Microplastics could be behind the change in the colour of the ocean in the Great Pacific Garbage Patch

This is a particularly interesting and original study, which suggests that microplastics may be contributing to the colour changes observed in the ocean around the North Pacific Subtropical Gyre. Based on two decades of satellite observations and simulations, the researchers have found that the water has become brighter in both blue and green wavelengths. This pattern is consistent with the accumulation of microplastics in the water column and is difficult to explain by natural causes alone. Most importantly, it opens up a promising avenue for indirectly monitoring the smallest microplastics from space – precisely those that are the most difficult to sample and the most likely to enter the food chain.

Could this be used to monitor microplastics from space? It is a real possibility, but it is not yet an operational tool. The signal will need to be validated through oceanographic campaigns that combine measurements of microplastic concentrations with simultaneous satellite observations. It will also be necessary to check whether the method works in other regions, particularly in coastal and more productive areas, where sediments, organic matter and phytoplankton also alter the colour of the water. This would be the case in the Mediterranean, which accumulates concentrations of microplastics comparable to those in the major ocean gyres, but whose coastal waters are optically much more complex. For this reason, satellite observation and direct sampling must be viewed as complementary tools. In situ observation networks, including those involving citizen science, will be essential for validating and interpreting these signals. If their usefulness is confirmed, this could represent a very significant step forward for the global monitoring of one of the marine pollutants that is currently most difficult to quantify.

Having worked in this field for almost a decade, I am not convinced by the study, although I recognise that I still need to analyse it in greater detail.

1. The study does not observe the plastic itself, but rather finds a statistical correlation between changes in the signal detected by the satellite and trends in microplastics observed in the field. On this basis, it proposes a causal relationship. However, existing studies generally agree that microplastic concentrations are not high enough to be detected by current space-based sensors, particularly when using satellites with low spatial resolution, as is the case in this study.
2. There are other factors that could be influencing the changing trends in the signal observed by satellites, ranging from methodological factors (for example, temporal drifts in sensor performance or atmospheric corrections) to ecological factors (for example, changing trends in phytoplankton composition). Therefore, the causal relationship is, in my opinion, debatable.
3. Current trends within the scientific community are moving towards the use of sensors with higher spatial resolution and towards monitoring extreme events involving the inflow and accumulation of marine litter on coastlines and in coastal waters. The present study overlooks this research (for example, Cózar et al., 2024, ‘Proof of concept for a new sensor to monitor marine litter from space’, published in Nature Communications).