Why Does Rubbish Collect in the Middle of the Ocean? Water Going in Circles

Surface currents are driven mainly by wind, and the pattern of prevailing winds pushes water westward in the tropics and eastward in the middle latitudes, which sets up a circulation around each ocean basin. The earth's rotation deflects moving water, which turns that circulation into a closed loop rotating clockwise in the northern hemisphere and anticlockwise in the southern. The same deflection also causes surface water to be pushed towards the centre of each loop, piling it up into a broad low mound a metre or so higher than the surroundings, and that mound is what makes the interior a place where floating things accumulate rather than a place they pass through.

Each system has a consistent structure with very unequal sides:

• •A fast narrow current on the western side, including the Gulf Stream and the Kuroshio
• •A broad slow return flow on the eastern side
• •An equatorial current flowing westward
• •A current flowing eastward at higher latitudes
• •A calm interior where water converges and sinks slowly
• •The western intensification is a consequence of the earth's rotation varying with latitude

Floating material entering the circulation is carried around and drifts gradually inward towards the convergence, where it remains because nothing carries it out. The result is not an island or a visible raft, which is the persistent misconception, but a diffuse region with elevated concentrations of fragments, most of them smaller than a fingernail and many suspended below the surface, spread across an area measured in millions of square kilometres. Concentration is high only by comparison with the rest of the ocean. Sampling with fine nets is how the accumulation is measured, and estimates of total quantity carry large uncertainties. Several such accumulation zones exist, one in each major gyre, with the North Pacific the most studied and the most publicised.

Removal from the gyres is difficult for reasons that follow from how the material sits there. The fragments are small, widely dispersed and mixed with plankton of similar size, so any collection method removes living organisms alongside plastic and processes enormous volumes of water for a modest yield. Much of the material is below the surface rather than on it. High-profile collection projects have recovered quantities that are substantial in absolute terms and small relative to the total, and their cost per tonne is high. Analyses consistently find that intercepting material at river mouths and preventing it entering the ocean is far more effective per unit spent, since a small number of rivers carry a large share of the input, and that reducing what is produced is more effective still.

The surface gyres are only part of ocean circulation and the rest operates differently. A deep circulation driven by density rather than wind moves water between basins on timescales of centuries, with cold salty water sinking in particular regions of the North Atlantic and around Antarctica and returning slowly elsewhere, and that overturning transports an enormous quantity of heat. The two systems interact but are not the same, and popular accounts frequently conflate them. Gyres also transport heat poleward on their western sides, which is why north-western Europe is far milder than its latitude implies. And they redistribute nutrients, with the interiors being nutrient-poor and correspondingly low in life, which is why those regions are sometimes described as ocean deserts.

Wind drives water westward in the tropics and eastward further north, and the earth's rotation closes that into a loop and pushes surface water towards its centre. Each gyre has a fast narrow western current and a broad slow eastern return. Floating debris drifts inward and stays, forming a diffuse region of small fragments rather than any visible island.