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The carbon cycle

All living things are part of the carbon cycle. Along with all the other plants and animals on Earth we are made of carbon. Carbon is continuously moving between plants, animals, soils, the atmosphere, rivers, oceans and sediments. The global carbon cycle can be divided into two categories: the geological, which operates over large time scales (millions of years), and the biological/physical, which operates at shorter time scales (days to thousands of years). Some of the carbon atoms in our bodies at this moment would have been constituents of the plants, animals and soils present on Earth many millions of years ago.

There are essentially four major reservoirs of carbon interconnected by pathways of exchange: the atmosphere, the terrestrial biosphere (which includes freshwater systems and nonliving organic matter, such as soil carbon), the oceans (which include dissolved inorganic carbon), and the sediments (which include fossil fuels):

(source: US Department of Energy)

The global carbon budget is the balance of the exchanges between these reservoirs. In addition to the natural pathways of carbon through the Earth system, human activities, particularly fossil fuel burning and deforestation, are also releasing carbon dioxide into the atmosphere. When we mine coal and extract oil from the Earth’s crust, and then burn these fossil fuels for transportation, heating, cooking, electricity, and manufacturing, we are effectively moving carbon more rapidly into the atmosphere than is being removed naturally through the sedimentation of carbon, ultimately causing atmospheric carbon dioxide concentrations to increase. Also, by clearing forests to support agriculture, we are transferring carbon from living biomass into the atmosphere (dry wood is about 50 percent carbon). Because of this, atmospheric carbon dioxide concentrations are higher today than they have been over the last half-million years or longer.

Moreover, scientists have recently found new evidence that the Earth’s natural feedback mechanism regulated CO2 levels in the atmosphere for millions of years - a system that human emissions have recently overwhelmed.

Richard Zeebe of the University of Hawai‘i at Manoa and Ken Caldeira of the Carnegie Institution’s Department of Global Ecology studied levels of carbon dioxide in the atmosphere over the past 610,000 years using data from gas bubbles trapped in Antarctic ice cores. They used these records, plus geochemical data from ocean sediments, to model how carbon dioxide released into the atmosphere by volcanoes and other natural sources is ultimately recycled via carbon-bearing minerals back into the crust.

When carbon dioxide levels in the atmosphere rise, the chemical reactions that break down silicate minerals at the Earth’s surface are accelerated.  Plants and soil organisms have a significant role to play in the weathering process - in warm, wet tropical conditions, life can accelerate granite and basalt weathering as much as 1,000 times relative to a bare, lifeless surface. Among the products of these reactions are calcium ions, which dissolve in water and are washed to the ocean by rivers. Marine organisms such as calcareous plankton and mollusks combine the calcium ions with dissolved carbon dioxide to make their shells (calcium carbonate), which removes both calcium and carbon dioxide from the ocean, restoring the balance.

The researchers found that over hundreds of thousands of years the equilibrium between carbon dioxide input and removal was never more than one to two percent out of balance, a strong indication of a natural feedback system. This natural feedback acts as a thermostat which is critical for the long-term stability of climate. During Earth’s history it has likely helped to prevent runaway greenhouse and icehouse conditions over time scales of millions to billions of years — a prerequisite for sustaining liquid water on Earth’s surface. “This suggests a natural thermostat which helps maintain climate stability” says Zeebe.  “The delicately balanced carbon thermostat has been a key factor in allowing liquid water, and life, to remain on earth.  If it weren’t for these feedbacks, the earth would look very different today”.

During that time the long-term change in atmospheric CO2 concentration was just 22 ppm (although there were larger fluctuations associated with the transitions between glacial and interglacial conditions).  By comparison, two centuries of human industry have raised levels by about 100 ppm.  “Before anthropogenic emissions were added to the equation, the system was nicely balanced,” says Zeebe. “But this has changed. The average man-made increase in atmospheric CO2 from fossil fuel burning and deforestation over the past 200 years is about 14,000 times faster than the long-term average change over the past 610,000 years.”

“We are currently releasing fossil fuels from long-term reservoirs that usually store carbon over millions of years,” says Zeebe. “The natural long-term feedbacks which remove carbon from the atmosphere are way too slow to keep up with the pace at which we release it. We cannot expect that those feedbacks will protect us from climate change and ocean acidification in the near future. This will take tens of thousands to hundreds of thousands of years. ”

Measurements of atmospheric carbon dioxide levels (going on since 1957) suggest that of the approximate total amount of 7.1 GtC released per year by human activities, approximately 3.2 GtC (1 Giga = 1billion, i.e. 1,000,000,000) remain in the atmosphere, resulting in an increase in atmospheric carbon dioxide. In other words, burning of fossil fuels and the destruction of biological carbon sinks has flipped the carbon cycle into dumping carbon dioxide into the atmosphere faster than it can be withdrawn.

Biological carbon sinks

A carbon sink is a reservoir which accumulates and stores carbon. At present we do not have the proven technology which can be scaled up to remove carbon dioxide from the atmosphere but this is achieved by green plants using photosynthesis; plants take in carbon dioxide and use it to store the energy of sunlight in the form of complex organic compounds.

Regrettably, in the pursuit of material growth and the belief in technology to solve all our problems, we forgot our respect for nature and the processes which sustain life.  By harnessing the energy of oil (and other fossil fuels), our species has been able to outcompete others for space and resources.  Currently it is estimated that humans have diverted 40% of the net primary production of the planet (the amount of useful green stuff that nature produces each year) to their own use, but in doing so we are undermining these natural processes which kept atmospheric CO2 levels within acceptable limits.  

The concentration of CO2 in the atmosphere has already increased by about 30% since the start of the industrial revolution, and about 25% of this increase comes from changes in land use.  This increase will continue unless we realign our activities with the natural regulation of carbon.  

The only feasible way we can hope to reduce atmospheric carbon dioxide levels from 385ppm to at least 350ppm is to enhance biological carbon sinks by restoring forests and other natural ecosystems (e.g. grasslands, wetlands) in the right places, and by using appropriate farming systems (e.g. organic farming, agroforestry) to provide wood products and build up soil carbon.  At the same time we need to prevent changes in land use which release carbon, such as deforestation and poor agricultural methods. All these steps will also enhance ecosystem services.

Ecological agriculture

Much of our industrial scale agricultural "production" is really consumption, spending the natural capital of the rich organic matter in soils and relying on supplements from artificial fertilisers to maintain fertility.  Consequently, there is little organic carbon left in the surface horizons of many farmed soils, often as a direct result of the loss of the soil itself, yet one of the most effective, inexpensive, and overlooked strategies available is to store carbon in the soil.

Thankfully, soil is a renewable resource. ‘Growing new soil’ is very much like ‘growing a tree’ - both processes require carbon dioxide, water and light to fuel the production of photosynthetic materials - and is a natural outcome of ecologically-sensitive farming methods, such as organic farming. Some of the carbon drawn from the atmosphere via green plants is exuded into the soil by actively growing roots and combines with weathered mineral particles to form new topsoil. Any organic manure also encourages topsoil formation through the activity of the right kind of microbes, which are encouraged by ecological farming methods.

In the US alone restoring soils in currently farmed land could capture as much as 20% of national annual carbon emissions. Not only is recapturing atmospheric carbon into soil and plant communities the easiest and least expensive method for mitigating climate change but it provides many other economic, cultural, and ecological benefits. The many feet of new soil would act as a sponge, soaking up rainfall and preventing excessive nutrients from entering rivers and lakes, which would reduce flooding and protect drinking water supplies, and if the food and fiber crops are used locally it would further reduce carbon emissions from transportation and distribution. Intensive organic systems can yield up to three times as much food on individual farms in developing countries as industrial methods on the same land.  Many conventional farmers who have changed to these methods are starting to report increased profits because of higher productivity and the reduced need for artificial fertilisers and other chemicals, which are ultimately derived from fossil fuels.

A healthy environment means more food and a greater capacity to survive natural disasters. That's why we choose carbon offsets from projects which work with natural processes and cycles. It does not mean abandoning all technology and modern knowledge - on the contrary, such systems are often more design and knowledge intensive.  By doing so we can obtain a better yield while benefiting the planet, and at the same time decrease dependence on fossil-fuel based inputs such as fertilizers and chemicals. Ultimately, this is a more resilient approach.