John O. Campbell
Since its beginnings, life on earth has faced periodic mass extinction from climate extremes caused by variations in the concentrations of atmospheric greenhouse gases. And this existential threat to life, may occur throughout the universe wherever life has been able to get a foothold. Recent research indicates that early microbial life on Mars may have been wiped out by the greenhouse gases it produced (1).
Soon after photosynthetic life emerged on earth, the first life-threatening climate event occurred in the form of a 300-million-year planetary deep freeze. Over the next 2 billion years, 4 further planetary freezing events occurred, sometimes called Snowball Earths, when the entire planet plunged into unmitigated freezing climates (2). During the first and most massive event, named the Huronian Ice Age, ice is thought to have completely covered the earth, and the shallower oceans froze from the surface to the bottom. During this single event, the earth remained in a deep freeze for approximately 300 million years, about 7% of its entire history (3).
The Huronian Ice Age is thought to be the first time Earth was completely covered in ice, and to have practically stopped photosynthesis.
It is ironic that photosynthesis, the revolutionary energy source allowing the spread of life throughout the planet, posed this initial lethal threat to life itself. Sufficiently concentrated, atmospheric oxygen, a by-product of photosynthesis, was poisonous to most forms of life existing at the time, including those photosynthetic forms producing oxygen. Thus, as photosynthetic oxygen came to form a significant portion of the earth’s atmosphere 2.4 billion years ago, it extinguished existing anoxic life forms in most niches. Although life eventually evolved and adapted to oxygen-rich environments, these adaptations required evolutionary time. Wikipedia describes the effects of this ‘Great Oxygenation Event’ (3):
As oxygen "polluted" the mostly methane atmosphere, a mass extinction occurred of most life forms, which were anaerobic and to whom oxygen was toxic.
In addition to oxygen poisoning, elevated oxygen levels also caused climate change leading to the Huronian Ice Age's deep freeze conditions. The presence of oxygen transformed the potent greenhouse gas methane into the less powerful greenhouse gases, water vapour and carbon dioxide. Atmospheric methane removal is thought to have triggered the Huronian Ice Age and its mass extinction of life. As Wikipedia explains (3):
The Huronian glaciation followed the Great Oxygenation Event (GOE), a time when increased atmospheric oxygen decreased atmospheric methane. The oxygen combined with the methane to form carbon dioxide and water, which do not retain heat as well as methane does. The glaciation led to a mass extinction on Earth.
Photosynthesis provided the fundamental energy source for practically all subsequent life on the planet and is perhaps its most outstanding bio-chemical achievement. However, its initial effects on the planet’s balance of greenhouse gases caused a 300 million years-long catastrophe for life’s existence.
As life suffered through the lethal effects of an oxygen-rich atmosphere, oxygen availability also provided life with two significant long-term opportunities. In addition to the beneficial role oxygen came to play in the efficiency of life’s metabolic processes, it also formed a protective atmospheric ozone layer. Ozone, a chemical variant of oxygen, screens out deadly ultra-violet rays from the sun, making the surface of our planet habitable.
In conjunction with the faint early sun, oxygen impact on greenhouse gases periodically plunged the planet into deep freeze conditions, with snowball earth events occurring four more times between 775 and 500 million years ago. During each of these events, both the initial freezing and eventual thaw involved changes in greenhouse gas concentrations. As Wikipedia describes it (2):
Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as the eruption of a supervolcano, a reduction in the atmospheric concentration of greenhouse gases such as methane and/or carbon dioxide, changes in Solar energy output, or perturbations of Earth's orbit. Regardless of the trigger, initial cooling results in an increase in the area of Earth's surface covered by ice and snow, and the additional ice and snow reflects more Solar energy back to space, further cooling Earth
Since the Cambrian explosion, 550 million years ago, when all of life’s multicellular designs began to rapidly evolve, five more events associated with planetary warming led to mass extinctions. These five major mass extinctions left single-celled life relatively unaffected but had their most significant impact on the more complex life forms present at the time, such as trilobites and dinosaurs.
Until recently, researchers believed that some of these mass extinctions, such as the late Ordovician mass extinction (LOME), did not involve greenhouse gas-induced planetary warming brought on by periods of extreme volcanism, but recent research indicates that warming and volcanism have been factors in all mass extinctions (4):
Rather than being the odd-one-out of the “Big Five” extinctions with origins in cooling, the LOME is similar to the others in being caused by volcanism, warming, and anoxia.
Notably, the Permian mass extinction, the most extreme in life’s history, killing off nearly 96% of all marine species and 70% of terrestrial vertebrate species, is now understood as resulting from the release of greenhouse gases during volcanism. As researchers describe it (5):
We are dealing with a cascading catastrophe in which the rise of carbon dioxide in the atmosphere set off a chain of events that successively extinguished almost all life in the seas
All of this is to say that life on our planet has faced continual existential crises since its beginnings. The mass extinctions experienced during life’s history, including numerous snowball earth events and the more recent five mass extinction events, were due to planetary cooling or warming beyond what most life could tolerate. Central to these fluctuations have been the level of greenhouse gases (GHG), primarily CO2, in the earth’s atmosphere. In other words, imbalances in atmospheric greenhouse gases have been a critical cause of most existential crises faced by life over its history.
Only when greenhouse gas levels are precisely balanced are temperatures moderate and life can thrive. Given the low intensity of sunlight reaching the early earth’s orbit, without the greenhouse effect triggered by CO2, the earth would be in a permanently frozen state. As researchers at NASA Goodard Institute for Space Studies describe (6) it:
Without the radiative forcing supplied by CO2 and the other noncondensing greenhouse gases, the terrestrial greenhouse would collapse, plunging the global climate into an icebound Earth state.
Despite many extremely hot and cold fluctuations, the climate regularly returns to an intermediate state sustaining life. This long-term thermostatic-like control of climate is regulated by physics rather than life. For example, the long-term sources of CO2 entering the atmosphere are varied, but their principal source is volcanic. The chemical weathering of rocks takes CO2 from the atmosphere and forms the primary mechanism balancing volcanic infusions of CO2. Cold climates with low levels of CO2 slow the chemical absorption, while warmer climates with higher levels of CO2 speed up it up. This physical thermostatic control has kept earth’s climate within bounds allowing life to thrive for much of earth’s history (6):
Having this physical thermostat in place is fortuitous for genetically based life could never regulate climate effectively by itself. Most species perished during each extreme climatic event but left behind a minority capable of clinging to existence and eventually adapting to the new circumstances. Effective regulation of planetary climate is far beyond the prowess of genetically based life; during its long history, life has repeatedly fallen victim to fluctuating climate instead of regulating climate to benefit its existence.
The evolution of human culture introduced a new player in this ancient and finely balanced dynamic. By burning fossil fuels, cultures are now producing CO2 at a rate of almost 1,000 times that of volcanism (7; 8), the major natural source of CO2. In just 150 years, cultures have overwhelmed natural mechanisms for absorbing CO2, leading to skyrocketing concentrations and inexorably rising global temperatures. Fortunately, unlike genetic life, which was blind to the deep freeze conditions that it would cause with the great oxygenation event, and bore the brunt of the resulting mass extinction, human culture is able to understand the threat posed by its unconstrained emissions of CO2.
Culture, unlike life, can and does regulate climate. The large-scale burning of fossil fuels has already resulted in an average global temperature increase of more than 1 degree Celsius during the preceding 200 years, and this is an example of culturally regulated climate change. We could also reduce planetary temperatures merely by reducing our rate of burning fossil fuels and by other technological means.
One great boon of the current climate crises is a massive increase in research focused on the relationship between greenhouse gas concentrations and climate. We now have extremely good climate models accurately predicting climate outcomes and this gives us the cosmic ability, to regulate planetary climate merely by controlling our greenhouse gas outputs.
References
1. . Early Mars habitability and global cooling by H2-based methanogens. . Sauterey, B., Charnay, B., Affholder, A. et al. s.l. : Nat Astron, 2022. https://doi.org/10.1038/s41550-022-01786-w.
2. Wikipedia. Snowball Earth. Wikipedia. [Online] [Cited: May 17, 2020.] https://en.wikipedia.org/wiki/Snowball_Earth.
3. —. Huronian glaciation. Wikipedia. [Online] [Cited: May 17, 2020.] https://en.wikipedia.org/wiki/Huronian_glaciation.
4. Late Ordovician mass extinction caused by volcanism, warming, and anoxia, not cooling and glaciation. Bond, David P.G. and Grasby, Stephen E. s.l. : Geological society of America, 2020, Vol. 48. https://doi.org/10.1130/G47377.1.
5. News, Staff. New Research Provides Comprehensive Reconstruction of End-Permian Mass Extinction. Science News. [Online] October 20, 2020. http://www.sci-news.com/paleontology/comprehensive-reconstruction-end-permian-mass-extinction-08965.html.
6. Atmospheric CO2: Principal Control Knob Governing Earth's Temperature. Lacis, Andrew A., et al. Issue 6002, pp. 356-359, s.l. : Science 15 Oct 2010, 2010, Vol. Vol. 330. DOI: 10.1126/science.1190653.
7. Werner, C., Fischer, T., Aiuppa, A., Edmonds, M., Cardellini, C., Carn, S., . . . Allard, P. Carbon Dioxide Emissions from Subaerial Volcanic Regions: Two Decades in Review. [book auth.] B. Orcutt, I. Daniel and R. Dasgupta . Deep Carbon: Past to Present. Cambridge : Cambridge University Press., 2019.
8. International Energy Agency. Data and statistics. IEA. [Online] [Cited: July 25, 2020.] https://www.iea.org/data-and-statistics?country=WORLD&fuel=CO2%20emissions&indicator=CO2%20emissions%20by%20energy%20source.
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