The concentration of carbon dioxide in the atmosphere has a key influence on observed climatic changes. Satellites can be used to measure the various parts of the Earth system that play a role in the cycling of carbon. Measurements of where carbon is stored in different forms, and the fluxes between these reservoirs, are vital for understanding current impacts and future changes.
21 February 2023
13 October 2022
By Hayley Evers-King and Rob Roebeling (EUMETSAT) and Ben Loveday (Innoflair)
The ocean carbon cycle
Carbon, in different forms, is cycled throughout the various parts of the Earth System. Carbon dioxide is emitted in to the atmosphere from both natural (e.g. decay of organic matter, volcanic eruptions etc) and human sources (e.g. the burning of fossil fuels, land use change etc). The concentration of carbon dioxide in the atmosphere is also effected by the oceans, and several mechanisms, including physical and biological processes, control exchange between these two parts of the Earth system.
The physical process of ocean carbon uptake is often referred to as the 'solubility pump', where carbon dioxide dissolves into sea water. This process is influenced by ocean temperatures, as carbon dioxide is more soluble in colder, less saline waters. Carbon dioxide that dissolves in to the surface ocean reacts rapidly with water forming a collection of ionic (charged) and non-ionic chemical species collectively known as "Dissolved Inorganic Carbon" (DIC). This includes bicarbonate and carbonate, as well as hydrogen ions (see Figure 1 for a schematic summary). The balance of the species formed in this carbonate system is influenced by various factors including pH, which is itself, in turn, influenced by the system, as hydrogen ions are formed during the conversion of carbonic acid to bicarbonate and carbonate. Generally, there is net positive charge in the carbonate system in the oceans, which leads to an increase in uptake of carbon dioxide from the atmosphere. It is this process that is leading towards the phenomenon of ocean acidification, as the ocean compensates for the increase carbon dioxide in the atmosphere.
Biological activity in the oceans also influences the movement of carbon from the atmosphere to the surface ocean, as well as within the ocean itself (i.e. from surface to deeper waters). Plankton, such as coccolithophores and pteropods, as well as corals, echinoderms, and bivalves, are all examples of organisms that depend on the process of calcification to build their skeletal structures, which contribute to "Particulate Inorganic Carbon" (PIC). By using carbon ions to form calcium carbonate these organisms can thus influence the carbonate system. However, they are also be effected by changes to it. In parallel, phytoplankton species, including calcifying organisms such as coccolithophores as well as many others, affect ocean carbon uptake through the process of photosynthesis. Carbon dioxide, in the presence of light energy and water, can be converted into carbohydrates and oxygen. It is this process that is the basis for nearly all food webs on Earth, as well as a main contributor to atmospheric oxygen concentrations that modern life has evolved in. Carbon dioxide captured in this way thus ends up forming ocean biomass, contributing to "Particulate Organic Carbon" (POC). This can eventually be respired, releasing carbon dioxide back in to ocean waters, and eventually to the surface. Particulate carbon, both organic or inorganic, can also sink and become locked away in sediments. Indeed the long-term accumulation of matter such as this is the source of much fossil fuel used in the modern era.
Using optical satellite data for measuring ocean carbon pools
How can satellites help us to understand this complex system and the changes it is experiencing? Satellites can measure the factors influencing the systems described e.g. temperature, wind speeds etc, but can also measure the pools of different carbon species in ocean waters.
Contributions to the PIC pool from calcifying plankton species like coccolithophores, can be observed with optical satellite data.
As an example, Figure 2 shows a true colour red-green-blue image of the Barents Sea from the Ocean and Land Colour Instrument (OLCI) aboard the Sentinel-3B satellite in July 2022. The bright white feature in the imagery is a coccolithophore bloom. These are readily identifiable thanks to the scattering characteristics imbued by the calcium carbonate shells of these organisms, which reflect a lot of light.
Figure 3 shows another example of a true colour image, this time of Barents and Norwegian Seas in May 2022. Here you can see that the colours are significantly different with more blue and green shades. This indicates varying contributions of different phytoplankton species, and whilst there may be some coccolithophores present, they are likely not present in as significant quantities as in the previous image.
The different colour shades and brightnesses of the reflected light measured by OLCI aboard Sentinel-3 can be used to provide more quantitative information on the carbon pools associated with these different types of plankton species.
Figure 4 (left) shows the derived chlorophyll-a concentration associated with the image in Figure 3. The OC4ME algorithm, used in the standard processing of OLCI data by EUMETSAT, compares the relative amounts of light reflected in the blue and green wavelengths of visible light to calculate the chlorophyll-a concentration. This works because large concentrations of phytoplankton are associated with high concentrations of the chlorophyll-a pigment. In waters where phytoplankton concentrations are the dominant source of optical variability, this is closely related to the ratio of blue-green light as a result of their photosynthetic activity. Biomass concentrations are, therefore, also closely linked to POC concentrations under these conditions, and, as a result, similar algorithms can be used to estimate POC concentration as well. Figure 4 (right) shows an example of this using the algorithm published in Stramski et al. (2008). The close correlation between chlorophyll-a concentrations and POC concentrations can be seen when comparing the left and right panels. These algorithms can also be applied to multi-sensor, merged time series of ocean colour data, such as in the example in Figure 5.
Similar approaches can be used to estimate PIC associated with coccolithophore blooms. Figure 6 (left) shows the chlorophyll-a concentrations derived using the OC4ME algorithm (as in figure 3; left), whilst the right panel shows the PIC concentration derived using the algorithms of Balch et al. (2005) and Gordon et al. (2001).
Although the patterns seen in the true colour RGB in Figure 1 are represented in the image of the corresponding chlorophyll-a product (Figure 6 (left)), it is likely that the chlorophyll-a estimates are inaccurate, as these algorithms were not derived using measurements in coccolithophore blooms, which present as significantly different to other phytoplankton blooms both in magnitude and shape of the reflectance. These are the facets of the optical signal associated with coccolithophores can be exploited with the specific algorithms used to produce Figure 6 (right). Close relationships can be seen between the bright waters identified in Figure 2 and the high PIC concentration estimated in Figure 6 (right).
What else can be measured from satellite? What more do we need to understand the oceans role in the carbon cycle?
Beyond ocean carbon pools of particulate organic and inorganic carbon, satellite data can play a further role in quantifying the carbon cycle in the ocean and in the wider Earth system. Much research is being conducted on the use of satellite data to assess pools of other forms of carbon i.e. the dissolved fractions. See Brewin et al., (2021) for a comprehensive overview of the current status of sensing of ocean biological carbon cycle. Satellite data can also be used towards measuring fluxes at the air-sea interface, and within the carbonate system (Shutler et al., 2019).
Upcoming missions, such as the Copernicus CO2M mission, which will be operated by EUMETSAT, will support quantification of emissions of carbon dioxide to the atmosphere. Further support to understanding the processes by which carbon is cycled in the Earth System will also come from advances in weather monitoring satellites (MTG and EPS-SG) such as through improved wind products over the oceans.
Relevance to ocean challenges, and further resources
As part of the United Nations Ocean Decade, ten specific challenges are being addressed, This work, and the data underlying it, support "Challenge 5 - Unlock ocean-based solutions to climate change". Data on ocean carbon pools and fluxes can help understand and quantify the oceans role in climate change , improving predictions and informing management decisions for mitigation and resilience both of ocean ecosystems and the Earth System at large. To support the UN Ocean Decade this case study has an accompanying Jupyter Notebook, which revisits the narrative and replicates most of the figures you see here. You can find this notebook in our ocean case studies repository, which you are free to clone for your own use or to support your own training. Alternatively, you can launch the notebook directly on Binder.