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How Plants Run the Carbon Cycle

view of tree from below
Half of the dry weight of wood is carbon
photo courtesy of Susan L. Shafer

What is carbon? Carbon is the black charred stuff on the grill. Carbon is graphite, a soft slippery mineral used for pencil leads and lubricants. Carbon is diamond, the hardest substance known. Carbon is element number 6, a comparative lightweight on the periodic scale of elements. Carbon makes up one half of the dry weight of wood. Carbon combined with oxygen is the atmospheric gas, CO2, carbon dioxide, a key component of the Earth's atmosphere.

The carbon cycle is complex, but understanding how it works is important in order to understand why the climate is getting hotter.

The Earth's atmosphere traps heat at the planet's surface. This is known as the greenhouse effect, a necessity for life on earth. However, when atmospheric concentrations of certain gases rise, they intensify the heat-trapping greenhouse effect. Carbon dioxide is a major greenhouse gas, and rising concentrations of CO2 are followed by rising heat levels. When the concentration of atmospheric CO2 becomes lower, temperatures also go down.

Today's CO2 level of around 380 parts per million (ppm) represents a rapid jump from the approximately 280 ppm level 150 years ago. The added CO2 comes primarily from the burning of fossil fuels - coal and petroleum. As CO2 levels have risen, so has the Earth's temperature, creeping slowly but steadily upward. In order to bring down the temperature, the carbon dioxide concentrations need to be reduced.

Basically, carbon is cycled and recycled as it moves from the atmosphere into the ocean, into the life cycles of plants and animals, through the soil, and back into the atmosphere. Two basic cycles are operating, the slow and steady geologic cycle, and the much faster biological cycle that depends mostly on plants.

Biological Carbon Cycle

chipmunk in a tree
Plants feed the whole chain of life
photo courtesy of Susan L. Shafer

Photosynthesis, first by marine cyanobacteria, then by marine algae and finally by green land plants, has dramatically changed the carbon cycle and ultimately the earth. Plants have run the biological carbon cycle for 3 billion years in the ocean, and for the past 400 million years on land, using the process of photosynthesis to convert atmospheric CO2 into carbon-rich carbohydrates and sugars to feed themselves. And they are very good at it. They not only feed themselves, but also feed the whole chain of animal life. They also produce oxygen as a byproduct of photosynthesis, an element essential for animal life.

As land plants grow, they accumulate more and more carbon. Woody plants, such as trees, can sequester - pull carbon out of circulation - for dozens to hundreds of years. The CO2 given off by decay of organic matter in the soil is released to the atmosphere, where it will be consumed by plants in the continuing cycle. Plants have helped keep CO2 levels from rising excessively because they keep using it to feed themselves. The carbon cycle has a number of self-regulating mechanisms that can compensate for small temporary increases in atmospheric CO2. For example, plants initially tend to grow faster under higher CO2 levels and their consumption of CO2 rises accordingly.

Too much CO2 in the atmosphere can intensify the greenhouse effect, trapping too much heat at the Earth's surface. More than 400 million years ago the earliest land plants lived in a very high CO2 atmosphere, enduring what must have been brutally high temperatures, high ultraviolet radiation and violent weather. By the time land plants achieved tree size, they had begun to change the atmosphere, ultimately making it habitable for land animals. CO2 levels dropped almost tenfold and oxygen levels rose dramatically during the first great spread of forests on Earth, a process that took place over many millions of years.

Some carbon eventually became sequestered as coal, formed from ancient forests, and as petroleum formed primarily from marine organisms. This carbon became buried deep in the Earth's crust, and might have remained there permanently sequestered in rock, had humans not begun using coal and petroleum for fuel. The burning of these fossil fuels releases carbon sequestered millions of years ago, and adds what is essentially new carbon, to the present carbon cycle.

Human Activity and the Carbon Cycle

Humans first began to influence the carbon cycle thousands of years ago when they figured out fire. The use of wood for fuel rose with the development of civilization, and in areas where population growth was especially rapid, the loss of forests sometimes led to a cascade of other environmental impacts. However the carbon released by the burning of wood was carbon that was part of the existing carbon cycle. Overuse of wood for fuel certainly had many detrimental effects, notably deforestation, but it did not add any new carbon to the Earth's carbon cycle.

The Use of Fossil Fuels

Around 300 years ago, humans began to exploit the much more concentrated heating capabilities of coal. Coal fueled the Industrial Revolution, largely replacing wood, and the depleted forests began to grow back. Toward the end of the 19th century, oil discoveries fueled the start of the transportation boom that was to dominate the 20th century. Coal used for fuel needed large amounts of storage space, and its use in transportation was limited to locomotives and ships. Oil was much more portable than coal, and could be used to fuel small vehicles that replaced horse-drawn carriages. It also burned more cleanly than coal, making it desirable for heating houses. Coal released vast amounts of particulates when it burned and large cities were perpetually covered in soot and shrouded with smoke from millions of coal fires.

Oil-fueled vehicles solved the problem of cities with tons of horse manure on the streets, and oil heat solved the problem of soot-filled air. Coal eventually became used mostly for generating electrical power in huge power plants, its particulate emissions reduced by more efficient combustion and pollution control technology. Throughout the 20th century, use of both oil and coal increased rapidly. The United States had ample coal reserves, but oil reserves were insufficient to meet the escalating demand. Oil imports rose steadily until the United States was importing over half of the oil it consumed. Global politics became tied to oil and were as volatile as the fuel itself. The fuel crisis of the 1970s was a shock to Americans, not only because of the actual shortage at the gas pumps, but also as a revelation that our country was dependent on global oil politics.

gassing up a car
Between 2004 and 2008 fuel prices
have risen 300%

The drop in oil prices beginning in the early 1980s made consumers happy, but it meant the end of most of the ongoing research in alternative energy. Oil prices in the United States stayed low for the next 20 years, encouraging even greater oil use. European countries, perhaps more aware of their vulnerability to global events, had kept oil prices high enough to discourage excessive use. After 9/11, oil prices still stayed artificially low, until the sudden rise beginning in 2004, which was belatedly recognized as a fuel crisis.

But another crisis from fossil fuel use was growing worse, a crisis that had been largely unnoticed except by scientists, some of whom had been alarmed at least since the 1970s by the steady rise of global CO2 levels. When fossil fuels are burned, they release carbon that had been sequestered for millions of years. As CO2 levels rise some of the effects include: global temperature increases, accelerated melting of glaciers and permafrost in arctic and antarctic regions, rise in sea levels and changes in weather patterns. Climate change can also have a profound impact on many plant and animal species by forcing them out of their historic ranges.

In order to bring CO2 levels down, it will require a reduction in carbon emissions and a major sequestration of carbon.

Among other efforts to deal with this complex environmental challenge, New York State has signed onto the Regional Greenhouse Gas Initiative (RGGI). RGGI, (pronounced re'jee), is a regional agreement among ten northeastern states to reduce greenhouse gas emissions from power plants.

Reducing emissions can keep CO2 from rising even farther, but to remove the excess CO2 already in the atmosphere, carbon sequestration needs to be greatly increased. New York's forests already sequester considerable carbon, and have the potential to greatly increase their sequestration with careful forest management.


More about How Plants Run the Carbon Cycle:

  • Geologic Carbon Cycle - The geological cycle is primarily driven by the movement of the Earth's tectonic plates and associated geologic processes.
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