our planet in the past, when this spinning lump of rock has flipped from a ‘Snowball’ Earth to searing temperatures devoid of ice, times when life has flourished and 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 label these planetary events as they discover them, in order to piece together a detailed history of what our planet looked like long before anything resembling our species even evolved. For example, they have named the relatively warm period during which dinosaurs dominated the land as the Cretaceous (from the Latin ‘creta’, meaning chalk, because the identifying fossils for the period were first found in chalk beds). Around 65 million years ago, a huge meteorite struck Earth, throwing up clouds of dust and rocks that dramatically cooled the climate and lead to the extinction of much of life on Earth, including the dinosaurs. Geologists can identify when this happened because of they see a sudden arrival of the metal iridium in the rock record, which was carried to our planet on the asteroid. It marks the boundary into a new geological time period, the Tertiary or Paleogene, when mammals came into their own.
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. Many are saying that we’ve entered a new geological time period called the Anthropocene, the Age of Man.
For geologists, this is no easy decision. To define a new time period, we would have to be making changes to the planet that would show up clearly a million years from now, or 100 million years. It means that our human footprint would need to be preserved in the rock record in the same way as the changes made 65 million years ago by the comet.
The rock record is composed of layers of deposited sediments building up and being compressed over time until they form solid rock. Organic matter – lifeforms like plants or animals – get trapped in the sediment layers and over time their chemistry changes as the original molecules of life get substituted by metals, silicates and salts. The physical form of a creature may be perfectly preserved, but instead of being made of proteins and bone or skin, they will be solid fossils cast in the rocks for perpetuity. Even tiny forms like pollen grains or delicate leaves and insects may be rendered perfectly identifiable through this process millions of years after they landed in the mud.
But what about the changes we’re making to our planet now? Would they be identifiable as human-caused, would they be significant enough to show up as a clear boundary in the rock record, and would they last over geological distances? 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?
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 of these changes would be readable in the rocks 10 million years hence.
At a farm in Maryland, ecologist Erle Ellis described 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 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, 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 and spread around. This is where our human mark will be most clearly tattooed 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 at the end of the Triassic extinction and the beginning of the Jurassic, when reptiles took over 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, 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 – 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.
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 weekly from Monday 22 October at 19.32 BST.