Carbon has gotten a bad rap. Joined by two oxygen atoms, it transforms into carbon dioxide, the greenhouse gas that human activities are adding too much of to the atmosphere.
But carbon also is the element that makes life on Earth possible. This article offers a fascinating look at the science, history, and culture of the versatile element. Carbon atoms appeared before Earth existed, and proved crucial to the evolution of the planet and its atmosphere explains research scientist Robert M. Hazen.
About the Authors
Carbon is the source of life on Earth. Just about everything you wear and use over the course of the day requires carbon atoms: the gas that heats your home, the nylon in your jacket, and your diamond wedding ring. Carbon atoms appeared before Earth existed, and proved crucial to the evolution of the planet and its atmosphere. Research scientist Robert M. Hazen offers a vast, literally cosmic perspective on the universe, the Earth, and human life, and illuminates the scale of the impact of climate change.
- The big bang generated carbon atoms, which eventually became one of Earth’s most common elements.
- The discovery of most of Earth’s carbon has occurred near the surface, but other resources may be deep in the planet’s mantle.
- Earth didn’t initially have an atmosphere, but it emerged rich with carbon.
- In an immense, planetary cycle, carbon moves from the air into the Earth’s deep interior and back.
- Climate change, and methane and carbon dioxide levels in the air, rise rapidly due to human activities.
- No atom more suits the manufacture of what human beings want than carbon.
- Some 4.5 billion years ago, Earth had all the basic ingredients of life.
- The emergence of life requires the development of “self-replicating systems.”
The big bang generated carbon atoms, which eventually became one of Earth’s most common elements.
Some 13.8 billion years ago, following the big bang, no distinctive atoms existed. A hot whorl of matter and energy cooled and expanded. The first elements to take form were helium and hydrogen, but heavier atoms such as oxygen, nitrogen, and carbon emerged as well.
A small but ultimately meaningful amount of carbon came into being along with other elements immediately after the big bang. Stars are the “engines of chemical evolution.” Stars’ staggering pressure and heat created carbon out of helium nuclei in greater and greater concentrations. Eventually, carbon became one of the most common elements in the universe. For millions of years, carbon remained mostly deep in the interior of stars.
“Carbon gradually increased in concentration, ultimately to become the fourth-most-abundant element in the universe.”
Gradually, carbon atoms rose to the surface of stars and projected out into the universe, especially when stars disintegrated and exploded in “supernovas.”
The discovery of most of Earth’s carbon has occurred near the surface, but other resources may be deep in the planet’s mantle.
Atoms connect to form complicated, beautiful crystals. Most of Earth’s carbon exists in minerals like diamonds. Carbon atoms bond quickly and easily. After the big bang, the original carbon atoms were immersed in vast quantities of hydrogen and formed CH4 – methane. Eventually, carbon bonded with oxygen to form carbon monoxide and carbon dioxide.
Diamonds were the first crystals in the universe to contain carbon, and graphite was the second. As Earth evolved, it developed hundreds of types of crystals. These “carbon-bearing minerals” proved crucial for the modern world. The manufacture of steel, cement and glass, and providing fertilizer for crops, all require them. They help create laundry detergent, medications, dynamite, and jewelry.
“The crust, mantle and core of Earth host vast quantities of carbon-bearing compounds: diamond, graphite and more than 400 other crystalline, carbon-bearing minerals that comprise the dominant repositories of carbon on Earth.”
The great majority of crystals containing carbon atoms reside in the Earth’s crust, relatively close to the Earth’s surface. Hundreds of miles beneath the Earth’s surface are the mantle and core, a largely inaccessible world of incredibly intense pressure and temperatures. It’s nearly impossible to examine what takes place below the “core-mantle boundary,” which is some 1,800 miles [2,900 km] beneath Earth’s surface and has temperatures of more than 5,400°F [2,982 °C].
Of the hundreds of known carbon minerals, only a few forms in the high temperatures and pressure of the deep interior, and diamond is one of them. When carbon atoms congregate in the Earth’s crust near the surface, graphite is more likely to form.
Earth didn’t initially have an atmosphere, but it emerged, rich in carbon.
In its earliest phases, Earth lacked an atmosphere. It was arid rock and dust. But as of more than four billion years ago, Earth’s current stratification into core, mantle, and crust had begun to develop. Only a couple of mineral elements, including iron and oxygen, comprise the greater part of Earth’s mass. Carbon atoms played a central role in the creation of planets. But unlike iron and oxygen, molecules like methane and carbon dioxide held most of Earth’s carbon. Earth’s carbon originated in space from solar winds and meteorites. Comets contributed carbon dioxide, carbon monoxide, and the water that became the Earth’s oceans.
“It’s all part of the grand cycle in Earth – the cycle that created and replenishes the air.”
A great deal of Earth’s carbon resources were in fluids deep inside the planet. These fluids pressed upward through the layers of rock, and eventually burst or seeped through the crust. The water became the oceans, and the gases became the air. Experts remain uncertain as to the makeup of Earth’s first atmosphere. Gases that are still in the atmosphere, like nitrogen, were there at the beginning, but others didn’t emerge for billions of years. Between meteorites and violent volcanoes, carbon entered the air and the Earth’s carbon cycle began.
In an immense, planetary cycle, carbon moved from the air into the Earth’s deep interior and back.
From the Earth’s surface crust, bathed in sunlight, carbon moves downward, and, by way of “subduction,” carbon moves into the Earth’s interior. Deep in the Earth’s mantle, carbon is a fluid that ultimately oozes upward through the soil or explodes out of volcanoes. Carbon atoms move with the oceans’ currents and the Earth’s atmosphere. Every living thing on Earth uses carbon, from bacteria to snakes, birds, and human beings. Like other chemical elements, carbon moves in cycles from one place where carbon atoms congregate to another: the air, the seas, rocks, plants, and animals. Carbon moves from the air into the remote interior of the Earth and then rises back up to the surface.
“Immense slabs of ocean crust ‛subduct,’ diving down hundreds of miles, carrying with them layers of sediment and basalt rich in carbonate minerals and the decomposing detritus of life.”
More than 99.9% of Earth’s carbon is in the crust and mantle. Much of this is in limestone or “biomass” like oil and coal. A relatively small portion is in basalt on the ocean floor, which moves down into the mantle at “subduction zones.” The carbon in the crust comes from air and water on the surface, and while much of it goes right back out, a small part moves deep down into the mantle.
Carbon rises back to Earth’s surface through volcanic eruptions. Mount Etna in Sicily is the world’s largest identifiable source of carbon dioxide, in part because the volcano’s lava rises through layer after layer of limestone, and a lot of it comes from deep in the mantle. In all likelihood, volcanoes, whether in Sicily or elsewhere, are nature’s largest contribution of carbon to Earth’s atmosphere. Given the significance of the human contribution, it’s important to discover how much carbon volcanoes contribute.
Climate change, and methane and carbon dioxide levels in the air, rise rapidly due to human activities.
The balance the carbon cycle creates can change over time and has changed over the course of billions of years. The advent of life itself affects the carbon cycle, introducing new ways to absorb and hold carbon. Human beings have been burning things for heat, for light and to cook their food since the Paleolithic era. For thousands of years, the human use of fire didn’t throw off the carbon cycle or leave a “carbon footprint.”
This changed with the Industrial Revolution and the technological advances that followed, which led people to use fossil fuels on a massive scale. Over the past 200 years, human beings extracted billions of tons of coal and oil from the Earth, and by burning, it released billions of tons of carbon dioxide into the atmosphere. The amount of carbon dioxide humans introduced into the atmosphere dwarfs the contribution that volcanoes make and changes the balance of the carbon cycle.
“For billions of years, Earth seems to have found a balance between carbon subducted deep into the interior and carbon emitted from volcanoes – processes that helped stabilize climate and environment.”
Climate change is real: It is driven by increases in carbon in the atmosphere, which is a consequence of human activities. Carbon dioxide and methane are greenhouse gases that trap the solar energy. The levels of carbon dioxide and methane in the atmosphere have been rising significantly as a result of humans burning billions of tons of fossil fuels like oil and coal. Scientists are in universal agreement that climate change is a fact and that human activities cause it.
No atom more suits the manufacture of what human beings want than carbon.
Human beings want immense houses, cars, airplanes, fancy food and clothing, and much, much more. To make all of this stuff, people need whole chains of atoms and molecules in every imaginable size and shape. The material required for making all the stuff that human beings demand has to be engineered and its properties rely on its chemical composition. Because it’s so adept at bonding with a variety of other atoms, no atom is in a better position to play a central role in manufacturing what human beings want than carbon.
“No chemical element plays the combinatorial game of bonding to other atoms better than carbon, whose chemistry is so unfathomably rich that scientists who spend their lives studying carbon have been given their own collective noun: ‛organic chemists’.”
To make things, human beings need energy, and carbon can provide it – with fire. Earth contains generous amounts of highly flammable hydrocarbons, like oil, coal, and natural gas. These three fossil fuels remain people’s most readily available and cost-effective sources of energy. Though burning fossil fuels is crucial to making many new materials, they can be put to even better use. One of the many good reasons to stop burning fossil fuels is their contribution to climate change. Many other sources of energy, like wind and sunlight, exist – but carbon is indispensable for creating the materials essential for contemporary culture.
Some 4.5 billion years ago, Earth had all the basic ingredients of life.
The “primeval Earth” of some 4.5 billion years ago was unfit for life. Stones rained down from space. Violent eruptions issued great flows of lava that streamed over the surface of the Earth. Sunlight, with no atmosphere to filter it, was deadly.
“The stage was set. Earth, Air, Fire and Water were about to organize themselves into something new: Life.”
Yet the rudimentary elements that life requires were in place. All life forms require water to survive and, probably, to emerge in the first place. And all life forms require energy, whether food or sunlight. Earth had both water and energy. Life also requires carbon atoms, and Earth’s geology, atmosphere, and water supplied vast stores of carbon.
The emergence of life requires the development of “self-replicating systems.”
The most important passage in the history of Earth is the creation of the biosphere.
“The story of life’s origins and evolution is an epic tale, best recounted in the language of carbon chemistry.”
Microbes that discoverers found in Greenland’s ancient rock show a life form on Earth 3.7 billion years ago. Some researchers believe life emerged on Earth 4.4 billion years ago, while others prefer a date closer to 3.9 billion years ago.
The most productive way to approach the origin of living things is through three chemical steps. First, the basic building blocks must emerge, like proteins, fats, and sugars. Second, those building blocks must assemble into larger structures that can serve a variety of functions, like preserving and transmitting information and growing. Finally, this new structure must figure out how to reproduce itself.
“Carbon cycling lies at the heart of the rapid changes that humans are imposing on Earth, both planned and unintentional.”
Like all living things, human beings are part of the carbon cycle. The carbon cycle sweeps up everything: Air becomes plants, plants feed animals, dead animals and plants feed microbes, and eventually, everything becomes soil again.
But human activity changes the carbon cycle. Human beings plunder the oceans and forests and upend the balance of life. The human impact on the global carbon cycle is vast and deep.
About the Authors
Robert M. Hazen is a research scientist at the Carnegie Institution of Washington’s Geophysical Laboratory, and the Clarence Robinson Professor of Earth Science at George Mason University. He also is the executive director of the Deep Carbon Observatory.