How do we know?

The scientific methods used to calculate the paths of floating debris through our oceans for 10 years after release are quite complicated. It is perhaps best summarised in the video below

Our oceans make up 70 per cent of the Earth's surface and are in constant motion. Driven by the sun and the wind our oceans develop mighty currents and eddies, some of which can take centuries to loop through all of our planet's ocean basins.

These currents also move through three dimensions. Many rise from the deep ocean near coastlines while other currents descend to the deepest parts of the ocean. These vertically descending currents are often, but not always, in the middle of the ocean in regions known as the five great gyres. These gyres are giant vortices spanning the whole ocean basin where water at the surface slowly spirals inwards until it sinks.

However, almost all plastic materials and lighter than water objects (such as those messages in a bottles) stay on the surface.

Since the late 1970s, ocean scientists have tracked drifting buoys but it wasn't until 1982 the World Climate Research Programme put forward the idea of a standardised global array of drifting buoys. These buoys float with the currents just like plastics except - like Twitter from the sea - they send a short message to scientists every few hours about where they are and the conditions in that location.

With this information, we have been able to create a statistical model of the surface pathways of our oceans. The plasticadrift website uses this model and generates an animation of the likely path and destination of floating debris over a ten year period, either into the future or into the past.

For the full details go to the papers Origin, dynamics and evolution of ocean garbage patches from observed surface drifters and Adrift.org.au - a free, quick and easy tool to quantitatively study planktonic surface drift in the global ocean. If you have any further questions you can contact Erik van Sebille.