Where currents flowing from the open ocean come up against narrow continental shelves, nutrient-rich, oxygen-poor water can be brought up into coastal waters and produce hypoxic zones (zones with low levels of dissolved oxygen) or even dead zones (zones with insufficient oxygen to support life, also called anoxic zones). Examples of this effect are found on the western coasts of America north and south of the equator, the western coast of sub-Saharan Africa and the western coast of the Indian subcontinent. Increases in the flow of some ocean currents may intensify this effect (chaps. Some greenhouse gases, especially chlorofluoro-carbons, have an impact on the ozone layer in the stratosphere.