If human beings disappeared from the face of the Earth, what would we leave behind us? There is such a mass of our detritus - roads, concrete tower blocks, dumps of plastic bags, nuclear power stations - that our passing as a species would surely leave an astonishing amount of non-biodegradable rubbish that would impede any future species. That much seems obvious - and yet to a geologist it is not obvious at all. Jan Zalasiewicz's thesis is that the traces left behind by Homo sapiens, as seen by a future race of geologists, would be much less than the dinosaurs left behind them.
When I picked up this book, I was instantly depressed. I don't like science fiction, and futurology seems to be the worst kind of science fiction - the meaningless imaginings of one person dressed up as a kind of science or social science. Admittedly, futurology from 1900 is great fun, when one sees the ludicrous notions they had about life in 2000. That former efforts have all been wrong suggests that current efforts are likely to fare no better. So why do it, and why read it?
But after I had read the first chapter of Zalasiewicz's book, I realised he had done something much better. He is not imagining how humanity might evolve in the future, or how technology might change our lives. His question about our long-term mark on the Earth is framed in classic geological terms, and he develops his thesis - that our mark will be modest - in rational and elegant terms.
Zalasiewicz imagines a time, some millions of years in the future, when intelligent beings chance upon the Earth and use their skills to determine how it works. There is no mention of future earthly life - is life entirely extinct? Perhaps life is there. The focus is on understanding the rock layers that have accumulated, and the dynamics of the Earth, with its molten core and mantle and mobile crust.
How would these future creatures identify our former existence? The first evidence is a thin mudstone layer containing abundant pollen of grasses, and representing a rather uniform aspect worldwide. This represents the time since the Industrial Revolution, documenting global deforestation and the loss of plant diversity.
No longer does the future palaeontologist find evidence of different species on all the continents, but only one or two strains of rice, representing of course the rapid spread of modern agriculture. Zalasiewicz terms this the "McDonaldisation of life ... the sudden appearance of floods of identikit pollen of crop plants around large parts of the globe".
The second marker of the human stratum is the sudden appearance of abundant shrimps, crabs and gastropods in marine layers. The depletion of marine fish stocks by overfishing means that the prey of those fishes becomes startlingly abundant. The disappearance of prolific cod, herring and even whales might be harder to detect - the species live on, so there is no extinction level.
But the sudden loss of reefs would be obvious as a third marker of the human stratum. Pollution, specimen collecting and other human activities are destroying reefs.
More important is ocean acidification from excess carbon dioxide in the atmosphere. Reef-builders live in skeletons of calcium carbonate, and this is dissolved in dilute acid, so great swaths of modern reefs are already dead, and more will die. Such reef crises do stand out in the rock record, and that would be one proud legacy of our time on Earth. The acid oceans would also see dramatic thinning of the shells of molluscs and other skeletonised creatures, and perhaps widespread extinction of those species, too.
But why the thin mudstone layer over the ocean floor? Ocean acidification destroys not only the calcium carbonate houses of living animals; it also destroys the top few centimetres of calcareous ooze on the ocean floor, leaving a residue of the inert material behind. To a future geologist, the thin, non-calcareous layer within successions of limestone below and above would point to a major, global acidification event.
There would also be traces of human civilisation, but perhaps not as many as we might expect. Clearly the concrete, bricks, asphalt, plastic and metal detritus would remain visible for some hundreds or thousands of years. But after millions of years, most of that would disappear. Glass, asphalt and bricks are made from the earth, and to the earth they would return. Coastal cities would soon be swamped by rising sea levels and progressively buried under mud. Cities sited away from the coast would not be buried but might collapse and be washed downhill.
Eventually even the most imposing of concrete buildings would collapse, and the concrete and steel frame would denature through chemical attack. The rusty steel would break into individual molecules, the sand and gravel in the cement mix would return to the earth, and the calcium, aluminium and other ingredients would form new compounds.
Nonetheless, a modern city would surely leave a layer a few metres thick in places of denatured concrete, metal and other materials. Some modern plastics are designed to degrade quickly, but most are not. Even so, plastics are made from hydrocarbons, and they will eventually degrade to thin black cuticles, like some of the long-chain organic molecules that made up the tough outer coats of ancient organisms. So those seemingly permanent markers of modern civilisation, such as concrete, steel and plastic, are not inert and will decay to a greater or lesser extent.
In tackling what might seem a rather minor conceit - what, if anything, will humans leave behind for future species to enjoy? - Zalasiewicz presents an elegant and authoritative primer on the earth sciences. In Victorian times, writers such as Charles Lyell, Thomas Henry Huxley and Gideon Mantell delivered the latest geological science to the public. Now, Zalasiewicz weaves modern thinking about the Earth's crust, geological time, plate tectonics, volcanology, palaeontology, oceans and atmospheres in an appealing narrative that makes everything clear. He refers to other planets, too, in asking questions such as: Why is the Earth covered thickly with sedimentary strata, and yet Mars is not? Why does only Earth have an atmosphere? Why can we see craters clearly marked on the surface of all other planets?
So many popular science books are rushed and awkward; the authors have little skill in writing, and the editors apparently abandon the effort to weave the prose into a smooth narrative, but this book is beautifully written. I like Zalasiewicz's characterisation of James Croll, autodidact and iconoclast, as "stubborn, eccentric, and Scottish".
It is hard to make zonal index fossils sexy, but Zalasiewicz more or less succeeds: "A biostratigrapher wishes upon any species the life of the wilder kind of Romantic poet: early brilliant success, a worldwide reach, and then a sudden death ... The sudden death is devoutly to be wished for. There is nothing quite so depressing to a biostratigrapher as a successful fossil that wholly overstays its welcome."
This is a wonderful, elegant short book. The directing theme of humanity's ultimate mark on the Earth is handled imaginatively and carefully, and gives pause for thought. It works even better, I would say, as an easy entree to modern Earth sciences.
Jan Zalasiewicz says his career has followed "no particular logic". He has pursued a rather irregular path between the study of truly ancient phenomena of the Earth (up to 3.4 billion years ago) to those that are more recent (the Ice Ages of the last couple of million years) and contemporary (human impact on the world's geology).
He would like to further explore the early years of geology, "when the realisation first dawned that the Earth's past is truly stranger and more dramatic than mythology or science fiction". He would also like to pursue a less conventional interest between earth sciences and music, another of his passions. He says there are links: "When humans first recorded music, they invented an entirely new - and perfect - kind of fossil (think of the preserved voices of Caruso or Callas). The development of sonopalaeontology as the most frivolous of subdisciplines would be highly agreeable."
The Earth after Us: What Legacy Will Humans Leave in the Rocks?
By Jan Zalasiewicz
Oxford University Press
Published 25 September 2008