The Great Oxidation Event: A cautionary climate change tale

In the midst of our summer living with COVID-19, a novel virus that attacks our respiratory system, and with people in our streets protesting excessive police force, literally wearing face masks bearing the phrase, “I can’t breathe,” it may not be surprising that I have been reflecting on the subject of respiration as it relates to Our Earth’s atmosphere.

As I write this, Our Earth’s atmosphere contains 413.5 ppm (0.04135%) CO2, and most of my writing is concerned with how changes in that small percentage control Our Earth’s energy balance and climate. But a far greater percentage, 20.9%, is molecular oxygen (O2), and of course this is vital to every breath we take.

Have you recently thought about why we breathe O2 and exhale CO2? How Our Earth’s atmosphere came to contain O2 in the first place?

It’s a fascinating story. In fact, it has been persuasively argued by many that the transition in Our Earth’s atmosphere I will describe, known as the Great Oxidation Event (GOE) or, sometimes, the “Oxygen Crisis,” is the single most momentous, impactful event ever to occur in Earth’s history. You see, Our Earth is about 4.5 billion years old and, for about half its life, there was no oxygen in the atmosphere. Zero.

“Try to imagine something so profound, so fundamental, that it changed the whole world. Think of something so revolutionary, that it forever changed the chemistry of the atmosphere, the chemistry of the oceans and the nature of life itself.”

-Prof. Donald Canfield, geologist, University of Southern Denmark, writing about the GOE in his book, Oxygen: A Four Billion Year History

Life got started on Our Earth without free oxygen. And it didn’t take long for it to get started, in geological terms. In fact, life got started on Our Earth just about 500 million years after its formation (maybe a bit earlier or a bit later). The required ingredients for life, at least life as we know it (based on carbonaceous DNA) seem to be hydrocarbon molecules and water in which they can dissolve. And life got started on Our Earth not long at all after the formation of its water oceans 4.4 billion years ago.

Image credit: Wikipedia. Crowd-sourced timeline of life on Earth, referenced in over 140 Wikipedia articles. Click here to access a version with clickable links to other articles.

So how do things live without oxygen? With some effort (and an expensive high-pressure submersible), we can still visit life forms on Earth much like the earliest ones that appeared. Like they did then, they exist in the deep oceans, near hydrothermal vents, volcanically active regions of the ocean floor where seawater co-mingles with mineral-rich magma and becomes superheated creating jets of mineral-rich steam. In the deep ocean, there is no oxygen and no light, and conditions are much like they were in the deep oceans of early Earth. The base of the food chain is formed by colorless bacteria that decorate the seafloor like fluffy snow. (Color has no use in the darkness there, so it hasn’t evolved.)

“Do you get claustrophobic?” the man asked. “No, not at all,” I lied. “Good,” he replied, “and whatever you do, don’t touch the red handle. That’s used only in emergencies.”

-Conversation between Prof. Canfield and the pilot of the deep-diving submersible Alvin, preparing to dive roughly a mile under the ocean to explore a preserved hydrothermal vent powered ecosystem containing organisms similar to the first living things on Earth, as recounted in Prof. Canfield’s book, Oxygen: A Four Billion Year History

They are methanogens, single-celled bacteria that derive their energy by combining hydrogen gas and carbon dioxide, both present in hydrothermal vent steam, to make methane and water. Here is their respiration equation, the chemical equation by which they derive energy to grow:

There’s no oxygen in their respiration equation, because there wasn’t any to be had. These bacteria are also from a category of bacteria called autotrophs, which means, by a set of more complex chemical equations I won’t get into here, they can produce all the mass they need to build themselves — carbohydrates, fats, etc. — straight from carbon dioxide and hydrogen. They work with simple stuff, because simple stuff was all there was when they got started. By observing methanogens, you can see the bare minimum required to make what we call life. You need a way to harness energy (respiration), a way to use that energy to transform stuff in your environment into whatever you’re made of, and (at least for all life we know of) water, which is the medium in which all these chemical transformations happen.

Other simple bacteria co-existed with methanogens on early Earth, living off their dead bodies and off sulfur compounds also issuing from the hydrothermal vents. But, by about 3.5 billion years ago, the random machinations of evolution (and about 500 million years of time) had made a significant innovation: Earth’s first form of photosynthesis. Anoxygenic phototrophs had evolved tiny pigment-driven bio-machines that could use light energy directly from the sun to combine hydrogen sulfide and carbon dioxide in the atmosphere, making their own biomass:

In any sunlit and wet part of the Earth, life no longer had to settle for eking out just 134 kilojoules of energy for every mole of CO2 and 4 moles of H2 it could find, then use that bit of energy to do other reactions to make itself. In fact, these little guys had, at a basic level, the same chemical equation for energy and mass building. And, in any sunlit part of the Earth, light energy was abundant. While the early methanogens and their cohorts slummed it near rare, mineral rich hydrothermal vents, the anoxygenic phototrophs inherited the entire wet, sunlit surface of the planet.

In short, life was good. If only hydrogen sulfide were a little more abundant. I mean, volcanoes were erupting, but not, like, every day. So evolution kept at it… About a billion years went by…

Then boom! Some little upstart cyanobacteria (otherwise known as blue-green algae) developed modern photosynthesis. For a billion years of random experimentation, the equation only looks a little bit different:

We’re still using light to form cell mass directly but, oh my! We don’t need a whiff of H2S here and a whiff of H2S there. The fuel is carbon dioxide and … water. We’re floating in food! Life was easy, and now in any sunlit, wet part of the Earth it was growing like crazy. But notice the key byproduct of this new respiration equation: oxygen.

So the now dominant blue-green algae spread across the globe, blithely spewing oxygen. This went on for about a billion years until, about 2.5 billion years ago, their waste oxygen began rapidly building up in the atmosphere. This was the Great Oxidation Event, arguably the most transformative period in Our Earth’s history.

For example, it led directly to you and me and to other complex, multi-cellular life, who learned to use the oxygen as part of aerobic respiration, the most energetically powerful respiration equation yet discovered by evolution:

If you compare the amount of energy generated by our oxygen-powered respiration equation with that of the humble methanogens that started life on Our Earth (above), you see that this is a luxurious amount of energy — over 3 and a half times as much as methanogens can produce! This plentiful energy made complex life possible, and powers the energy-hungry brain with which you’re reading this.

So the success of the oxygen-producing cyanobacteria transformed Our Earth’s atmosphere, setting in motion the Great Oxidation Event that put the oxygen in the air that makes life possible for us lucky humans. What a success story! Except, here’s the rub.

IT WAS A LIFE-ENDING DISASTER FOR ALMOST ALL SPECIES OF THE CYANOBACTERIA THAT DID IT, AND ALMOST ALL OTHER LIFE ON EARTH AT THE TIME.

To almost all life on Earth back then, oxygen was a toxic poison. The same hyped-up chemistry that makes oxygen reactions so energetic makes oxygen highly reactive with, well, everything life is made of. For example, it oxidizes, degrades, and wrecks the very DNA that contains the blueprints for life. Also, further changes in the atmosphere occurred. The newly present oxygen oxidized methane (a potent greenhouse gas) in the atmosphere to carbon dioxide (a less potent greenhouse gas). This caused geologically rapid global cooling, resulting in a Snowball Earth that lasted for 300 million years.

The Great Oxidation Event wrought a mass extinction of almost everything living on the planet. When the evolutionary dust settled, the few surviving species had evolved protections that enabled them to live in an oxygenated world. For example, sexual reproduction is an evolutionary invention that repairs oxidation errors in DNA each generation. But that’s another story.

So that’s the story of how we came to breathe oxygen. And how complex life came to flourish across Our Earth’s seas and land, sustained by a virtuous balance of producers that harvest energy from the sun, consuming carbon dioxide and giving off oxygen as a waste product, and eaters (like us) that eat the producers, breathe their waste oxygen, and give off carbon dioxide as a waste product.

Source: Wikimedia Commons

But it’s not that simple, is it? Because, unlike other living things, we have developed a civilization. And our civilization has its own respiration equation that greatly amplifies the energy available to us by consuming the buried remains of long-dead plants and animals that have accumulated over the millions of years in this story.

Unlike your personal aerobic respiration equation, our civilization’s respiration equation is returning to the atmosphere carbon dioxide not from plants growing now, but from ancient (fossil) plants. So that carbon dioxide is not part of the virtuous cycle that has governed our atmosphere’s composition since we evolved as a species. The waste CO2 is building up in the atmosphere, and quickly.

Our success as a species is transforming the composition of Our Earth’s atmosphere on a geologically rapid timescale.

Sound familiar?

“The GOE makes it clear that, at an earlier point in Earth’s history, life fully and completely changed the course of planetary evolution. It shows us that what we are doing today … is neither novel nor unprecedented. But it also tells us that changing the planet may not work out well for the specific forms of life that caused the change.”

-Dr. Adam Frank, astrophysicist, in his book, Light of the Stars

Thoughts on this cautionary tale during COVID

Meme on social media (origin unknown)

COVID-19 reminds us of some valuable things. It reminds us that the routines and luxuries of our lives should not be taken for granted. It reminds us that many of the things we count on in our daily lives — from food and water to the mail — rest on complex systems of civilization we have created. And it reminds us that those systems are more fragile than we are used to thinking.

As all of us, regardless of wealth and status, are stalked by a virus that doesn’t read our resumes or care about our bank account balances, COVID-19 reminds us that we are all in this together. In sundry and crucial ways, we count on each other.

The privations of COVID-19 give us time to think. And I think some time thinking about how we want to come out of this — what kind of human civilization we want to have — would be time well spent. I do not think we should want to “return to normal.” By any objective measure, normal is not working and normal will not remain, well, “normal” for much longer. We are cruising pell-mell into a time period entirely analogous to the Great Oxidation Event, and most scientists who study it believe Our Earth’s 6th mass extinction is already underway.

Unlike the mindless cyanobacteria that caused the Great Oxidation Event, we have these big, oxygen-consuming brains with which to think. Plan. Solve. Create. For example, it would be nice if our civilization’s respiration equation looked less like intensified animal respiration and more like photosynthesis. Of course, this is exactly what the solar panels we’ve created are: synthetic photosynthesis. We’ve already used our big brains to develop most of the solutions we need.

We’re just not using our brains to put them to work at nearly the speed required to avoid some really scary outcomes for our species.

So now we have a chance to think about what our “new normal” should look like. It’s a good time to reflect on some assumptions that have been part of our daily routine in our “old normal.” For example, we expect an economy fundamentally based on extraction of valuable resources from Our Earth to continue growing, at an ever faster rate, indefinitely. The assumption is embedded in our daily news, which plays happy music if the stock market grows (typically meaning many of our businesses are growing at a faster rate than they were the same time last year), and otherwise plays sad music.

Hold that thought.

I brew beer at home. It starts by boiling a soup of grain extracts (called “wort”) in a pot. Once the wort is cooled down, you throw a little bit of yeast in. Yeast, a kind of fungi, uses an enzyme called zymase to digest the sugars and carbohydrates in the wort to make ethanol and carbon dioxide.

Several hours after you pitch the yeast in, they enter an exponential growth phase. The beer foams like crazy from all the carbon dioxide being released; you have to have a “blow off” tube on the top of the brewing vessel to let all the foam escape. But the yeasty party is ultimately doomed. The ethanol they are making is poisonous to them, and they are trapped with their toxic ethanol waste inside a closed vessel. Depending on the strain of yeast, once the ethanol reaches between about 5 and 12 percent, the conditions become toxic and most of the yeast die and fall to the bottom of the brewing vessel.

Beer in my closet, with blow-off tube, somewhat after the end of the exponential growth phase. A mass extinction of yeast is underway inside that closed vessel. You can see their little bodies beginning to pile up on the bottom.

We are doing the same right now. Our civilization has a number of waste products, the most worrisome being fossil carbon dioxide. Which, as ignorant-sounding politicians occasionally remind us, is not “toxic” in the classical sense. But, if allowed to accumulate in our atmosphere unabated, it’s certainly capable of ending our civilization by driving the conditions of our planet (a closed vessel) far outside of those in which we evolved.

“We are in the beginning of a mass extinction, and all you can talk about is money and fairy tales of eternal economic growth. How dare you!”

Greta Thunberg, 16 year old Swedish climate activist, speech at the UN Climate Action Summit, New York, Sept. 23, 2019

So, every batch of beer I brew reminds me that the assumptions underlying our economy are unsustainable. Nothing grows exponentially forever inside a closed system. I certainly don’t have all the answers, but here are some things I think we should be thinking about as we consider what future “normal” we want to have.

  • How can we keep the best parts of our way of life (significant personal freedom, the value and efficiency of competition) while making it more sustainable?
  • How can we pay ourselves and each other not to do some things we shouldn’t do?
  • As we get better and better at automation, how can we pay ourselves and each other not to work as hard at the now automated tasks we used to do ourselves?
  • How can we find a healthy balance between the benefits of competition and the divisiveness and inhumanity of an economy that awards most of its benefits to a handful of people who control the increasingly automated capital?
  • How can we get past our tendency for xenophobia and start thinking of ourselves as a global species with a shared future?

Of course, it goes without saying that I think we should be investing aggressively in the energy transformation needed to avoid the worst consequences of climate change. According to the International Panel on Climate Change, we have about a decade to be making serious headway on that. For the most part, we have all the solutions we need. The only thing standing in our way is excuses.

And, it seems to me, working together on that (while being necessary for our survival) would also be a good vehicle to start answering the bigger questions.

#rescuethatfrog

Related reading:

A scientist calling for informed action on global climate change. Check out my fact-rich website and blog, rescuethatfrog.com.

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