Cosmos, with Neil de Grasse Tyson, discussed climate change last night. The entire discussion was limited to the greenhouse effect, carbon dioxide (with a brief mention of methane), and comparisons of Venus and Earth - one of which had a runaway greenhouse effect early on and now has a life-inhospitable environment, the other of which did not. One implication was that our current situation on Earth is nearing the threshold for a runaway greenhouse effect, which will result in dire consequences for life (including humans).
Now, I do not want this post to put me into the "denier" camp, or anything. The correlation between atmospheric carbon dioxide (and methane, which is less discussed in the media but is also very important) and global temperatures is well established. If we continue to pump carbon dioxide and methane into the atmosphere at current levels it will likely be catastrophic. But this is not the first time an organism, or collection of organisms, has been responsible for devastating climate change. We are just the most recent in a line of atmosphere-changing events precipitated by life.
Early on in Earth's history, its atmosphere was carbon dioxide and methane rich, with very little diatomic oxygen gas. (Currently, the atmosphere is about 20% diatomic oxygen, often mentioned as a prerequisite for life. This takes a very narrow, eukaryote-centric view of life.) Early life forms were chemotrophic, reducing inorganic materials for energy. Once photosynthesis evolved (about 2.8 billion years ago), oxygen gas was released and carbon dioxide pulled from the atmosphere. Free oxygen initially reacted with dissolved metallic elements in the ocean, then oxygenated the atmosphere, oxidizing the methane (and metals on the surface). The net result was a decrease in atmospheric carbon dioxide and methane with an increase of oxygen gas - i.e. a removal of potent greenhouse gases from the atmosphere (about 800 million years ago, after metallic minerals became oxygen-saturated). This may have led to the cooling of Earth (snowball earth). It almost certainly led to the evolution of eukaryotic organisms which require oxygen for metabolism. (Prior species were all anaerobic, and were in fact poisoned by oxygen. The oxygenation of Earth due to photosynthetic organisms may have caused a large-scale extinction of early life forms.) This chain of events likely prevented Earth from the runaway global warming that struck Venus. It also allowed the Earth to obtain an ozone (triatomic oxygen) layer, creating suitable conditions for life to colonize land without being destroyed by UV radiation.
Fast forward a few hundred million years to about 374 million years ago. The late Devonian extinction may have been helped by the evolution of land plants, with the rapid increase in biomass due to colonization of the land pulling more carbon dioxide from the air, and leading to rapid cooling.
Next, the Permian-Triassic extinction (252 million years ago). This is the largest known mass extinction in Earth's history. New results indicate that it may have been triggered by a sudden release of methane from methanogenic (i.e. methane producing) microorganisms who had just evolved the methanogenic capabilities (similar to how evolution of photosynthesis resulted in a huge release of oxygen). Methane is a potent greenhouse gas and such a release would have started rapid global warming, leading to mass extinctions. Such a catastrophe would presumably be similar to what would happen with our current global warming due to the sudden release of carbon dioxide.
As a note, causes for the Triassic-Jurassic extinction are little understood (thought to be ocean acidification, which would be related to sudden increases in acidifying compounds), and the Cretaceous extinction was likely caused by rapid climate change due to blocking of sunlight by dust particles created by an asteroid impact.
Do we see patterns here? Mass extinctions on Earth are generally caused by climatic shifts, which are in turn caused by rapid changes in the equilibrium levels of certain chemical compounds, precursors and solar energy.
If you get nothing else from this post, understand this. The biosphere is continually in flux. Levels of carbon dioxide, oxygen, methane, etc. have been changing since life first evolved, and life on Earth is responsible for the current cycles and equilibrium patterns (if photosynthetic organisms all went extinct, the oxygen in the atmosphere would rapidly disappear) of many elements. Life evolve within the confines of its environment, and if the equilibria change too quickly for evolution to react, you get mass extinctions. This has been going on for millenia, and humans are only the latest organism to participate. The difference is we are the first organism with the intelligence to determine that this is a choice - we can continue on our current path and be extinct (along with numerous other species) in some future time (likely farther in the future than anyone alive today will experience), or we can try to prevent the next mass extinction event from being human-caused and prevent suffering of our descendents.
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