Science Puzzle
Why Did the Ozone Hole Open Over Antarctica?
High in the atmosphere, a thin layer of ozone gas shields life on Earth from much of the Sun's harmful ultraviolet light. In 1985, British Antarctic Survey scientists reported that a huge hole was opening in it every spring.
The culprits were chemicals called CFCs, used in fridges and spray cans, almost all of them released in the northern half of the world, where most people live. So why did the hole appear over Antarctica, as far from them as it's possible to get?
The Answer
Antarctica's extreme winter cold sets up the chemistry that destroys ozone, and the returning spring sunshine sets it off.
CFCs last for decades, and winds spread them evenly around the whole world. High in the atmosphere, sunlight breaks them up and releases chlorine. Most of the time that chlorine is locked away in fairly harmless compounds.
During the long, dark Antarctic winter, a ring of fierce winds called the polar vortex circles the continent and seals off the air above it. Inside, the high atmosphere gets so cold, below about minus 78 °C, that rare icy clouds form there. On the surface of their tiny ice particles, the locked-up chlorine is turned into forms that come apart easily. When the Sun rises again in spring, its light splits them into free chlorine atoms, which attack ozone. Each chlorine atom works like a tiny machine, destroying ozone molecule after ozone molecule, thousands of them, before it's finally locked away again.
The Arctic is warmer and its vortex is weaker, so its ozone losses are usually much smaller. In 1987 nations signed the Montreal Protocol, which phased out CFCs. The hole is slowly healing and is expected to recover around the 2060s, one of the great success stories of science-led action.
The principle: Atmospheric chemistry. Reactions depend on conditions as well as ingredients; the extreme cold, ice clouds and returning sunlight over Antarctica turn widespread pollutants into a concentrated, seasonal ozone hole.