The IUCN (International Union for Conservation of Nature) stated that “plastic pollution is the most widespread problem affecting the marine environment”. Most of this pollution ends in the plastic oceans.
Plastics take centuries to degrade and contaminate the entire food chain. Each year, up to 13 million tons of plastic end up in the oceans because it is not recycled. Current estimates indicate that there is about 150 million tons of plastic found in the ocean. In 2018, the UN environment program placed the issue of plastic in the ocean among the six environmental emergencies of greatest concern.
Active methods of plastic removal are required to address this issue. Being able to detect and track plastic litters is crucial to enable these methods. There are three main approaches to track them: numerical modelling, in situ observation and earth observation satellite images.
Numerical models
Most of ocean plastic comes from rivers. Knowing the main points of origin and using data on ocean currents and on vertical mixing of water layers, it is possible to model the probable trajectory of plastic litter and the areas where they will stagnate and concentrate. The Copernicus Marine Service provides ocean current models that are used for this purpose.
Figure 1 : Visualization of Copernicus Marine currents numerical models, extracted from https://myoceanlearn.marine.copernicus.eu/
In situ observation
Local measurements are regularly performed to understand and quantify the issue’s importance. With statistical processing of the collected data, these measurements campaigns allow to build and calibrate the plastic litter tracking models (see for instance van Sebille et al.).
Figure 2 : Maps of microplastic count (left column) and mass (right column) distribution for one of the studied statistical models, extracted from the work of Van Sebille et al.
Earth observation satellites
99% of ocean plastic lays at the bottom of oceans, whereas only 1% floats on the surface. Remote sensing from space therefore suffers from a structural limitation that will never make it a self-sufficient solution to the problem. However, with their ability to cover large et inaccessible areas, using satellite images is an innovative approach that can complement modelling and in-situ observation. While still at the research and development stage, many research groups carry out experiments. They have to work with the tools at their disposal, which are not initially designed for this task.
The European Earth Observation Copernicus program offers different options here. First is the satellite Sentinel 3, dedicated to ocean observation. It is though designed to monitor large-scale oceanic processes. Its 300 meters resolution does not make it an adequate tool for plastic monitoring.
The Sentinel-2 satellite is an interesting alternative. It provides images in the visible part of the spectrum with a 10 to 20 meters resolution. Small pieces of plastic are not noticeable. However when they aggregate into large clusters they become more likely to be detected by satellites with such spatial resolution.
Sentinel-2 has been studied as a solution to ocean plastic monitoring by many research groups (see the work performed by a group from 25 different institutions, or this work carried out by a group from the Plymouth Marine Lab in 2019). However It has been initially designed for land monitoring. While its orbit covers all the earth, images are not collected on the wide open ocean areas. Images are only available on continental coastal areas and around islands. It is still though a very interested approach showing interested results in the covered areas.
Figure 3 :Example of plastic detection on a coastal are using Sentinel-2 images. Extracted from the work of the Plymouth Marine Laboratory
Use of satellite observation data
Earth observation satellite programmes mainly focus on land. Coverage of the oceans is limited to the observation of global phenomena, with a relatively low resolution (~300m).
Higher resolution observations are available over coastal areas.
However, most plastic waste is deposited on the ocean floor. It is therefore not observable by satellites. Those that remain on the surface can accumulate in clusters visible from space.
Space observation as a complementary solution to plastic oceans
Observation from space should be considered as a complementary solution. It can be used to detect plastic waste in coastal areas before it is dumped.
Earth observation is thus a useful data source for flow analysis and waste tracking models. It provides a wide and systematic spatial coverage that ideally complements the more accurate but local and punctual in-situ observations.
Evolution of observation means
Finally, it is important not to stop thinking about the role of space observation in ocean pollution control. Indeed, satellites are evolving, and so are observation tools. Improved resolution could allow for more detailed observation of plastic waste in the oceans. Going from 300 m resolution to 20 m with global coverage of the oceans would be a major advance. This would make it possible to position space observation at the centre of the tools and means for sustainable action in the oceans.
Setting up this type of mission takes time, but the plastic crisis in the oceans could lead space observation down this path.
Authors : Fabien Castel, Rémi Nassiri
