Microplastic aggregation and movement are significantly influenced by marine circulation and weather conditions. Microplastics are particles that measure less than 5 mm and are a growing concern. They not only become more applicable as other plastic marine litter disintegrates into tiny particles, but also interact with species in a variety of marine habitats.

A research by Natalie Welden and Amy Lusher published in Integrated Environmental Assessment and Management, examines how global climate change and the influence of changing ocean circulation impacts the distribution of marine microplastic litter. It forms part of an exclusive invited section on microplastics.

The ability to predict areas of plastic input and deposition would enable the identification of at risk species, and it would allow for efforts to reduce and remove plastic debris at targeted locations. The current uncertainty as to the effects of global warming on our oceans is the greatest challenge in predicting the future patterns of plastic aggregation in relation to global circulation. Natalie Welden, Open University

Landfill runoff, littering, and loss at sea are the chief pathways via which plastics enter the ocean. It is projected that plastic waste from coastal countries will grow almost 20-fold by 2025. The plastic’s density determines if it stays in surface waters, sinks to deep-sea sediments, or becomes beached in estuaries and coastal areas. In addition, marine circulation and weather conditions have a significant role to play in the distribution. For instance, the circular systems of ocean currents, such as the California Current in the Pacific or the Gulf Stream in the North Atlantic, influence the movement of plastics from their point of release to remote areas where they can collect in central ocean regions known as gyres. Unusual huge quantities of marine debris have been seen in these zones, such as the Great Pacific or North Atlantic garbage patches.

However, oceans are presently undergoing a marked period of uncertainty caused by global climate change. For instance, ice melts in Polar Regions is projected to have a range of effects on the spreading on marine plastics. Something swimmers are well aware of - it is easier to float in saltwater compared to in a swimming pool. Drop in the density of seawater at sites of freshwater input is estimated to decrease the relative buoyancy of marine debris, speeding the rate at which plastics sink. Correspondingly, areas of high evaporation, because of the increase in temperature, will undergo increased water densities, resulting in plastics continuing to remain in the water column and surface waters.

To further add to the complexity, alterations in sea surface temperature may also impact the scale and patterns of precipitation, especially cyclones, tropical storms, and tornadoes. Global warming increases alongshore wind stress on the ocean surface. Severe storms, flooding events, and increasing sea levels also indicate that more debris littering coastlines will become available for transport in the seas.

The hope is that future models of climate-ocean feedback are producing more accurate predictions of circulation patterns. This is vital in forecasting and mitigating potential microplastic hotspots and 'garbage patches'.