The era in which humans have had the power to alter the conditions for all life on Earth is widely thought to have begun with the Industrial Revolution 250 years ago.
This era has been dubbed the “Anthropocene”, the age in which humankind has become the dominant influence on the planet, causing widespread mass extinctions in the process. But this era arguably began earlier – much, much earlier – than we think.
In a recent paper I argue that humans began dominating the planet not at the Industrial Revolution, or with the advent of farming 10,000 years ago, but deep in the mists of time, back when Homo erectus tamed fire 1.8 million years ago.
Of course our potential influence on the globe is more powerful now, in this age of greenhouse emissions and nuclear technology. But I argue that the crucial step came with fire. That was the moment when humans first learned to unlock large amounts of energy for their own benefit.
Millennia of development
Martin Rees, the British Astronomer Royal, has an image he uses during presentations of Earth as seen by an extraterrestrial witness. During the past 8000 years signatures appear of green cultivation, the lights of big cities, and later an atmosphere clouded with aerosols and greenhouse gases, intermittent nuclear explosions and satellites fired into outer space. And from the perspective of astronomy, geology, the history of life and human evolution, Martin Rees asks a poignant question: Is this our final century?
Of all the factors which allow life on Earth one stands out: the presence of liquid water. And water has been vital for human development. Since the Neolithic and throughout history, cultivation and agriculture-based civilisations concentrated along rivers, such as the Nile, Euphrates and Yellow River, or above groundwater reservoirs, such as in the Yucatan Peninsula, Mexico.
Our development relies on availability of water. Water, in turn, depends on the the hydrological cycle and therefore the climate, including annual river rhythms controlled by melt and freeze relations in source mountain glaciers, the effects of forests on microclimate, soil erosion, and - in some parts of the world such as the Indonesian islands - on volcanic regimes.
Earth’s water is permitted by the planet’s unique orbital position around the Sun, its active tectonic and volcanic nature and its evolving atmospheric composition, which regulates surface temperatures in the range of about -90C to +58C.
The atmosphere, mediating the carbon, oxygen, nitrogen and sulphur cycles, acts as lungs of the biosphere. It allows the existence of an aqueous medium where metabolic microbiological processes occur. These range from chemo-bacteria around volcanic fumaroles, to nanobes in deep crustal fractures, to near-surface phototrophs.
The histories of the atmosphere and of life are inherently interdependent.
Earth started with a Venus-like atmosphere, dominated by CO2, CO, SO2, N2O, CH4, H2 and likely H2S. The sequestration of CO2 and the build-up of nitrogen — a stable non-reactive gas — have led to intermittent ice ages from at least as early as around 3 billion years ago.
Atmospheric change and extinctions
We now know periods of gradual evolution were interrupted by abrupt events which transformed the physical state of the atmosphere-ocean system and the state of habitat of plants and organisms and resulted in mass extinction of species.
We mostly think of mass extinctions of species in terms of extreme shifts in the climate, usually from events such as volcanic eruptions, asteroid impacts or massive methane release.
There have been many such extinctions through Earth’s history, but the phenomenon of a biological species perpetrating a mass extinction of species through the fastest-rate climate change recorded, at least for the last 65 million years, is not easily reconciled with the principle of natural selection inherent in Darwinian evolution.
The great late dinosaurs, having survived for nearly 200 million years, vanished unaware through an asteroid strike that raised global temperature by 7.5C. But Homo sapiens has over only a couple of centuries raised mean global temperature by near 2C and is proceeding toward 4C (see figure below), despite warnings from its own scientists.
The sixth mass extinction
The Anthropocene – the era of man – has been defined by Crutzen and Steffen in terms of the onset of the industrial age and by Ruddiman in terms of Neolithic agriculture. In this article I refer to the mastery of fire about 1.8 million years ago as the “early Anthropocene”.
When a species - such as humans - learns to master ignition and energy output, it leads to an increase in entropy in nature by orders of magnitude. This sets up a biological and cultural chain reaction and a blueprint for that species’ future.
The genus Homo evolved in relatively sheltered sub-tropical rift valleys. Unique among all genera, we learned how to ignite and transfer fire and through this to modify extensive land surfaces of Earth. This had potentially profound consequences for the composition of the atmosphere, a process culminating in the Anthropocene and leading toward the sixth mass extinction of species.
Nature includes species whose activities are capable of devastating environments. Toxic viruses, methane- and hydrogen sulphide-emitting bacteria, fire ant armies, locust swarms and rabbit populations can lay waste to their surroundings. Host-destroying organisms include species of fungi, worms, arthropods, annelids and vertebrates such as oxpeckers and vampire bats. The mastery of fire enabled the genus Homo to magnify its potential to harness and release energy by orders of magnitude, potentially adding its name to that list. From the mid-20th century, the splitting of the atom allowed humans to trigger a chain reaction potentially devastating much of the biosphere.
Since the onset of the industrial age in the 18th century and accelerating since the mid-1980s, the release of more than 560 billion ton of carbon (GtC) through industrial emission and land clearing has triggered unprecedented developments in the terrestrial climate at a rate which, with rare exceptions, is faster by an order of magnitude than natural geological warming events.
Whereas comparisons can be made with the “Paleocene-Eocene Thermal Maximum” of about 55 million years ago, the scale and rate of modern global warming may compare more closely with those triggered by major volcanic and asteroid impact events. The non-linear nature of current climate change, multiple feedbacks and their synergy are driving the climate to uncharted territory and possible tipping points.
A species able to magnify its entropy effect in nature by orders of magnitude, as the genus Homo has done through mastery of fire and the splitting of the atom, would need to be a perfectly wise and controlled species, lest its invention gets out of hand.