Sentinel-3 helps understanding the carbon cycle in the ocean – Interview with Shubha Sathyendranath

Sentinel-3A, has been delivering outstanding observations for almost two years and very soon, on 25 April, the B- unit is scheduled for launch, ensuring the full deployment of the Copernicus Sentinel-3 mission.

Thursday, 26 April 2018

Copernicus is the European Union's Earth Observation Programme, looking at our planet and its environment for the ultimate benefit of all European citizens. It is coordinated and managed by the European Commission.

The European Union has entrusted EUMETSAT to operate the Copernicus Sentinel-3 marine mission and to deliver marine data services to Copernicus service providers and users, supporting the development of a broad range of real-time marine applications and services in Europe.

OLCI, the Ocean and land colour instrument on board the satellite, provides valuable information about the sediment flow from rivers, the distribution along the coastlines or harmful algae blooms.

Ocean colour measurements also help tracking the microorganisms that form part of the carbon cycle.

Dr Shubha Sathyendranath, a scientist in the Remote Sensing Group at Plymouth Marine Laboratory explains why these measurements are important.

OLCI has multiple sensors in the visible and infra-red regions of the electromagnetic spectrum, which can be used to study phytoplankton (microscopic plants) in the ocean. We want to observe the seasonal cycle of phytoplankton in the ocean, as they are an important part of the ocean’s carbon cycle. The only way we can observe these plants at the global scale is through satellite-based detection of the pigments that they all contain, of which the major one is chlorophyll. OLCI is designed for measuring chlorophyll concentration from space. We can combine chlorophyll concentration with other information, also from OLCI, such as the amount of light reaching the sea surface, to calculate the rate of photosynthesis in the ocean: a process by which phytoplankton take up dissolved carbon dioxide in the ocean and convert them to organic material.
OLCI is also designed to study biological fields over land.

What is the cycle all about?

The ocean carbon cycle is an important part of the Earth’s carbon cycle, which we need to understand in the context of climate change. The planet is warming because of an increase in the green-house in the Earth’s atmosphere. Carbon dioxide is introduced into the atmosphere by the combustion of fossil fuels. Some of it is transferred into the ocean by diffusion. In the ocean, some of the dissolved carbon dioxide is consumed by phytoplankton through photosynthesis, to produce organic material. Globally, we estimate that phytoplankton consume some 50 GT of carbon annually through this process (which is comparable to net photosynthesis by terrestrial plants and trees). With satellite data from OLCI and similar sensors, we can now study marine photosynthesis on a regular basis. Some of the phytoplankton sink to the deep ocean, where they decompose and liberate carbon dioxide again. Some of that newly-released carbon dioxide may be brought back to the surface by the ocean circulation. This cyclic sequence of events is called the biological pump, which is a major part of the ocean’s carbon cycle. We cannot understand the Earth’s carbon cycle without considering the components of the cycle through the ocean.

We know that different types of phytoplankton have different roles in the ocean carbon cycle. The higher spectral resolution of OLCI is designed to help us identify key phytoplankton types from space, using differences in their spectral signature.

The benefits of monitoring the carbon cycle

We monitor the ocean carbon cycle because it is an important part of the Earth’s carbon cycle, which we must understand if we are to forecast future climate and future states of the ocean. We know that the Earth is warming. As a gas, carbon dioxide’s behaviour in the ocean depends on the temperature. So when we monitor the ocean carbon cycle, we are better able to contribute to discussions about the future states of the ocean and the future climate of planet Earth.

Another reason to study phytoplankton is that the entire marine ecosystem of the upper ocean relies on phytoplankton for their food, either directly or indirectly. We want to learn how information on phytoplankton from OLCI can be used to understand fluctuations in fisheries related to climate variability in the ocean, as well as the relationship between climate and marine biodiversity.

The difference Sentinel-3A has made for carbon cycle monitoring

Ocean colour is recognised as an Essential Climate Variable by the Global Climate Observation System, because of the important information that we can obtain about phytoplankton and their role in the Earth’s climate system. To contribute to climate studies, we need to monitor ocean colour systematically, consistently and over multiple decades. Within ESA’s Climate Change Initiative, we have now generated some 20 years of time series of ocean-colour products, by laboriously stitching together data from multiple satellites, including MERIS on ESA’s ENVISAT, and other NASA sensors.

The advent of OLCI on Sentinel-3A has marked a step change in the work of OC-CCI: we are now at the beginning of an operational era when we can count on a sensor with high spectral resolution (important for generating improved chlorophyll products and for distinguishing between major phytoplankton types) in orbit for the next couple of decades, which will improve tremendously our ability to generate climate-quality time-series data on phytoplankton. The presence of two sensors in orbit at the same time will reduce gaps in data significantly.

What will change when we have the full mission deployed?

There are three major aspects of the Sentinel 3 missions, that we ar looking forward to: (1) Improved spectral resolution, compared with what is available today (key to improving quality of ocean-colour products and for generating novel products relevant to climate and carbon cycle); (2) Commitment to long-term deployment for at least two decades into the future (important for long-term time-series studies in the climate context); and (3) Availability of two identical sensors in orbit at the same time (important for reducing gaps in data).

Integrated approach required for climate monitoring

When we study important questions associated with climate and climate change, it is essential that we implement an integrated approach. That is to say, it is not sufficient to study various components of the climate system in isolation. We need to understand how the various components interact with each other. Along with ocean colour (and information on phytoplankton and their activities derived from ocean colour), other variables such as temperature, winds, sea ice, sea level, aerosol and clouds in the atmosphere, green-house gases in the atmosphere are recognized as Essential Climate Variables. These variables interact with each other, and respond to changes in the environment around them. To understand how climate variability and change impact our living planet, we need to understand the connections across various climate variables, and how they influence each other. A major value of the Copernicus programme is that the Sentinel missions provide us access to information not only on phytoplankton, but also a large number of other, relevant Essential Climate Variables, thereby facilitating an integrated approach to studying the climate problem.

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