The Anthropocene

The new synthetic materials do far more than make new kinds of rock. Earth on its own produced about five thousand natural minerals, all but a few hundred very rare. But mostly since the mid-20th century, the number of new synthetic inorganic crystalline compounds – minerals in all but formal name – have exploded to exceed 200,000 types, making Earth one of the most mineralogically diverse planets in the cosmos. These are not just rare and exotic substances, but also simply purified metals like steel, copper, aluminum and titanium. Rare to absent in nature, now present in millions of tons, they are typically shaped into complex manufactured items, destined to be fossils, or more precisely technofossils, relics of our extraordinary activities. At the same time, wind and water carry new synthetic polymers, used in industrial plastics, to virtually every corner of the Earth. Other synthetic organic compounds, including pesticides and the ‘forever chemicals’ (per- and polyfluorinated alkyl substances, or PFAS), have also become almost omnipresent in water, soil and sediment – and in living organisms such as ourselves. As we grow ever more food, levels of reactive nitrogen and phosphorus have doubled at the Earth’s surface in the last 70 years.
Chemical changes to the atmosphere are equally striking. After thousands of years of near-stability, levels of carbon dioxide began to rise significantly in the mid-nineteenth century, then soared from the mid-20th century. This long-lasting greenhouse gas now stands at more than 40% above levels before 1850, and higher than at any time for at least 3 million years. The recent rise is phenomenal – more than 100 times faster than when the Earth left the last Ice Age. It may be the fastest large surge in concentration of this gas in Earth’s history. Each growing tree and seashell registers its chemical fingerprints, via changes in isotope patterns: a fast-forward variant of similar, if less abrupt, perturbations of the geological past. The mass of this extra CO2 – about a trillion tons, mostly from fossil fuel burning – is equivalent to a layer of the pure gas about a meter thick around the surface of the Earth, and thickening by two millimeters each month. Meanwhile, even more potent than carbon dioxide, atmospheric methane shows a similar astonishing rise in the mid-20th century, more than doubling to reach levels not known for at least 800,000 years.
Methane and carbon dioxide as heat-trapping gases have pushed the energy balance of the Earth out of equilibrium, so it absorbs more heat than it radiates. Most of the extra trapped heat, something like 15 zettajoules (15,000,000,000,000,000,000,000 J) each year, is warming the oceans. The effect is similar to pouring about 5 billion mugs of hot tea into the sea every second. And it far exceeds the energy, about half a zettajoule annually, that we obtain by burning fossil fuels.
Some of the extra heat is held within the atmosphere, to give the ongoing planetary warming that we feel, now globally 1.1oC more than in pre-industrial times for mean surface temperature, though much higher for polar and alpine regions. The Earth warmed by about six degrees C after the last Ice Age, but at a much slower rate than for today – and the warming is still in its early stages. The Earth’s radiative equilibrium is still evolving: complex feedback loops govern the Earth’s climate so it will take time for the added greenhouse gases to have their full effect. The world’s desert regions, delicate ecosystems and home to nearly 6% of the world’s population, are among the areas most sensitive to perturbation by this human-driven reconfiguration of climate – even as they are invaded by agricultural expansion, invasive species and solar power arrays.
Just as global temperature lags behind changes in the atmosphere, so sea level, from the thermal expansion of ocean water and the melting of land ice, lags behind temperature rise. Nevertheless, after the last 3,000 years when global sea level changed no more than some 10 centimeters, an acceleration is now underway, with sea levels rising at about 0.5 centimeters each year. Land and sea are beginning to change.
Changes to Earth’s biosphere, Earth’s living skin, are complex, but far-reaching because the biosphere punches far above its weight on the planetary stage. For instance, during hundreds of millions of years before humans evolved, the total (dry) mass of all life on Earth was around two trillion tons: that mass maintained more than a quadrillion tons of oxygen in the atmosphere via photosynthesis. Since the dawn of agriculture some 11,000 years ago, this mass of life has shrunk, by something like half now, as farmers progressively replaced dense forest with crops. Humans have driven species to extinction, like mammoths, mastodons, and giant ground sloths, and globally introduced other species, including rats and cats, domesticated cattle, pigs and chicken. For almost all of that time, these changes impacted only the land. The oceans, covering two-thirds of our planet’s surface, remained mostly pristine until the acceleration of whaling in the 17th century. Since then, industrial fishing has removed some 90% of big fish from the sea and driven many species to the brink of extinction, trawling has scraped and churned millions of square kilometers of ocean-bottom ecosystems, and climate change is degrading the world’s coral reefs.
The last three-quarters of a century have seen a profound acceleration in change. The gradually falling biosphere mass, as deforestation continues, is now surpassed by the weight of ‘anthropogenic mass’ – the mass of all of our built items including buildings, roads, and vehicles – which has skyrocketed, increasing more than ten-fold since the mid-20th century.7 This extraordinary expansion of human-built structures adds another sphere to the planet’s roster of lithosphere, hydrosphere, cryosphere, atmosphere, and biosphere: the “technosphere,” all our technological constructions and the institutions bound up with them. Humans and their institutions do not so much control the technosphere as they are caught up in it, and now utterly depend on it. The technosphere has emergent properties and dynamics of its own, and is evolving hyper-rapidly.
Within the bland concept of ‘anthropogenic mass’ lie arresting and sobering details: the mass of all the plastic manufactured, for instance, now outweighs all animals, both land and sea. Of the mass of terrestrial mammals, about a third is now human, and most of the other two-thirds is livestock; the world’s remaining wild creatures comprise just 2%.8 Of the mass of all birds in the world, the standard supermarket chicken, bred since the 1950s to be a fast-growing and quickly-slaughtered giant, now accounts for two-thirds of the mass of all birds in the world.9
The last three-quarters of a century has also seen an intensification of species translocation unique in Earth history, affecting every continent and ocean.10 Species introductions have blurred boundaries between biogeographic realms that had been separate and distinct over geological time. In many parts of the world – in San Francisco Bay, for instance, a nexus for shipping – introduced ‘invasive’ species outnumber native ones. In the same span of time, species extinction rates have mounted a hundred-fold or more over background levels. Human action has fragmented or destroyed wildlife habitats and a heating planet is pushing species out of their habitable zones and diminishes whole ecosystems on land and in the sea. Of all the changes to the Earth System in the Anthropocene, it is those to the biosphere that are most clearly irreversible.
This cascade of planetary changes shows that Paul Crutzen’s intuition was correct – that a new kind of Earth has recently emerged and continues to diverge from the generally stable conditions of the Holocene.11 What makes the Anthropocene justify its geological ‘-cene’ ending, signifying a new epoch, is that, in myriad ways, the Earth itself is recording, and reacting to, these changes. Recently formed strata around the world, on lake beds or sea floors, in peat bogs, in layers of growing coral skeletons, in stalagmites or in layers of polar ice, and in the thick layers of rubble and landfill around urban areas, collect calling cards of the Anthropocene. These include a wide array of signals, including fly ash from fossil fuel burning, pesticide residues, microplastics, technofossils and – the sharpest and most global signal of all – plutonium and other artificial radionuclides generated by nuclear tests between 1945 and 1963 and spread worldwide through the atmosphere. These highly distinctive strata, a link with Earth’s long history preserved in ancient rocks, are the basis for current attempts to define the Anthropocene formally so it becomes an official and universally understood part of the Geological Time Scale.12
Whether this effort will succeed and pass through all of the committee stages and formal votes in this most bureaucratic of processes, remains uncertain. The evidence that the Earth has changed – suddenly, fundamentally, and in many senses irreversibly – is now extensive and undeniable. But the idea that a geological epoch – an interval usually several million years in duration – can be little more than 70 years long is still an unfamiliar and even uncomfortable notion for some geologists.
Nevertheless, whether formalized or not, the Anthropocene is real both as planetary process and as enduring signals in strata. Its climatic effects will reverberate for tens of thousands of years into the future. The Earth’s biology (and hence its future fossil record) is showing changes comparable in some ways to those following the meteorite impact that, 66 million years ago, ended the Mesozoic Era. In the Anthropocene case, humans are in effect the meteorite, sharply altering the course of planetary history. The effects are the same upon the Earth, whatever their cause: whether tectonic shifts, volcanic outbursts, or human actions. We humans would find these transformations easier to analyze and discuss if Homo sapiens were not involved. But we are, front and center.
The driving forces of Anthropocene changes thus are socio-political. They are bound up with the technosphere, and more complex than, say, the dynamics of meteorite impact or volcanic eruption (although these are complex enough). They reflect different actions by different groups with different consequences. To understand how the Anthropocene truly emerged, and in what direction it might be heading, requires the most multidisciplinary of collaborations among people of many backgrounds. Only similarly broad coalitions of communities can achieve a decent and sustainable future on our hotter, biologically degraded, and less stable planet.