Life on Earth is carbon based. Thanks to the presence of substances such as water, nitrogen and carbon dioxide, all of which are essential for life, microbial ecosystems were able to develop 3.8 billion years ago, thus beginning to add the biological component to a system that was, until then, only physical and chemical. How and why this happened provides an ongoing challenge to our understanding, mapping out the frontier between science and faith. What we do know is that early microorganisms first bound carbon through anoxygenic photosynthesis on what was an extremely hot planet Earth, struck directly by the sun’s radiation, with no oxygen or ozone in the atmosphere to provide a protective layer against harmful UVA rays. These early forms of microorganism lived in shallow waters that filtered some of the radiation from the sunlight and provided floating nutrients such as iron and phosphorus.
About a billion years later, cyanobacteria began to use water for photosynthesis, releasing oxygen gas as a waste product, leading to an enormous transformation of the Earth. Certain rocks bear evidence of having supported cyanobacteria 2.7 billion years ago, but the oxygen they produced did not accumulate in the atmosphere right away, being either taken up in reactions with other substances or metabolised by microbes that fed on the cyanobacteria and used the oxygen they produced to break down this now abundant carbon-based, organic food, thereby recycling the nutrients and breathing out CO2. It was only 300 million years later that oxygen levels in the planet’s atmosphere started to rise, leading to what is known as ‘The Great Oxidation’: an irreversible revolution both in the planet’s atmosphere and at its surface caused by the amassing of biologically produced molecular oxygen. However, it still took two billion years for the oxygen levels in the atmosphere to reach their proportion of 21% of the overall atmospheric gases that we are familiar with. After millennia of planetary development and adaptation, the biosphere finally began to reach the characteristics of the ecology that we know now.
It is because of the biological life on this planet that the atmosphere sees the ongoing exchange of gases that has been managed and regulated at harmonious proportions for the last 500 million years. Today, the plants on Earth extract around three thousand tonnes of carbon from atmospheric carbon dioxide every second, both on land and in oceanic ecosystems. Cyanobacteria are still most often the dominant primary producers among the phytoplankton at the ocean’s surface, whilst the first early forms of bacteria and archaea that weren’t able to survive in oxygenic environments still exist in the deep, anoxygenic parts of the planet. These organisms kept themselves concealed not only in order to survive but to form partnerships that are still extremely necessary, if not indispensable: their colonisation of animals’ guts, for example, provides a crucial and symbiotic relationship.
The concentration of carbon dioxide in the atmosphere is much less than that of oxygen, but it influences the climate through what is known as the greenhouse effect. Up until humanity’s mass industrialization, our interference in the planet’s systems was not enough to create significant disturbances in atmospheric composition. Since then, however, the use of natural resources has scaled to levels that interfere directly with the carbon cycle.
The Carbon Cycle is the exchange of carbon between the important carbon reservoirs of the planet’s biosphere, atmosphere, soils and oceans. A starting point for this exchange could be considered to be the point at which carbon dioxide (CO2) is taken from the atmospheric reservoir and is used by primary producers during photosynthesis. Through this process, carbon becomes part of organic bodies: stored within plants, supporting living biomass and travelling through the food chain. It can be released back into the atmosphere via respiration and decomposition, or can be stored as organic matter – dead or alive.
Over the long-term course of the carbon cycle, across hundreds of millions of years, organic sediment was stored in long-term reservoirs occurring on the ocean floor in the form of carbonate sedimentary rock, or else was turned to fossil. Fossil fuels are the remains of living organisms that have been transformed over time into solids (coal), liquids (oil) and gases (methane; other gases in smaller quantities such as propene and ethane), and which are formed mostly of carbon and hydrogen bonds. When these are burned, the carbon and hydrogen react with oxygen in the air to release energy, carbon dioxide and water vapour.
Life depends on the transformation of energy and matter, the process of which involves – and has evolved – some incredible specialization and adaptations. Recycling elements, fixing nutrients by transforming inorganic elements into organic matter, obtaining energy at cellular levels: these are abilities that have been developed and been passed on across generations, resulting in the intricate biodiversity that maintains the balance of our planet’s systems at all levels, from microbial chemistry to water provision. We are the only species creating waste products at such a level that they cannot be reused harmoniously within or by the environment. It is time for us to revert the many deep disruptions we are still causing to the Earth’s complex system of exchanges, starting with the most pressing issue: the overheating of the planet due to anthropogenic activities. Hopefully, as the way out of the current climate crises involves ecosystem restoration, we might use this opportunity to re-assess and find a better role to play in the cooperative intricacy of our planet.
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> Green carbon: the carbon removed by photosynthesis and stored in the plants and soil of natural ecosystems.
> Blue carbon: the carbon captured by photosynthesis in the world’s oceans, representing more than 50% of the green carbon (this rate in 2009 was 55%). Blue carbon is stored in the biomass and soils of coastal ecosystems – mangroves, salt marshes and sea grass meadows – and also captured by phytoplankton colonies.
> Brown carbon: the carbon released in the atmosphere by human activity. The burning of fossil fuels, wood burning and cement manufacture are estimated to inject from 7 to 10 Gt C (Giga tons of Carbon) per year in the atmosphere in the form of CO2.
> Black carbon: particles resulting from impure combustion. These have a large effect on radiation transmission in the troposphere, both directly and indirectly via clouds, and also reduce the snow and ice albedo (the proportion of radiation reflected by a surface as opposed to that which is absorbed by it). Black carbon is the second largest contributor to global warming, next to brown carbon. Black carbon tends to remain in the atmosphere for days or weeks whereas CO2 remain in the atmosphere for approximately 100 years.
> Anthropogenic climate change: the change in climate caused by the rising content of greenhouse gases and particles in the atmosphere as a result of human activities: burning fossil fuels, cleaning and burning natural vegetation, agricultural emissions, including those from livestock. Also caused by the degradation of natural ecosystems, reducing their ability to bind carbon through photosynthesis (green carbon) and store it for long term periods. There are critical thresholds of anthropogenic climate change beyond which dangerous and irreversible changes will occur.
> Carbon sequestration: the up-take of CO2 into a reservoir over long time scales – decades, centuries or even millennia.
> Carbon sink: is any process, activity or mechanism that removes a greenhouse gas, an aerosol or a precursor of a greenhouse gas or aerosol from the atmosphere. Natural sinks for CO2 are, for example, forests, soils and oceans (providing they are healthy and able to function actively). A forest’s biomass accumulates carbon over decades and centuries. The carbon captured by living organisms in oceans is stored in the form of sediments that, in the natural course of geological history, could remain stored for millennia. This is the case for sediments within coastal ecosystems but also in deep waters, where they’re derived from the sinking of organic debris.
> Soil organic matter: the term used to describe organic constituents in soil: tissues from dead plants and animals, the by-products produced as these decompose and the soil’s microbial mass.
> Soil organic carbon: the carbon component of the soil organic matter
> Carbon inventory: an accounting of carbon gains and losses emitted to or removed from the atmosphere/ocean over a period of time. Policy makers use inventories to establish a baseline for tracking emissions trends, developing mitigation strategies and making policies.
> Carbon stock: the total amount of organic carbon stored in an ecosystem, typically reported in megagrams of organic carbon per hectare (MgCorg/ha) over a specified soil depth. It is determined by the sum of relevant carbon pools.
> Carbon pool: a reservoir that absorbs and releases carbon.
As an example, for a Mangrove ecosystem, the relevant carbon pools are:
> The Ocean as Carbon Sink: Oceans represent the largest long-term sinks for carbon. Even though the plant biomass in the ocean is less than 1% of that on land, the carbon trapped via photosynthesis in the oceans’ ecosystems is almost the same as that on land. Oceans are very efficient carbon sinks not only in this capacity, but also due to the mixing of CO2 itself with the seawater and the sinking of carbon contained in dead biological matter that accumulates on the ocean floor. These various processes are known as ‘pumps’, and can be classified as follows: