Fifty-six million years ago, the Earth warmed far above the temperatures we experience today – and ice at the poles melted.

Could the event – the Paleocene-Eocene Thermal Maximum (PETM) – be evidence of global warming caused by an extinct civilisation which ruled Earth before mankind? It sounds like the kind of crackpot conspiracy to be found on ‘ancient alien’ UFO sites, but scientists have seriously investigated the idea. Professor Adam Frank of the University of Rochester writes in The Atlantic that the investigation could offer insight into how to find alien life – or about our impact on our planet. Frank writes, ‘There is a conundrum here. If an earlier species’s industrial activity is short-lived, we might not be able to easily see it. ‘The PETM’s spikes mostly show us the Earth’s timescales for responding to whatever caused it, not necessarily the timescale of the cause.

‘So it might take both dedicated and novel detection methods to find evidence of a truly short-lived event in ancient sediments. In other words, if you’re not explicitly looking for it, you might not see it.’ Frank says the researchers thought about what our civilisation might leave behind in rocks for future civilisations to find – plastics in the ocean, evidence of fertiliser use, and (most of all) evidence of fossil fuel use. Frank says that civilisations might – through their collapse – trigger the conditions to create more fossil fuels, for future civilisations to use. Frank says, ‘Our work also opened up the speculative possibility that some planets might have fossil-fuel-driven cycles of civilization building and collapse. If a civilization uses fossil fuels, the climate change they trigger can lead to a large decrease in ocean oxygen levels.