The Carbon Cycle & The 6th Extinction

The Carbon Cycle

At the beginning of the Archean Eon 4 billion years ago, life emerged upon Earth in a process called abiogenesis; ie., “origin of life”. Several evolutionary processes at the molecular level of increasing complexity formed various chemicals in the early ocean, which led to cellular processes like self-replication and membrane formation. Bacteria eventually evolved and began adding oxygen to the existing volcanic gaseous atmosphere. (For a time, the bacteria went a bit crazy, creating “The Great Oxygenation Event”). Fungi, plants, and animals evolved…some crawling onto land. Plants took root, making homes for the evolving insects, amphibians, reptiles, birds, and mammals. At the atomic level of all this is carbon. Carbon loves to bond with other elements, and is the best element able to form the long chains needed for complex molecules, like DNA. So, Earth is host to carbon-based life…

…and The Carbon Cycle.

Living things breath. Plants “inhale” carbon dioxide, add water, and with the sun’s energy use a process called photosynthesis to make sugars. The plant “exhales” oxygen. The plant grows, using up atmospheric carbon. The plants get eaten by animals. Their life processes…breathing, waste-production…release the carbon back into the air and soil. Plants and animals die, get eaten by carrion eaters and macro- and micro-organisms, releasing more carbon. The remaining soil-based carbon is sequestered for 100’s of millions of years, forming reservoirs of peat, coal, oil shale, natural gas, and oil deposits. 

Atmospheric carbon also dissolves into the oceans, which hold up to 50 times more carbon than the atmosphere. The tiny plants of the oceans, phytoplankton, eat some of the carbon via photosynthesis. Other things eat the plankton, other things eat the other things, etc.; all these things eventually die, drifting to the ocean bottom. The layers of sediment form an organic-rich mud and the pressure compresses it into organic shale and limestone. Through millions of years the layers continue to sink. Heat and pressure transform the shale into oil shale. Oil percolates from the shale, upwards through the other relatively porous layers and groundwater, until it meets a solid rock layer. Thus, another method of carbon sequestration. Carbon also get stored in the rock layers thus formed. This is all naturally in balance….

…WAS naturally in balance.

The actions of anthropogenic (human) activities…burning of carbon-based materials (fossil fuels) removed from the sequestered carbon reservoirs…releases carbon back into the atmosphere as CO2 which disrupts the carbon cycle in ways severely detrimental to human existence, and the existence of most other things.

Let’s begin with the air we breath.

Land-based plants and the oceans are currently managing to take up about 80% of the carbon generated anthropogenically (by humans). The rest stays in the atmosphere for thousands of years. Scientists have found that CO2 causes 20% of Earth’s greenhouse effect, water vapor 50%, and clouds 25%; other things make up the rest. 

Earth’s atmosphere is a thin blanket of gases and tiny particles where water vapor levels are controlled by temperature: higher temps equal more water. CO2, and other materials, heats the air by absorbing solar energy then releasing it. So as CO2 atmospheric levels rise, water concentrations increase due to the increase in temps. The increase in water concentration adds to the greenhouse effect, further increasing temps, further increasing water levels, further increasing the greenhouse effect, etc. The Earth is heating up on a global scale: Global Warming.

As carbon enters an environment, it combines with tiny water droplets to form carbonic acid. As the air cools, retained water get released it as rain..all precipitation begins as rain…and the rain holds the aforementioned carbonic acid, adding to the acid caused by other pollutants, creating “Acid Rain”. Acid rain disrupts plant growth and aids in mineral breakdown. As rock is degraded, the carbon stored there 100’s of millions of years ago gets released, adding more carbon to the Earth’s carbon cycle. So of all the greenhouse gases, CO2 is the gas that regulates the Earth’s temperature more than any other, thus controlling the both size of Earth’s Greenhouse Effect and the speed of Global Warming. And Earth’s atmosphere is getting wetter, creating closer ties with the oceans.

The Oceans, you say?

As carbon as CO2 enters the oceans, the carbonic acid created is soon converted into bicarbonate, lowering the ph level and, worldwide, causing Ocean Acidification. Higher atmospheric CO2 levels are causing an increase in these actions, resulting in an increase in species mortality; various sea creatures suffer depressed metabolic and immune response rates. Oysters, clams, and shallow and deep sea corals experience higher mortality rates. 

Plankton, the basis of the ocean food chain, which happens to add 50% of Earth’s oxygen to our air, also suffers. Studies have shown negative effects of ocean acidification on plankton. Zooplankton species have calcium-based protective “shells” which do not properly form in an acidified environment. Phytoplankton photosynthesize and need chlorophyll; chlorophyll health suffers in an acidic solution. Some species of phytoplankton cannot survive in acidic and warm environments…and that’s what’s now happening. So, plankton diversity may decrease and species’ population will redistribute as they adapt to changing conditions. The implications of population redistribution and decreased diversity will disrupt the ecosystem and the food chain in ways now not fully understood. 

So, we’ve disrupted the carbon cycle, adding atmospheric carbon faster than the normal carbon cycle can address it. Acid Rain, Greenhouse Effect, Global Warming, Climate Change, Ocean Acidification, Bug Die-Off, Diminished Species: all terms coined in my generation. The latest term?…

The Sixth Extinction

Five mass extinction events during the past 540 million years have happened, all involving worldwide extermination of marine species over the course of thousands to millions of years. Each event was preceded by major changes in Earth’s carbon cycle…a change we currently seem to be experiencing. Some scientists point to carbon cycle data suggesting that the sixth event may very well be happening now. The difficultly is that previous cycle data have spanned thousands or millions of years, while the current data base involves only a century or so. The magnitude of our current carbon cycle needs deeper study…

…which has been done. 

Daniel Rothman, professor of geophysics in the MIT Department of Earth, Atmospheric and Planetary Sciences and co-director of MIT’s Lorenz Center, has published an article in Scientific Advances, identifying “thresholds of catastrophe” in carbon cycles which, if exceeded, would lead to an increasingly unstable atmosphere, and as his data shows, mass extinction. 

Rothman found that the critical rate for catastrophe is related to a process within the Earth’s natural carbon cycle, in which organic carbon sinks to the ocean bottom and is buried and becomes sequestered. If this rate of natural sequestration is exceeded by carbon being added to the cycle…by burning fossil fuels for example…the carbon cycle becomes unstable. 

His study shows that historic mass extinction follows if one of two carbon cycle thresholds are crossed: long cycles with relatively slow carbon level changes that happen faster than Earth environments can adapt; and short cycles with a relatively quick, high level change. The latter change is what we are now experiencing; excess carbon circulating through the oceans and atmosphere, resulting in global warming and ocean acidification. 

His study indicates the crossing of the carbon threshold by 2100, using projections in the most recent report of the Intergovernmental Panel on Climate Change. IPCC considers various politically-enacted carbon emission-limit policy scenarios in their models, which all show that by 2100, the carbon cycle will either be close to or well beyond the threshold for catastrophe.

“This is not saying that disaster occurs the next day,” Rothman says. “It’s saying that, if left unchecked, the carbon cycle would move into a realm which would be no longer stable, and would behave in a way that would be difficult to predict. In the geologic past, this type of behavior is associated with mass extinction.”

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