Plastic is one of the best materials ever invented, but it doesn’t belong in the ocean. Large pieces of it can entangle turtles, birds, sharks, and other marine animals. Tiny bits of plastic, the result of the degrading actions of waves and sun, can linger for decades; once they get into the food chain, they, too, can adversely affect marine life. Many scientists and concerned citizens alike think that humankind should try to clean up oceanic plastic. But before we can best start the process, we need to understand how plastic moves through the ocean.
The motion of the ocean
The seas are constantly moving. Part of the reason for the incessant motion is that the winds blowing over the ocean surface exert forces on the water itself. Those forces help create a complex system of surface currents that span the global ocean.
Oceanographers long thought those currents were rather sluggish. But recent advances in observational technology—satellites, free-floating drifters, and other devices—have revealed that the ocean is full of vortices. Those vortices, or eddies, range from a few hundred meters to several hundred kilometers across. The oceanic equivalents of storm systems, eddies pack a lot of energy. They also disrupt currents and thus significantly alter the pathways of water and the plastic it carries along.
The ocean is so chock-full of eddies, and the eddies seem so disorganized, that it is very difficult to predict exactly how a piece of plastic would move through the ocean. That difficulty can be quantified. In December 2013, I was part of a team that did an experiment in which we deployed pairs of free-floating drifters from the stern of a research vessel traveling the Southern Ocean from New Zealand to Antarctica. The drifters floated just like plastic, but they packed a GPS device and satellite phone so they could be tracked. Each pair of drifters started out 13 meters apart as we tossed them into the water. In the following weeks, however, their separation grew to several hundred kilometers in a seemingly unpredictable way, a finding that shows just how dispersive the ocean is. Many groups have done comparable experiments in many parts of the ocean, and they all found results similar to ours.
Although ocean currents are unpredictable and complex on small scales, the flow does self-organize into large-scale patterns. As a result of those patterns, plastic particles tend to accumulate in the middle of the subtropical oceans, in what are often called garbage patches.
The subtropics are the regions between roughly 20° and 40° latitude in the Northern and Southern Hemispheres. There are five subtropical ocean basins, each of which has a strong, poleward-flowing western boundary current. In the first half of the 20th century, oceanographers applied Newton’s laws to fluids on a rotating planet—that is, to the ocean—and found that with the Coriolis effect and frictional forces they could explain the subtropical basins’ large-scale circulation patterns, or gyres.
Figure 1 illustrates the circulation in a Northern Hemisphere ocean gyre along with a key phenomenon that contributes to the accumulation of plastic garbage in subtropics: Ekman transport, the flow of water perpendicular to the wind direction, in part due to drag forces in the ocean. Because of Ekman transport, westerly winds (that is, blowing from west to east on the poleward side of a subtropical gyre) cause water to flow toward the equator. On the other hand, easterly winds on the equatorward side of the gyre cause water to flow toward the pole. As a result, water converges at the middle latitudes of the subtropical basin and spirals into the center of the gyre.
The water approaching the center of the gyre eventually has to exit, and it does so by flowing downward, sinking to depths of a few hundred meters. Plastic brought to the center of the gyre by the constantly inspiraling water doesn’t flow downward with the escaping water because it is too buoyant. Instead, it stays behind, and the accumulating plastic forms a garbage patch.
Modeling plastic movement
Ekman convergence explains why the plastic garbage patches are located roughly in the middle of the subtropical gyres, but it can’t explain many other features of garbage patches. Why are most of them actually somewhat off center (see figure 2)? Why are some bigger than others? Can plastic drift from one patch to another? What time scales characterize the movement of plastic in the ocean? Addressing such questions requires an understanding of how the ubiquitous eddies impact the large-scale gyre circulation and how those influences affect the actual flow of plastic in the ocean.
From a statistical analysis of the trajectories of all the free-floating drifters ever deployed in the world oceans, my colleagues and I created a website (http://www.adrift.org.au) that allows anyone to virtually track plastic or any other passively floating object. Clicking at different places on the interactive map will show that the time scale for plastic to reach a garbage patch ranges from months to decades. In all cases, plastic movement is much more unpredictable and dispersive than the simple, sluggish flow predicted by subtropical gyre theory. That is the effect of the eddies.
No islands of trash
Theoretical calculations and simulations based on free-floating drifter observations both predict that plastic floating in the ocean will end up roughly in the middle of the subtropical gyres, in areas as much as a few thousand kilometers in diameter. And indeed, levels of plastic concentration at the gyre centers are much higher than anywhere else in the open ocean. But the popular term “garbage patch” might suggest an island of trash in the middle of the ocean, and that picture would be wrong.
Instead, oceanographers who have gone out to sea to count plastic in the garbage patches have found that the density of plastic particles hardly ever exceeds one piece per square meter. And as shown in figure 2, most of those pieces are tiny, less than a few millimeters in size. So, although garbage patches in the sense of areas with elevated concentrations of plastic do exist, the patches are perhaps best described as a very thin soup of plastic and plankton.
Because plastic can take years to reach the garbage patches, and because global plastic production and concomitant plastic pollution are growing rapidly, a lot of plastic is still en route to the garbage patches. The patches, however, coincide with “deserts of the ocean,” regions with low nutrient levels. Most sea creatures live closer to the coasts, where a much greater quantity of nutrients and plankton can support a complex food web. Thus it could well be that plastic closer to the coastlines, on its way to the centers of the gyres, poses the biggest risk to marine organisms. Think about that the next time you see trash on a beach.
This work was supported by the Australian Research Council grant DE130101336.
Erik van Sebille is an associate investigator at the Australian Research Council Centre of Excellence for Climate System Science at the University of New South Wales in Sydney, Australia.