The carbon cycle

Earlier on, you had a closer look at the water cycle in the Mediterranean region. In this video and the associated article here, you will learn about the different carbon cycles and their importance to Earth’s climate.

As part of the way Earth works as a system, carbon is continuously passed between the ocean, the land and the atmosphere. This involves a range of different processes, some of which can be observed by satellites. Human activity is disturbing these natural processes and causing a rise in atmospheric carbon dioxide. Satellites and ESA’s Climate Change Initiative are helping to improve our understanding of the carbon cycle and its role in climate change. Source, ESA

The global carbon cycle

The global carbon cycle is the process by which carbon is exchanged within and between the atmosphere, land, and ocean (Houghton, 2003). Any shift in the carbon cycle that causes carbon to be transferred from one reservoir to another causes more carbon to be stored in the other reservoirs. In other words, carbon is continuously exchanged between various ecosystems and the atmosphere. Therefore, to better understand the carbon cycle, understanding carbon sinks and sources and processes leading to carbon sequestration is essential.

Sinks of carbon are natural (e.g., plant, ocean and soil) or artificial (e.g., landfills, carbon capture and storage) reservoirs that accumulate and absorb more carbon than they release. In contrast, carbon sources are those that release more carbon than they absorb into the atmosphere through natural (e.g., volcanic eruptions) or artificial (e.g., burning fossil fuels) processes. Carbon sequestration refers to the process of capturing and storing carbon dioxide (CO₂). Carbon sequestration can be classified into three types, namely geological, biological and technological.

• Geological sequestration is a natural process in which carbon and CO₂ are stored in geological formations (fossil fuels) (Marini, 2006).
• Biological sequestration is a natural process in which CO₂from the atmosphere is stored in vegetation, soils, and aquatic environments.
• Technological sequestration is the artificial process of trapping and storing carbon or CO₂ (Riahi et al., 2004).

Figure 1: Simplified schematic of the global carbon cycle. Numbers represent reservoir mass, also called ‘carbon stocks’ in PgC (1 PgC = 10¹⁵ g of carbon) and annual carbon exchange fluxes (in PgC per year). Source IPCC, 5th Assessment Report (Click to expand)

There are two carbon cycles, namely fast and slow Carbon Cycles:

• The Fast Carbon Cycle is the rapid exchange of carbon between the atmosphere and the Earth’s surface (land and ocean). The turnover time of the carbon in this fast cycle, defined as the mass of carbon divided by the exchange flux, ranges from a few years for the atmosphere to decades and millennia for the major carbon reservoirs of the land vegetation and soil and the various domains in the ocean (IPCC, WG1AR5). The most well-known actors in the fast carbon cycle are plants and phytoplankton with their photosynthesis and respiration processes (see figure 2). On land, during photosynthesis and respiration, O₂ and CO₂ are exchanged in nearly a 1:1 ratio. In the ocean, only a comparably small amount of O₂ is dissolved in the ocean, whereas the oceanic content of CO₂ is much larger due to carbonate chemistry (IPCC, WG1AR5).

Figure 2: Carbon uptake and photosynthesis in a seagrass meadow, also applicable to land vegetation. By Cullen-Unsworth L, Jones B, Lilley R and Unsworth R, CC BY 4.0. (Click to expand)

• The Slow Carbon Cycle takes place in rocks and seabeds and is considered a long-term process that takes millions of years to complete (Dasgupta and Hirschmann, 2010). Rain initiates the transition of carbon from the atmosphere to the lithosphere and hydrosphere by transferring carbon into the land and ocean through acidic rainfall. This dissolves the rock’s surface and deposits carbon compounds onto the ocean floor. The ocean sediments degrade into carbon-rich rock, which then migrates into the tectonic margins over millions of years. These carbon compounds eventually might return to the atmosphere through volcanic eruptions.

Nature tends to balance CO₂ by absorbing it through photosynthesis and returning it to the atmosphere via respiration. For example, Figure (3) shows the overall perturbation of the global carbon cycle caused by anthropogenic activities, averaged globally for the decade 2009–2018 (Friedlingstein et al., 2019)

Figure 3: Schematic representation of the overall perturbation of the global carbon cycle caused by anthropogenic activities, averaged globally for the decade 2009–2018. See legends for the corresponding arrows and units. Source Friedlingstein et al., (2019) (Click toe expand)

Up next

Congratulations! You have reached the end of this activity. In the next activity, we will introduce the concept of climate change and how the climate is described. We will start by reviewing what climate change is and which variables are considered essential to characterise the Earth’s climate.