15 to 35 kms above the Earth’s surface, a layer of ozone has allowed life to thrive on Earth by protecting it from the ultraviolet radiations from the Sun. But the rising levels of pollution since the industrial revolution, particularly in the last one century, has resulted in the depletion of ozone in the stratosphere.
Appearance of hole over Antarctica
This is an annual phenomenon that takes place from September to early December when the westerly winds circulate all over the continent. The frigid temperature during this time, in combination with the Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) contained in the manufactured chemicals that are emitted from the surface of the Earth, trigger ozone depletion. High altitude clouds are formed as the temperature goes down. Chlorine and bromine react within these clouds in a photochemical process and destroy the surrounding ozone layer. The following entry of ultraviolet rays increases the chances of skin cancer and blindness in humans and sun damage in animals.
The appearance of ozone hole over the Antarctic mobilized international community into signing the Montreal Protocol in 1987. Good news is that the year 2019 recorded the smallest ozone hole in Antarctic since 1984, however, the detection of a rare ozone hole in the Arctic in the late March of 2020 has called for a re-evaluation of the progress made by Montreal Protocol.
In the Montreal Protocol, it was decided to phase out all the substances that cause damage to the ozone layer. It came into force in 1989 and considered the most successful model of international cooperation, especially when other important environmental treaties like Kyoto Protocol have failed to everybody’s disappointment. While all the chlorine and bromine containing substances are controlled by the Montreal Protocol, other ozone depleting substances like nitrous oxide are yet to be regulated under the treaty.
Appearance of hole over Arctic
The formation of ozone hole in the Arctic is a rare event given the fact that it usually doesn’t get cold enough in Arctic stratosphere for an ozone hole to form. It is almost three times the size of Greenland and probably the biggest ever detected in Arctic. Last time such an incident was recorded in 2011. What has changed the situation this year is the phenomenon of polar vortex.
What is A polar vortex ?
Formation of polar vortex involves the accumulation of extremely cold air inside a ring of extremely strong westerly winds, also known as the Polar Night Jet. It appears in the upper stratosphere of both the poles every winter season. A strong polar vortex with high-altitude clouds is a fertile condition for the formation of the ozone hole.
Such conditions are normally not witnessed in the Arctic, which usually has fluctuating temperatures, but the story has been different this year. Powerful winds have caused cold air to get trapped in the polar vortex. This is the reason ozone depletion in the Arctic primarily takes place inside the polar vortex. Fortunately, the temperature has been increasing in the Arctic since the last few days in April that has allowed the ozone hole to close.
The location of an ozone hole has nothing to do with the location of the source of CFCs on Earth. Given the global pattern of winds, the chemicals are mixed from everywhere. But what makes the ozone holes appear only on polar regions is the low temperature these regions provide. Nonetheless, there is a consensus among scientists that the behavior of ozone depletion in Arctic is different from the one in Antarctic as a matter of timing of the occurrence.