Ozone, a highly reactive gaseous molecule consisting of three oxygen atoms, forms a layer when found in the Earth’s stratosphere; the part of the atmosphere about 10 – 50 kilometres above sea level. The existence of the ozone layer is essential to protect all life on Earth as it effectively absorbs the Sun’s harmful ultraviolet (UV) radiation. Without the ozone layer, this radiation would cause genetic damage resulting in issues such as skin cancer in humans and growth problems in plants.
In 1985, scientists working for the British Antarctic Survey discovered that the ozone layer over Antarctica had become depleted. Ozone monitoring had begun in 1957. The data showed that ozone levels had reduced in spring throughout the 1980s. Following this, scientists at NASA confirmed that this depletion of ozone extended to the whole of the Antarctic continent. By 1992, the hole in the ozone layer was the size of North America.
This concerning breakdown of ozone is caused mainly by the release of chloroflurocarbons (CFCs), chemicals that were previously widely used in aerosols. Once CFCs reach the Earth’s stratosphere, they are exposed to the Sun’s UV radiation, which causes them to breakdown into substances including chlorine. Chlorine reacts with ozone to form oxides which combine with sunlight to destroy ozone. The presence of free chlorine in the stratosphere allows for the continued breakdown of ozone molecules.
The destruction of the ozone layer has been particularly prominent over the Antarctic. The low temperatures in the region produce polar stratospheric clouds, which enhance free chlorine through chemical reactions on the cloud surfaces. This, in addition to the long periods of sunshine during the spring and summer seasons, has caused up to 65% of the ozone to be destroyed, compared to 20% in the Arctic and about 5-10% in other world regions.
Many people were worried about the depletion of the ozone layer and the implications this could have for human health. In 1987 an international agreement to protect the ozone layer, called the Montreal Protocol, was signed. This agreement has led to a reduction in the release of CFCs into the atmosphere. The Montreal Protocol was the first treaty in history to achieve universal ratification – meaning that it has been ratified by every member state of the United Nations. Its success has led to the recovery of the ozone layer over Antarctica and less ozone depletion worldwide in the past few years. The long atmospheric lifetime of CFCs mean that it could be 2060 or 2080 before the damage is fully repaired, but the swift action taken at an international level has helped to avoid a global catastrophe.
Professor Susan Solomon ForMemRS is an expert in atmospheric science and winner of the Bakerian Medal and Lecture 2018. Join Professor Solomon on 8 March at the lecture Meeting the scientific and policy challenges of the Antarctic ozone hole: a global success story, to hear how public engagement and international policy saved the Earth’s ozone.