In Earth’s upper atmosphere (stratosphere), what is called the ozone layer contains about 90 percent of the planet’s ozone. Since the 1980s, scientists measuring the ozone layer over the North (Arctic) and South (Antarctic) Poles have noted “holes,” or areas that do not contain as much ozone. It is thought that the holes are caused mainly by chlorine from human-produced chemicals, and the chlorine levels are high in the poles’ stratospheres. In particular, the three main ingredients for ozone destruction are chlorine from human-made chlorofluorocarbons (CFCs), very cold temperatures, and sunlight—all of which are found at both poles.

The fear has always been if either of the poles’ holes became larger or more persistent—thus diminishing or depleting the planet’s ozone layer—it could lead to increased health problems for humans, such as skin cancer, cataracts, and weakened immune systems. In addition, an increase in ultraviolet radiation could reduce crop yield and disrupt aquatic ecosystems, especially affecting the marine food chain in the upper layers of the oceans.