When I teach Intro to Oceanography this semester and last year, one of the stories that made the greatest impression on my students (and on me, too) was the sad account of the “Great Pacific Garbage Patch.” This is an area of the ocean surface in the North Pacific covering between 700,000 square kilometers (270,000 sq mi) to more than 15,000,000 square kilometers (5,800,000 sq mi), larger than the size of Texas. It is composed nearly entirely of plastic trash, fishing nets, and other floating garbage, at least 80% which comes from the land and is non-biodegradable. The denser areas of garbage are so large they can be seen from satellite views.

Why is so much garbage concentrated in one area? It all has to do with the oceanic currents that I teach my Oceanography students about. The centers of each portion of the ocean is surrounded by a big circular current called a gyre, which usually runs in a clockwise fashion in the Northern Hemisphere and counterclockwise in the Southern Hemisphere (due to the Coriolis effect). In the North Pacific, a huge gyre completely surrounds the center of the northern ocean, with the warm fast Kuroshio Current coming up from the tropics past Japan, and the cold slow California Current coming down the eastern edge from Alaska, connected by currents (North Equatorial, North Pacific) which travel east or west to connect these in the equator and in the polar regions. This spiraling current tends to accumulate a mound of water in the middle (due to the Ekman spiral effect) that is rather stagnant and does not mix or blend with the boundary currents very well. It is also at a latitude with permanent high-pressure over it, so there’s no strong air currents to move it in any particular direction. Consequently, the centers of ocean gyres are slow and stagnant and tend to accumulate stuff that floats into them, and cannot escape. In the North Atlantic, the stagnant center of the gyre is called the “Sargasso Sea” after the huge floating patches of Sargassum seaweed that floats in the region for decades. There are garbage patches of smaller size in most of the other oceanic gyres as well. But in the North Pacific gyre, what stays put is garbage, the largest such patch in the world. (continue reading…)