A Rare Earth
Some Beautiful Ironies Allow Life to Persist
It seems fitting that this final essay posted to the Less Heat More Light site shares some personal reflections on just how rare and vulnerable this habitable Rock might be. As we hope for a rational global discussion on climate change, anything that encourages a sense of wonder and humility towards our home planet can’t hurt.
An Appreciation of Good Popular Science Writing
Good friends began my career-long fascination with popular science writing with a gift of Carl Sagan’s amazing book, Cosmos. Across the 40+ years since that gift, I have come to appreciate the ability of some scientists to see well beyond the strictures of their disciplines and academic departments, and to see, as some define genius, similarities, not differences.
Sagan’s book carries us from the origins of the universe through biological evolution to nuclear winter and global change. His ability to see similarities is famous now even in the meme universe based on this statement: “This oak tree and me, we’re made of the same stuff.” He called all the major chemical elements of life “star-stuff” as all those heavier elements that make our lives possible were formed in supernova explosions.
Sagan was also deeply engaged in the search for intelligent life elsewhere in the universe, as well as the contemplation of how intelligence would be manifest, and how long such intelligence might be able to survive itself. Links to the Drake Equation and to Nuclear Winter under Sources below offer some sobering perspectives on that latter point.
And Simpler Definitions
Simple definitions make for easier detection. Just as J.E. Lovelock defined life at the planetary scale as the presence of an atmosphere in chemical disequilibrium (see Gaia reference below), Sagan and others defined intelligence as the ability to generate radiowave signals that would exhibit information content against the random background noise of the universe.
This search took popular form in the film, Contact, based on a novel of the same name by Sagan, but also a very real presence in a number of research programs designed to search for and detect those signals, should they reach us. SETI (or the Search for Extraterrestrial Intelligence) was manifest most concretely in the radioastronomy program at the Arecibo Observatory in Puerto Rico, the main antenna of which was destroyed by a hurricane in 2017.
While SETI may be in abeyance, NASA has a very real program to locate Earth-like planets in other solar systems that might also be capable of supporting life – however defined. To date, the Exoplanets program has identified more than 5,600 candidate rocks out there. The sensitivity of measurements required to locate these bodies is a wonder of modern technology, and is described on the relevant websites (see links below).
The primary requirements for identifying these candidate planets are that they be solid (rocky) and not gaseous (like Jupiter), and that they be in the “Goldilocks” zone, not too near, and not too far from their sun, such that liquid water might be able to persist.
Those requirements seem very Earth-centric to me. They assume that life, in its most elemental characteristics, would be as it is on Earth – centered on the manipulation of carbon-based compounds in a water medium. If we ever do find life out there, it would not be surprising if it took some form that we had not even considered as a possibility.
But all of this is just an elaborate lead-in to the main theme of this essay.
As we forcibly and dramatically change the global environment here on Earth, a sense of humility, caution and appreciation might be helpful. The creation and persistence of life here depends not only on carbon, water and moderate temperatures, but on a number of other characteristics that support life and shield us from some extreme threats. Even with these in place, the continuation of life as we know it here was a near-miss thing.
Three Beautiful Ironies That Allow Life to Flourish
In this essay format I can’t address all of the coincidences that occur to me as crucial for sustaining life on Earth, but let’s pick three: The ozone layer in the upper atmosphere, the “window” in the absorbance of sunlight by the atmosphere that makes both sight and photosynthesis possible, and the still-controversial idea of “Snowball Earth.”
Oxygen was a minor constituent of the Earth’s atmosphere for the first 3.8 billion years or so. Only around 550 million years ago did photosynthesis become a dominant process, with the resulting accumulation in the atmosphere of the waste product of this process – free oxygen gas. As oxygen grew from less than 3% to the current 21% of the atmosphere, anaerobic life yielded to aerobic life (including large animals like us that require oxygen for metabolism) and the evolutionary dynamo termed the Cambrian Explosion, sometimes called the Biological Big Bang, began. Life claimed dry land and diversification of life forms did indeed explode.
All because plants were more effective at generating oxygen than animals and microbes were at using it, as we do, for the respiration that drives all of our metabolic processes.
Which leads to our first example of the tenuous hold of life on Earth.
Oxygen Supports and Protects Life on Land
Oxygen as O2 is fairly stable in the atmosphere, allowing it to reach the current concentration. But, under the intense solar radiation received at the top of the atmosphere it can be separated into two atoms, one of which can then rejoin the two-atom version to produce ozone (O3 - three atoms of oxygen). Ozone has the property of absorbing almost all of the high-energy, life-damaging ultraviolet (UV) radiation received from the sun. The “ozone hole” controversy centered on the potential loss of this protective shield. If the sun’s output of UV radiation could reach the surface, damage to soft tissues (like skin and eyes) and to genetic materials (like DNA) could seriously disrupt evolution and life.
So, without one unique property of photosynthesis (generation of oxygen) and the unique properties of oxygen and ozone, life on land, at least as we know it, might well be impossible.
It is a beautiful irony that intense UV radiation generates the ozone in the upper atmosphere that then protects us here at the surface from that same radiation.
Oxygen and Nitrogen Create a Window for “Visible Light”
The second example involves photosynthesis as well, and also our ability to see. Our atmosphere is 78% nitrogen gas as the very unreactive, two-molecule, triple-bonded form (N2). With oxygen (O2) at 21%, that leaves only 1% for all the greenhouse gases and pollutants we emit. We know that greenhouse gases, even in minute concentrations, modify the energy balance of the Earth and are now causing global warming by absorbing radiant heat energy being emitted from the surface. But how did that energy get to the surface?
It seems to me an astonishing coincidence that the two major gases in our atmosphere are essentially transparent in what we call the visible range. All of the ROYGBV spectrum that we can detect with out eyes occurs in a very narrow window within the total range of radiation received from the sun. This also happens to be the region in which most of the energy from the sun reaches the Earth.
We call this the “visible range” because this is where evolution has led us, and most other animals, in developing the ability to see. If we could see only UV radiation, our world would be dark, because the ozone layer absorbs nearly all of this form of “light.” But then why would such an ability evolve?
Plants operate in these same visible wavelengths. Photosynthesis is driven primarily by radiation in the red and blue portions of the “visible” light spectrum. This is why healthy leaves usually appear green to us. Red and blue light are preferentially absorbed to drive carbon fixation and photosynthesis, while green light is preferentially reflected.
If the atmosphere had some different composition of gases that absorbed radiation in what we term the visible range, perhaps some other form of “sight” and some other form of photosynthesis, would have evolved. Or life could be total different, if it existed at all.
Finding and exploring other planets that support “life” could reveal modes of development and energy capture that we cannot even imagine. NASA’s exoplanet site allows for this possibility as well.
The third example is a “what if.”
What if Snowball Earth Had Persisted?
Environmental topics tend to come and go in popular media. I have used published cartoons to follow those trends. In the 1980s it was all about acid rain, with the ozone hole as a close second. Global climate change may dominate now (and maybe a relative dearth of cartoons emphasize that this is not a “funny” phenomenon), but even in the age of climate change, the Polar Vortex became a cartoon topic in 2013-15 as sudden outbreaks of record cold arctic air descended on the Eastern U.S. The absence of the drought relief expected for California due to a strong El Niño in 2015 was explained by “The Blob” – a patch of unusually warm water in the North Pacific. The Blob versus El Niño made for great cartooning!
At a somewhat lower level, the idea of “Snowball Earth” has been something of a media sensation recently – there are even anime or manga renderings on the topic.
Has the Earth really ever been entirely ice-covered? If so, how did it happen and how did it “recover?” Building on puzzling geological evidence and some early speculations, a Russian climatologist, Mikhail Budyko, proposed a simple model of the energy balance of the Earth that predicted either a total absence of ice sheets, or complete ice coverage.
At the heart of this concept was the relative effectiveness of ice and water in absorbing the energy delivered in sunlight or reflecting that energy back into space. Water absorbs nearly all sunlight; pure ice reflects almost all of it. If ice coverage begins to increase, increased reflection of sunlight by that ice should cause additional cooling, leading to more ice, etc. The technical term for this process is “positive feedback” (but it has also become the stuff of a solstice folktale!).
Geological evidence suggests not one but possibly 2 or 3 periods of massive ice coverage of the Earth, all prior to the Cambrian Explosion (so more than 550 million years ago). Some recent ideas propose that volcanic eruptions, which would have penetrated the ice cover and loaded the atmosphere with greenhouse gases, eventually began the retreat of ice coverage. Once the retreat began, a different positive feedback, with less ice and less reflectance leading to more warming, would accelerate further melting. This two-way positive feedback is at the heart of Budyko’s prediction of either an ice-free or a totally ice-covered Earth.
Humility and Appreciation for a Life-Sustaining Earth
I find each of these three global environmental stories to be fascinating in their own right, but in an era of rapid and accelerating climate change driven by human activity, I feel they offer an important perspective as well.
There is nothing out there in the universe that guarantees the habitability of the Earth for complex life forms like us. Without the combination of UV absorbance and “visible light” transparency by the atmosphere, life would be very different, if it existed at all. Significant swings in the global climate system have driven Earth to the brink of total ice coverage. While we are clearly in NASA’s definition of the “Goldilocks Zone,” so are Mars and Venus, where early planetary development led to conditions we would consider inhospitable for life as we know it.
A recent paper on the evolution of the atmosphere of Venus suggests that the planet had a chance to be like Earth, but that a period of violent tectonic activity, perhaps driven in part by high atmospheric temperatures, released unfathomable amounts of carbon dioxide and water vapor, leading to the runaway greenhouse gas conditions that now create surface temperatures of 870 degrees Fahrenheit or about 465 degrees Celsius. Another outcome our planet has avoided.
By some combination of events, Earth has sustained life for billions of years. In Gaia (please read the original version) Lovelock presents the argument that life is actually what has kept the planet suitable for life – another compelling irony.
All this says to me that our currently supportive climate is not a foregone conclusion. As we approach climate change with enhanced scientific understanding, and, hopefully, some political sophistication, it might be well to realize that there are no solid boundaries out there that limit the extent of climate change.
As we look to the future, we might approach the climate system with a bit of humility and respect, and also realize that our current emphasis on what might happen by 2050 or even 2100 could be myopically shortsighted. We have induced changes that will continue, and probably accelerate, for several centuries.
And as we look for life elsewhere in the galaxy (or universe) we might imagine that it will take more than a rocky planet in the Goldilocks Zone to sustain life. I like Lovelock’s definition of life at the planetary scale as using solar energy to create disequilibrium in the atmosphere of a planet (as photosynthesis has done), but life succeeding on those rocks and producing that atmospheric disequilibrium, might require many additional convenient accidents of atmospheric, chemical and climatic development like those presented here.
Life might be rarer than we expect.
Sources
Thanks to Karen and Kirk Nordstrom for the gift of the book Cosmos that inspired that career-long interest in popular science writing.
The original edition of Cosmos is:
Sagan, C. 1980. Cosmos. Random House, New York, 365pp
The original Gaia is:
Lovelock, J.E. 1987. Gaia: A New Look at Life on Earth. Oxford University Press, 157pp
The Contact poster was downloaded from Wikipedia where it was posted under “fair use” conditions. Original copyright by Warner Bros.
Even with the major radio telescope lost, the SETI program continues. The website is here:
https://www.seti.org/
There are also a number of citizen science projects on this theme:
https://en.wikipedia.org/wiki/Breakthrough_Listen
https://setiathome.berkeley.edu/
https://www.planetary.org/sci-tech/seti
Information on the Drake equation and Nuclear winter can be found here:
https://en.wikipedia.org/wiki/Drake_equation
https://en.wikipedia.org/wiki/Nuclear_winter
Properties of radiation absorbance of the atmosphere and on the role, formation, and break down of the ozone layer can be found here:
https://en.wikipedia.org/wiki/Sunlight
https://en.wikipedia.org/wiki/Ozone_depletion
https://en.wikipedia.org/wiki/Ozone_layer
The figure on atmospheric absorbance is modified from:
https://en.m.wikipedia.org/wiki/File:Atmospheric_Transmission.svg
The Cambrian Explosion is described here:
https://en.wikipedia.org/wiki/Cambrian_explosion
NASA’s Exoplanet program, including how candidates planets are identified, can be accessed here: The image of the habitability zone is from the first link below.
https://exoplanets.nasa.gov/search-for-life/habitable-zone/
https://exoplanets.nasa.gov/news/1673/whats-out-there-the-exoplanet-sky-so-far/
https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/#
Background information on Snowball Earth including an introduction to Budyko’s model can be found here:
https://en.wikipedia.org/wiki/Snowball_Earth
https://astrobiology.nasa.gov/news/sustaining-aerobic-eukaryotes-on-snowball-earth/
The snowball Earth image is from:
https://www.space.com/9461-snowball-earth-scenario-plunged-planet-million-year-winters.html
with credit to NASA
The article describing a possible early Earth-like Venus modified by violent tectonic activity is here:
And an educational NASA site on Venus is here:
https://www.nasa.gov/general/why-is-venus-so-hot-we-asked-a-nasa-scientist-episode-39/