“As the World Breathes”(The Carbon Dioxide Cycle & Evolution of Earth’s Atmosphere)

By Steven Saburunov, Earth magazine, January, 1992

The real questions is not: “Why is there so much carbon dioxide in our atmosphere today?” but “Why is there so little?” Earth’s two sister planets, Mars and Venus, have atmospheres composed almost entirely of carbon dioxide (CO2). And there is every reason to believe our original atmosphere was almost pure carbon dioxide. Where did it go? It went into formations such as those found in Bat’s Head, Dorset, England (See cover picture). It went down to the bottom of the seas, becoming part of the crust and mantle. Almost all of our original atmosphere has been locked in Earth’s rocks. In short, we are standing on our ancient atmosphere.

Mars and Venus formed at the same time as Earth and from similar elements, yet neither one of them learned to breathe in and out: Venus exhaled all of its CO2 while Mars still holds it breath.

Only Earth is still breathing…Just as in the story of Goldilocks and the three bears, Venus is way too hot, Mars is way

too cold and Earth is just right. Venus never learned to inhale CO2 into its rocks and overheated early in its history. Mars never learned to breathe CO2 back out of its crust and is freezing without a warm blanket of atmosphere.

As mentioned, while Earth’s original atmosphere had roughly as much CO2 as Venus’ atmosphere today (96.5%), Earth has removed this huge burden of CO2 and locked it up in the rock beneath our feet, giving us a pleasant world.

The story behind why our atmosphere turned pleasant takes 4.6 billion years – the entire history of Earth as a planet. By examining a few chapters in that story, we will better understand the greenhouse effect as well as two important Earth cycles: the geochemical carbon cycle and the organic carbon cycle.

The greenhouse effect allowed both the organic and the geochemical carbon cycles to begin. The carbon cycle most familiar to Earth readers is the organic cycle: we participate in it whenever we breathe. We inhale oxygen and exhale carbon dioxide. Then green plants return the favor by taking in our carbon dioxide and returning oxygen to the atmosphere.

But the Earth has its own breath, breathing in CO2 and then returning it. This geochemical cycle is so gradual that only now are we beginning to appreciate its power. Between the two cycles, the Earth has achieved a balance, taking in just as much CO2 as it releases.

The carbon cycle started in the earliest years of our planet, so let’s examine what was happening in the atmosphere and on the surface then.

It appears that even during Earth’s early years, from 4.6 to 3 billion years ago, the planet had all the essential ingredients for the carbon cycle: oceans, atmosphere, continents and plate tectonics. It also had the Sun, but with an important difference: the Sun was only 70 % as bright as it is today. If the Sun were only that bright now, icebergs would clog the Panama Canal (located in the tropics).

And therein lies the first mystery. With such a dim Sun, Earth might have remained a solid ball of rock and ice until about 2 billion years ago, waiting for the Sun to warm. Yet geologists tell us that our oceans have always been liquid, that they have never been a solid pack of ice stretching from the North Pole to the South Pole, dim sun or no.

The answer to this enigma is the greenhouse effect.Carbon dioxide is nature’s thermostat, which

controls the temperature in the greenhouse. Acting like glass in a greenhouse, CO2, controls the balance of radiant energy Earth receives from the Sun and radiates it back into space. The greenhouse effect explains why on a sunny, sub-zero day, we can stand inside an unheated glass structure and be warm, even get a suntan. Light enters through the clear glass and warms our skin. Just as hot charcoal glows red, our skin glows with infrared radiation as it is warmed. This wavelength is invisible to our eyes, but we can feel it on our skin. But infrared cannot pass back through the glass, so the room warms. In the atmosphere, sunlight streams through CO2 like xrays cut through paper, but the infrared finds it difficult to do the same as it tries to radiate back into space.

Carbon dioxide is a necessary part of today’s atmosphere as well as the primordial sky. If all of the CO2 were stripped out of our heavens, the radiant energy balance would change and global temperatures would drop some 70˚ F (35 ˚ C). Conversely, if no radiant energy escaped, Earth would roast as Venus does now. Carbon dioxide, along with water vapor, methane and several other greenhouse gases, allows just enough energy to be radiated back into space, keeping Earth the way we know and like it best.

A calcite crystal forms from a drop of water in this underground cave. This slow process has locked our ancient atmosphere in rock.

During the first few billion years of Earth’s history, the greenhouse thermostat was turned all the way up for maximum warming. This early warming system allowed life to develop much earlier than it might have otherwise. This premier greenhouse was well-timed because it allowed the primordial oceans to stay liquid. In stark contrast, the oceans of Venus boiled away into their dense atmosphere, then escaped into space. Geologists are still puzzling over what Mars did with its seas.

But Earth enjoys warm oceans of liquid water, the only planet in the solar system to do so. One reason liquid oceans are so critical to life is that they give us weather, especially the wet kind. Freed by the greenhouse to roam the world, oceans were able to evaporate prodigious amounts of moisture into the skys, generating clouds, hurricanes, storms and rivers in a process called the hydrological, or water, cycle. A thundering water cycle allowed the weather to begin in earnest.

What weather it must have been! Winds blasting across open plains with no trees to slow them and no vegetation to prevent erosion. There was little oxygen (O2) except for a smattering on the face of the oceans that the unfiltered radiation from the Sun would ionize from the water. With so little oxygen, thre was not ozone (O3) layer, so the planet was scorched by dangerous, purplish ultraviolet (UV) light, its high-energy beams so powerful that they tend to change the chemical content of cells, killing them or causing mutations.

Our planet, in short, was a world void of life but full of potential.

Life had plenty of excuses not to start up at all under these conditions, but it did anyway! Its strategy for survival was to run away, hiding from all of the burning ultraviolet light by putting a layer of ocean between itself and the sun. It used about 30 feet (10 meters) of ocean as sun screen. The first cells were not too fancy (they did not even have a nucleus) and were dependent on whatever minerals were floating by for food.

Nevertheless, life continued peacefully under the waves for about a billion years. Then something happened that would change the Earth forever: chlorophyll. The immediate benefit of this green pigment was that plants could take whatever meager sunlight existed at those depths and use it to manufacture their own food right on the spot. The organic cycle began when, through this process, called photosynthesis, individual cells began to make their own food from sunlight, water and CO2 rather than try to get by on low-energy flotsam. (In microfossils, scientists have found the imprints of

primitive chlorophyhll-based organisms similar to blue green bacteria that we know today. The Warawoona Formation, located in Western Australia, is known for these fossils believed to be more than 3 billion years old.)

The teeming oceans led a tranquil life for years on end. Then something remarkable started to happen about 3.8 billion years ago (bya). Ocean life changed the environment. Green organisms gave off oxygen as a waste product, just as they do today. It took this green life lounging in the ocean depths a couple of billion years to add significant amounts of oxygen to our skies, but they did. From 2.5 to 2.0 bya, the first oxygen produced was absorbed by ocean water and seafloor sediments. Production of O2 by prokaryotes would’ve initially been offset by oxidation of BIFs, etc. By 2.0 bya, oxygen began to leave the oceans but was quickly absorbed by land rocks and soil. It has only been for the last billion years that oxygen finally started to collect in Earth’s atmosphere.

These higher levels of oxygen must have wreaked havoc on the landscape. Rocks and minerals that were resistant to weathering in a CO2 atmosphere now were being ravaged by oxygen’s terrible activity. Starting in the oceans, oxygen began to saturate ocean water and reacted quickly with just about everything. Iron, for example, that had been dissolved in the ocean water for 2 billion years, suddenly started to oxidize (“rust”) about 2.5 bya. These iron oxides were not soluble in water and precipitated (settled out) on the ocean floor as massive beds of chert and iron-rich minerals known as the Banded Iron Formations (BIFs). These ocean beds, created as the oxygen atmosphere formed, were raised by tectonics and are now Earth’s most important sources of iron ore. They are common in rocks around 2.0 – 2.5 bya, but nearly unknown in younger rocks, so this ore is an example of a non-renewable resource.

Forests “inhale” CO2 and produce oxygen as a byproduct during the photosynthesis stage of the organic carbon cycle. So, tree store a lot of CO2 that would otherwise be in the atmosphere.

Banded Iron Formations formed as dissolved iron was oxidized (turned into iron oxides that precipitated out of ocean water).

Redbeds, rocks that form when land rocks and soil are oxidized by oxygen in the air, are unknown in rocks formed more than 2.3 bya, but are common in rocks less than 1.8 by old. This means that there must have been substantial O2 in the atmosphere by that time.

The increasing levels of oxygen not only ravaged the oceans and the land, they created the ozone layer that blocks harmful ultraviolet radiation. This provided a marvelous benefit for life: Ozone blocked much of the raw ultraviolet light from the Sun. With the dangerous ultraviolet light problem solved, the green plants found they could live closer to the surface of the ocean. The production of oxygen snowballed: the more ultraviolet light was shut out, the more life increased, which in turn produced more oxygen. In a few billion years it was safe to go to the beach, so to speak. Exactly when our atmosphere became oxygen-rich is estimated to be a brief 500 million years ago.

This, then is the chapter on the origins of the organic cycle.

[For the next section, view the diagram “The Thriving Carbon Dioxide Cycle” and the accompanying PowerPoint “Evolution of the Atmosphere”.]Meanwhile, as the greenhouse was keeping the oceans liquid so the organic cycle could build up, the geochemical cycle was finding the water cycle convenient for its own ends. Long before the organic cycle began, all the necessary ingredients for the geochemical cycle were found in abundance: liquid oceans, a bit of weather to move the clouds around, plenty of exposed rock and a seemingly unlimited amount of CO2

in the atmosphere.A single geochemical cycle removes one molecule

of CO2 from the atmosphere and locks it in rock. To do this, water containing CO2 (raindrops) dissolves a Calcium silicate mineral (usually feldspar, a silicate mineral found in basalts and granites and a regular ingredient of mountains). The dissolved stone is then free to flow off the mountain, into caves, rivers, lakes or the ocean. Somewhere along the way, the carbon is locked up into a new variety of rock: calcium carbonate. [ CaO + CO2 → CaCO3 ]These carbonates often end up on the bottom of the oceans but can be found in limestone caverns as stalactites and stalagmites.

Marine organisms take Calcium and Carbon dioxide out of solution to make shells and skeletons. Their remains collect in layers on the ocean floor to form limestone.

The silica-rock weathering process proved to be the key to the success of the geochemical cycle. The task of removing CO2 from the skies and building rock was enormous. The original atmosphere was not only loaded with CO2, it was jampacked! By adding up all the CO2 locked in the crust and mantle of Earth, scientists project that the original atmosphere had so much CO2 that the air pressure was 60 times greater than today. It makes our present day atmosphere appear thin.

If all the carbon in our present-day atmosphere is counted as one unit, the measure locked up in rock is 100,000 units! The amount of carbon tied up in recoverable fossil fuels is about 5.5 units, while the amount dissolved in the ocean is much, much greater. In a planetary perspective, there is roughly as much carbon in the rocks of Earth as there is in the atmosphere of Venus (which has an air pressure 90 times as great as ours).

Besides locking CO2 in rock, the geochemical cycle restores CO2 back into the atmosphere, primarily in subduction zones where the ocean’s crustal plates dive under the edges of continents. In the middle of the ocean, upwelling molten magma drives the renewal process, pushing the plates apart at the mid-ocean ridges.

The ocean floor is on the move, speeding along at almost two inches per year in places. That’s not too fast on the Autobahn, but it is Mach nine on the geologic speedometer. It is so fast, that the enormous geochemical “engine” takes only 200 million years to turn over. The young ocean floors are only a mere few hundred million years old, while the mature continental rock is often billions of years old. The oldest surface (land) rocks found so far are 20 times older than the oldest rock on the ocean floor.

The fact that ocean plates slide means that all the carbon locked up in ocean floor rocks and sediment has a chance to get free again. The subducted plates grow hot from the incredible friction generated by sliding under a continent. The subducted plates begin to melt, forming magma. This magma becomes lighter than the cooler rock and starts making its way to the surface where it releases CO2 back to the atmosphere through volcanoes. If the red-hot lava appears to be boiling, that’s because it’s boiling off CO2 and other gases.

The Earth breathes out through volcanoes, as with Mt. Pinatubo in the Philippines.

If heat and pressure are applied to carbonate rocks, there is a possibility that CO2 will be released. As plates collide because of tectonic forces, the rock can fold and bend. Called metamorphism, this process of bending and twisting rock generates heat and pressure, enough to produce a chemical reaction. After the CO2 gas escapes, the rock may return to what it was to begin with – feldspar. The geochemical process can begin again if the metamorphosed rock is exposed to weathering.

As the gas rises toward the surface, it dissolves easily with groundwater. This is why some spring water, even though it is not as hot as the deep molten magma, is naturally carbonated (Think Perrier). Two hundred years ago around the start of the Industrial Revolution, CO2 levels were at an all-time low, about 285 parts per million (ppm). The following 200 years of industry have increased levels of CO2 by 40 % to present levels of 399 ppm (as measured in 2014 at NOAA’s Mauna Loa Observatory in Hawaii). Yet CO2 continues to be released by the geochemical cycle and the burning of fossil fuels.

Studying these past levels and process is useful because the information gained may help predict future climate changes. Although computer simulations are powerful tools to predict increasing levels of CO2, the ideal solution would be to find a planet somewhere, jack up the levels of CO2 and see what happens! Although there are no planets handy to play with, we do have a perfect laboratory: Earth through the last 4 billion years. The only limitation is that the experiments have all been conducted; now all that’s left is to find the ancient lab books and look through them.

Geologists and climatologists are doing just that. Traces of the ancient atmosphere are literally written in stone. Each layer of sedimentary rock is like a page in the lab book, revealing the climate of that particular age. The pages of this ancient lab book are still being found and deciphered, with many discoveries remaining to be made. But we have already have confirmed that when the temperature went up, the atmospheric CO2 levels were higher as well. Likewise, when global temperatures fell, so did levels of CO2.

Each layer of sedimentary rock is like a page in a lab book. This limestone outcrop in Central Texas tells us that a shallow ocean covered Texas during the Cretaceous period.

One page of this lab book suggests that unusual volcanic activity in the mid-Cretaceous caused the increased temperatures of that time. During the Cretaceous period, when the dinosaurs thrived, global temperatures were significantly warmer by about 10˚ C. These warm temperatures allowed life to boom, with the green belt extending from the equator to the Antarctic. We are still using the coal and oil that originated in this fertile environment. High levels of volcanic CO2 (3,500 ppm – almost 10 times higher than today’s) appear to have maintained the global temperatures during the 80 million years of the Cretaceous.

Will an increase in CO2 cause global warming?? All other factors remaining constant, yes. But how fast, how soon, and at what threshold – these questions remained locked in the ancient lab book.

Nevertheless, even though we don’t know all the scientific language these lab books use, we can easily read some parts in any language. Simply by skimming through the chapters, as we have done here, it is easy to read between the lines that, because of the actions of two great natural cycles, we are living in a paradise.