Lifeboat Foundation

Global warming is bad. But just how bad could it be, worst case? Could it make the Earth hotter than a self-cleaning oven, like it did Venus? Venus is even hotter than Mercury even though Mercury is closer to the sun, because of Venus’s greenhouse effect. But there seems little reason to fear such a runaway greenhouse effect on Earth. Aside from the fact that it has never happened here before, the Earth may simply not have enough solar energy and greenhouse gas (carbon dioxide and methane) to start the runaway positive feedback process that happened on Venus. Some day that may change, however — the sun is getting hotter as it grows older, and greenhouse gases, perhaps exotic and powerful ones, could potentially be manufactured and released by hostile invading extraterrestrials, robots, or apocalypse-minded humans. But let’s ignore this scenario as unlikely for now (so that we could claim to be optimists, if not for the following paragraphs). Is there any other apocalyptic global warming scenario still to worry about? Something that is not only known to be theoretically possible, but has actually happened? Say, a stinking poison that contaminates the atmosphere and waters of the entire Earth, not only wrinkling noses worldwide but killing off almost all living things? Welcome to the gray, dead plains (often warm and balmy), oxygen-starved waters, green skies and repellent smell of hydrogen sulfide poisoned Earth.

Hydrogen sulfide, H2S, is a gas. Chemically similar to H2O (water) but with a sulfur atom in place of water’s oxygen, it is not a necessity of life like water, but very poisonous. Much less than 1 part per million (ppm) in the air is detectable as an odor like rotten eggs. 10 ppm is a typical occupational exposure limit. 1 part in 1000 in air can cause rapid death. As a young man I kept several 1-gallon milk jugs of green algae-containing water, which I fertilized with vegetable peels and such. It worked great, but there was one slight problem: some vegetables contain substantial amounts of sulfur, which can lead to H2S dissolved in the water especially in the muck at the bottom. I finally dumped all the algae water down the toilet rather than move it to another apartment (a decision with which you are welcome to disagree). Some were smellier than others, and I ended up with a modest case of hydrogen sulfide poisoning. Main symptom: a mental “slide show” of colorful crystalline images, presumably the result of H2S-caused inhibition of cellular respiration in the brain. Like humans, most animals and plants are poisoned by H2S.

How might H2S come to poison the Earth? Like it did in the past. The dinosaurs are thought to have perished in a mass extinction event triggered by an small asteroid, several miles in diameter, crashing into the ground near the town of Chicxulub on the Yucatan peninsula in Mexico, 65 million years ago (mya). But the worst extinction event of all time is believed by many to have been caused by H2S. This was the much earlier Permian-Triassic (or P-Tr) extinction event of 251.4 mya — about 20 million years before any dinosaur was even a gleam in its mother’s eye. The vast majority of plant and animal species then in existence went extinct, both in the sea and on land. The P-Tr event is often called the “Great Dying.” A similar process could play out in humanity’s future, potentially ending it. Here is how.

The causal process begins with global warming. While massive volcanism in Siberia is thought to have triggered global warming by releasing carbon dioxide into the atmosphere back then, human burning of fossil fuel is doing it now. This warming is melting sea ice which darkens the ocean surface, causing more sunlight to be absorbed and worsening the warming trend. As the oceans warm, methane hydrate crystals deep underwater will warm too, which may cause methane to be released into the atmosphere. Methane is a greenhouse gas like carbon dioxide, except many times more powerful. Such a release of methane from the ocean floor is a likely though still controversial cause of the global temperature spike called the Paleocene-Eocene Thermal Maximum of 55.8 mya, during which average global temperatures soared over 10°F.

Heating of the Earth’s surface causes the top layer of the polar oceans to warm disproportionately. Normally, cold air in the polar regions chills and oxygenates the surface waters, and salinates them by evaporating some water and leaving the salt, which makes the remaining water saltier. The cold and extra salt makes these waters denser, so that they sink and flow along the bottom, causing a planet-wide current of oxygen-rich water called the thermohaline circulation that connects the bottoms of the oceans. Global warming affects the polar regions the most, and warmer temperatures there can slow and potentially even halt the thermohaline circulation, thereby slowing or stopping oxygen from getting to the ocean depths.

Back in the Great Dying, it is hypothesized that after the thermohaline circulation stopped, the oxygen in the deep ocean waters was used up by the organisms that live down there. But some microbes don’t need oxygen gas dissolved in the water. Bad news — they get their oxygen instead from oxygen-containing sulfur compounds, and release the villain…hydrogen sulfide (just as they did at the bottoms of those algae water-containing plastic milk jugs). But it gets worse. The hydrogen sulfide slowly accumulated in the ocean waters, poisoning many of the remaining oxygen-breathing organisms. That explains why the extinction event was so devastating to marine life. Things went from awful to even worse. So much hydrogen sulfide accumulated that it started leaking from the water into the atmosphere. Because such a low concentration of hydrogen sulfide is needed to create a bad stink, if this happens during the human era the first blatantly obvious sign will be the smell of rotten eggs. It will be everywhere. Though unpleasant, it is not harmful until the concentration grows. As it accumulates in the atmosphere though, the smell will go from bad to worse, and eventually the increasing amount of H2S will start poisoning land life. And the sky will turn green. That can explain the devastation to land life during the Great Dying. And maybe it could happen again.

Recommendations

No need to buy a gas mask just yet. Thing won’t start getting really bad during our lifetimes. But this could be an existential risk to our species. Thus, scientific study is important. A serious risk is that things we do in our lifetimes may be the trigger for an extinction event later. It should be obvious that it would be the height of irresponsibility to let that happen. Yet there will always be forces of irresponsibility. One may hope that those forces fail to win or their victory may be a Pyrrhic one indeed.

Reference

There is a lot of both popular and scientific literature on this topic. A well-known full length work that bridges the gap between those literatures is P. D. Ward, Under a Green Sky, HarperCollins, 2007.

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