A Flammable Biosphere
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The advent of plants on land surfaces since about 420 million years-ago created an interface between carbon-rich organic layers and an oxygen-rich atmosphere, leading to recurrent fires triggered by lightning, volcanic eruptions, high-temperature combustion of peat and, finally, ignition by humans, constituting the blueprint for the Anthropocene. For a species to be able to control ignition and energy output, leading to increase in entropy in nature higher by orders of magnitude than its own physical energy outputs, the species would need to be perfectly wise and responsible. No species can achieve such levels.
KeywordsAtmospheric Oxygen Geological Timescale Soil Carbon Storage Optical Refractive Index Extensive Fire
Charcoal, a proxy for fire, occurs in the fossil record from the Late Silurian ~420 Ma. Scott and Glasspool (2006) document Silurian through to end-Permian charcoal deposits with reference to the frequency of Paleozoic fires in relation to atmospheric oxygen concentrations. As atmospheric oxygen levels rose from ~13 % in the Late Devonian to ~30 % in the Late Permian, fires progressively occur in an increasing diversity of ecosystems. Late Silurian to early Devonian charcoal indicates the burning of diminutive rhyniophytoid vegetation. There is an apparent paucity of charcoal in the Middle to Late Devonian coinciding with low atmospheric oxygen (Fig. 2.2). Fires become widespread during the Early Mississippian (Lower Carboniferous) and in the Middle Mississippian. During the Pennsylvanian (Upper Carboniferous) oxygen rose toward levels above 30 %. Charcoal is recorded in upland settings and is important in many Permian mire settings, suggesting the burning of even moist vegetation. The decline of oxygen levels through much of the Mesozoic (250–65 Ma) to below 15 % (Fig. 2.2) and its gradual resurgence through the late Mesozoic and Cenozoic limited the effect of fire.
Atmospheric CO2 levels are buffered by the oceans (~37,000 GtC), which contain about x46 times the atmospheric CO2 inventory (~800 GtC). The solubility of CO2 in water decreases with higher temperature and salinity and the transformation of the CO 3 [−2] ion to carbonic acid (HCO 3 [−1] ) retards the growth of calcifying organisms, including corals and plankton. Plants and animals work in opposite directions of the entropy scale, where plants synthesize complex organic compounds from CO2 and water, producing oxygen, whereas animals burn oxygen and expel CO2. Disturbances in the carbon and oxygen balance occur when changes occur in the extent of photosynthetic processes, CO2 solubility in the oceans, burial of carbon in carbonate and organic remains of plants and oxidation of carbon through fire and combustion.
Above a certain level of atmospheric oxygen fires would constrain the development of forests, constituting strong negative feedback against excessive rise of atmospheric oxygen (Watson et al. 1978). Conversely a decline of oxygen reduces the frequency and intensity of fires. The association of fossil charcoal with fossil trees suggests O2 levels continued to be replenished, whereas the upper oxygen limit of Phanerozoic atmospheres is uncertain. Robinson (1989) pointed to Paleobotanical evidence for a higher frequency of fire-resistant plants during the Permo-Carboniferous, supporting distinctly higher O2 levels at that time. Model calculations of the interaction between terrestrial ecosystems and the atmosphere by Beerling and Berner (2000) suggest the rise from 21 to 35 % O2 during the Carboniferous resulted in a decline in organic productivity of about 20 % and a loss of more than 200 GtC (billion ton carbon) in vegetation and soil carbon storage, due to burning in an atmosphere of ~300 ppm CO2. However, in a CO2-rich atmosphere of ~600 ppm carbon fertilization of the soil productivity increases lead to the net sequestration of 117 GtC. In both cases these effects resulted from strong interaction between O2, CO2 and climate in the tropics.
- Belcher CM, Yearsley JM, Hadden RM, McElwain JC, Rein G (2010) Baseline intrinsic flammability of Earth’s ecosystems estimated from paleo-atmospheric oxygen over the past 350 million years. Proc Nat Academy Science. doi 10.1073/pnas.1011974107