Background SCOR WG 153: FLOTSAM

Background and motivation for the working group on Floating Marine Litter

The importance of environmental plastic pollution and current knowledge gaps

Contamination by man-made debris is increasingly reported in marine habitats around the world. An estimated 70-80% of marine litter is made of plastic polymers, and that percentage is probably much higher at the sea surface of the open ocean. Because they do not readily biodegrade, plastics may persist in the marine environment for years to decades or longer, longer than the time-scales of many of the ocean processes typically considered in physical oceanography. An estimated ~8 million metric tons of mismanaged plastic waste entered the oceans from land in 2010 (Jambeck et al. 2015), with additional sources including natural disasters (Maximenko et al., 2015) and accidents (Trinanes et al., 2016), and inputs are expected to rise with the continued acceleration in global plastics production (Plastics Europe 2016).

Marine litter poses a variety of environmental and socioeconomic risks, which will be mitigated only with a substantial, sustained and integrated effort from individuals, industry, governments, and intergovernmental organizations at local to regional and global scales. In October 2015 the G7 Science Ministers highlighted marine litter, especially plastics, as a major ocean health issue, and the International Association for the Physical Sciences of the Ocean (IAPSO) and the Scientific Committee on Oceanic Research (SCOR) published conclusions regarding those issues and recommendations for future action by G7 countries (Thompson and Maximenko, 2016).There is no single solution; rather, a variety of local and regional solutions will be required to effect change (Hardesty et al. 2017).

Marine litter occurs all over the world from densely inhabited to remote areas, from the seafloor to surface waters. However, our knowledge of the abundance and distribution of plastic in the open ocean is limited, with most prior work having focussed on floating microplastics (millimeter-sized particles and smaller) measured with plankton nets.

Floating microplastic debris is found in seas around the world, from oceanic subtropical gyres (e.g. the so-called ‘garbage patches’) where concentrations exceed 600,000 pieces per km2 (Law et al. 2010), to inner seas (e.g. Suaria et al. 2016, Chubarenko et al. 2016, Chubarenko and Stepanova, 2017) to more remote regions such as the waters of the Arctic (Cozar et al., 2017, Bergmann et al., 2016) and the Antarctic (Barnes et al., 2010; Ryan et al., 2014), where far fewer plastic particles are observed. It has become clear that humanity's discarded litter is spreading throughout our seas and oceans (e.g., Pham et al., 2014; Jambeck et al., 2015; GESAMP, 2016) and ocean models of surface transport predict that marine debris should ultimately be expected everywhere (Van Sebille et al., 2015).

A number of international working groups have focused on ocean plastics, but the focus is often on impacts of plastics to marine organisms and ecosystems. With the goal to assess the risks of plastic debris, they frequently highlight the need for increased knowledge about its abundance, distribution and transport. A necessary step is to get an estimate of the amount of plastic in the ocean, including knowledge about its origins, where it is accumulating, and its transport pathways. This is a complex problem for a variety of reasons, including challenges in sampling, both in situ and remotely, as well as in modelling.

If 8M tons of plastic are added to the ocean annually and plastic is expected to be around for decades or even centuries, why don't we find these large amounts in the ocean (e.g. Ryan, 2015)? Estimates of floating litter to date only tally up to order of 100,000 tons of floating microplastics (Cozar et al, 2014, Eriksen et al. 2014, van Sebille et al., 2015), with only an order of 10,000 tons removed by coastal clean-ups [1]. What missing knowledge can explain this multiple order of magnitude mismatch? Emerging research in physical oceanography may help elucidate marine debris distribution patterns and transport processes. Bringing together scientists with expertise in plastic marine debris with those focused on ocean observations, remote sensing, and numerical modelling in a single SCOR working group will create a powerful collaboration that will advance our understanding of marine debris in the open ocean.

The Challenge

The major challenge of this WG is to explain the distribution patterns, trends, and pathways of plastics in the open ocean.

Limitations of our understanding of the transport of floating plastics result from technical gaps as well as gaps in our knowledge of the near-surface ocean dynamics. These gaps include:

  • coarse vertical and time resolution of debris modelling
  • high-frequency processes and their non-linear interactions (Stokes drift, inertial oscillations, diurnal cycle in the upper-ocean stratification)
  • lack of data on critical parameters (e.g. fluxes) that could support next-generation models of plastics at sea, and limited observations of surface currents (remote or in situ) that could help calibrate/validate such models.

In addition, there is a dearth of knowledge on the typical features of marine debris, including floating lifetime, settling, fragmentation, degradation, and ingestion by organisms, which may alter the debris properties affecting its transport. Questions to be addressed include:

  • How variable is marine litter composition in time?
  • How big are the differences in the composition between different regions?
  • What characteristics of plastic debris should be used in models (different degradation and fragmentation rates for different polymers, average particle size, etc.)?
  • How does the composition of microplastics correspond to the composition of macroplastics?
  • How do the dispersion and physics that apply to floating debris differ between macroplastic, microplastic and nanoplastic?
  • What are the roles and impacts of the biological pump on transfer of pollutants to deep waters at different scales?
  • What are the temporal trends in composition of microplastics associated with changes in macroplastic production?

Drift models have been used to describe marine litter distribution and transport, but improvements are required to adequately simulate pathways of marine debris ranging in size from microplastics to large objects. This includes improved models of ocean motion and definition of the dynamics of buoyant objects in a turbulent sheared flow, together with characterization of properties of plastic debris.

Distribution of floating marine litter has been studied since the 1970s using plankton net tows and visual selection of plastic particles in collected samples. Preliminary efforts have been made to standardize collection procedures and sample analysis protocols. Yet global, or even regional, in situ sampling at high resolution is not feasible, which calls for development of remote sensing instruments. At present, only optical data are readily available and they are only capable of detecting very large debris items.

Prospective satellites and airborne sensors may be able to measure various indices related to plastics and other types of floating debris and quantify their abundance on the ocean surface. The scientific recognition on this topic is still in its infancy and the key issues to be addressed and the full potential of remote sensing are still to be fully discussed in the scientific community. In 2016, the European Space Agency (ESA) released a call for proposals on remote sensing of Marine Litter (RESMALI). In the same year, the US National Aeronautics and Space Administration (NASA) sponsored a workshop on Mission Concepts for Marine Debris Sensing[2] and included marine debris research in the scope of NASA’s Interdisciplinary Research in Earth Science (IDS) program[3]. Satellite remote sensing can best contribute to the marine debris field through new missions to measure surface velocities, as well as implementation of existing and development of new sensors (optical, hyper-spectral, SAR, etc.) to track larger objects or detect the presence and quantify the concentration of smaller particles.

In this SCOR WG we will:

  • address the problem of floating litter at the global scale by disentangling coastal processes (with their short timescales) from the open ocean low-frequency processes.
  • improve the application of ocean circulation models to the drift of debris at sea.
  • assess and promote the use of remote sensing tools to study floating debris at sea.