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.
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