The discovery of fire: initial steps toward anthropogenic climate change
The evidence for a rapid shift in state of the terrestrial atmosphere-ocean system over the last two centuries (see figure 1) requires a deep time perspective, beyond events of the day. Tracing the original blueprints of anthropogenic effects on the terrestrial environments takes us back at least a million years to the time when – according to research released this month – the first compelling evidence for human use of fire was found.
Of all life forms which ever existed, only the genus Homo acquired the skill of igniting and transporting fire. This gave Homo not only warmth, cooking and protection from animals, but a high degree of power over nature. Prehistoric hominids burned large parts of the biosphere and, more recently, combusted carbon and hydrocarbons derived from fossil biospheres up to 400 million years old.
The high intelligence underlying human inventions has been variously attributed to a large brain size (Chimpanzees ~395 grams; Australopithecus apheresis ~430 grams; Homo ergaster ~850 gram; Homo sapiens ~1350 grams) and a high brain/body mass ratio (~1/40).
However, sperm whales brains weigh ~8000 grams and elephant brains over 5000 grams. Mice have a brain/body mass ratio similar to that of humans (1/40) and small birds a higher brain/body mass ratio (1/12). A more confident parameter of human intelligence is the high ratio of the neocortex (frontal intelligence lobe) to medulla (lower “mammalian” part of the brain stem) in the human brain (Lemurs ~10; monkeys and apes 20-50; humans 105).
Theories which try to explain the uniqueness of humans invoke its bipedal nature, language and the use of stone and bone tools. In these respects, however, pre-Homo sapiens hominids were hardly unique. Many animals are bipedal and some use tools. Termites design articulate nests, insects have a sophisticated language (such as the bee’s dance), meerkats make special calls, whales and dolphins echo-locate and birds have navigation systems.
However, Homo’s ability to ignite fire constitutes an exclusive blueprint not shared by any other life form, with far-reaching consequences. This facility was allowed by the potentially flammable terrestrial environment where hominids emerged, inhabiting plant-rich land surfaces surrounded by phytoplankton-rich oceans where photosynthesis produces an oxygen-rich atmosphere and plant decay results in formation of carbon-rich surface deposits and derived peat and coal deposits.
The evolution of land plants in the late Silurian (~420 Ma: vascular plants such as Cooksonia and Baragwanathia) and in the Permian (299–251 Ma: Cycads and Ginkgo) led to the accumulation of carbon as cellulose in trees and grasses, soils and bogs, methane hydrate and methane clathrate.
During tropical greenhouse gas-dominated eras (Silurian-Carboniferous – 443–299 Ma; early Mesozoic – 251–65 Ma) there were extensive fires from lightning, volcanic eruptions and underground peat fires. Diagnostic optical refractive indices allow scientists to estimate fire frequency (see figure 2) from charcoal remains. In the Permian atmospheric oxygen exceeded 30%, a level at which even moist vegetation becomes flammable, as represented in charcoal concentrations as high as 70% in coal.
When humans harnessed fire, it elevated the species’ oxygenating capacity by many orders of magnitude as we utilized the solar energy stored in plants. As the use of fire, and subsequently of combustion, have grown, this increased planetary entropy (in physics – a measure of the degree of disorder and chaos of a system) to levels approaching those triggered by the geological events, including those resulting in the major mass extinctions in geological history. The splitting of the atom achieves yet higher levels of entropy.
Likely the mastery of fire has been driven by necessity. There were abrupt environmental shifts when mean global temperatures varied during glacial-interglacial shifts by about ~5°C and local temperatures by larger amounts. Humans had to find refuge in relatively protected sub-tropical shelters, such as the East African rift valleys.
Early Paleolithic evidence for human-lit fires includes hearths containing charcoal, burnt bones and red clay shards heated to 400°C and higher temperatures. Widespread use of fire in the late Paleolithic is indicated by charred logs, charcoal, reddened areas, carbonised grass stems and plants and wooden implements hardened by fire.
A likely advantage of cooking was the enhanced supply of protein, allowing an increase in brain size (Homo ergaster ~850 grams; Homo sapiens ~1350 grams). Over hundreds of thousands of years, gathered during long nights around camp fires, captivated by the flickering life-like dance of the flames, humans developed curiosity, imagination, insights, cravings, fears, premonition, legends, aspiration for immortality and beliefs in deities and gods. Oldest expressions of cultural and spiritual creative minds may date back to 350,000 years ago, although this remains unconfirmed.
As climate conditions stabilized in the early Holocene, around 8000 years ago, agriculture and production of excess food allowed these ideas to be manifested through both the creative and destructive activities of civilizations.
The stabilisation of climate allowed cultivation of crops, enhanced by smelting of metals and crafting of ploughs. Extensive burning and land clearing associated with agriculture, from about 10,000 years ago, culminated with the combustion of fossil fuels.
Bill Ruddiman suggests the rise in CO₂ in the mid-Holocene reflects land clearing, fires and agriculture, defining the onset of an Anthropocene era. He wrote, “A wide array of archaeological, cultural, historical and geologic evidence points to viable explanations tied to anthropogenic changes resulting from early agriculture in Eurasia, including the start of forest clearance by 8000 years ago and of rice irrigation by 5000 years ago.”.
However, other authors define the onset of the Anthropocene at the dawn of the industrial age in the 18th century. They attribute the mid-Holocene rise of greenhouse gases to natural perturbations during the interglacials, for example the 420-405 thousand year old Holsteinian interglacial. According to this definition the Anthropocene is characterized by greenhouse gas emissions levels exceeding those of any earlier geological period (see Figure 3).
Since the 18th century, burning fossil fuels and clearing land increased atmospheric carbon content by 237 billion ton carbon (GtC). The present atmospheric carbon concentration is 820 GtC at present, an increase of some ~39% relative to the original level of 590 GtC. Of the additional CO₂, approximately 42% stays in the atmosphere which, together with other greenhouse gases, led to an increase in the atmospheric energy level of ~+3.2 Watt/m2 and of potential mean global temperature by +2.3 degrees Celsius (see figure 1). Approximately -1.6 Watt/m2, equivalent to -1.1°Celsius, is masked by industrially emitted sulphur aerosols.
The significance of human mastery of fire in terms of the consequences of enhanced entropy has been underestimated. Human respiration dissipates two to ten calories/minute. A camp fire releases more than 100,000 calories/minute, but the output of a 1000 megawatt/hour power plant expends more than 2 billion calories/minute and nuclear fission orders of magnitude higher. This amounts to an increase in entropy on the scale of geological events.
While complexity increases in conurbations, the rise in atmospheric energy and heat due to the release of greenhouse gases associated with exothermic combustion results in a series of extreme weather events (http://www.ipcc-wg2.gov/SREX/), droughts, floods and storms, degrading natural habitats.
According to Ancient Greek mythology, fire was stolen from the gods by the titan Prometheus, who breathed it into human clay figures. From our modern perspective, this legend and related stories in other traditions acquire a special meaning.
For an intelligent species to be able to explore the solar system planets but fail to protect its own home planet defies explanation. For a biological species to magnify its entropic effect on nature by orders of magnitude, developing cerebral powers which allow it to become the intelligent eyes through which the Universe explores itself, hints at yet unknown natural laws which underlie life, consciousness and complexity.