Many of the pieces of plastic suspended in the water eventually drift out to the open sea. Sooner or later, most of them are caught by the great rotating ocean currents, also known as gyres. Once swept up, the plastic moves deeper and deeper into the gyre, as a result of which the litter becomes concentrated at the centre. Because of this effect, over the past several years the amount of litter at the centres of the gyres has grown so rapidly that these areas are now also referred to as garbage patches.
However, contrary to what many images in the media would seem to suggest, the litter is not densely packed; the first thing you see when you look at a garbage patch is mostly water. According to scientific surveys, even in the centre of a gyre, “only” between 50 and 60 pieces of plastic drift in an area of one square kilometre; most of it is small pieces of debris ranging from one to two centimetres in length.
At first blush, that may not seem like that much garbage, but compared to the North Sea, the effect of the concentration in the garbage patches becomes clear: although the North Sea is much closer to land (and therefore to the sources of pollution), on average it is home to only 30 to 35 pieces of plastic per square kilometre – only half as many as in the garbage patches, which are several thousands of kilometres from land.