Scientists have identified unprecedented levels of depletion of the ozone layer above the Arctic this spring. These levels were comparable to those found in the Antarctic, where a hole has formed every spring since the mid-1980s. The hole over the Arctic is approximately 2 million sq km and is similar to the one over the Antarctic

Experts suggest that prolonged spells of cold weather, in addition to man-made chlorine-based chemicals like refrigerants and consumer aerosol (which were banned by the 1989 Montreal Protocol and are being slowly phased out of use), are responsible for the destruction of the layer and these are at their most active. Approximately 80 percent of the layer at a height of about 13 miles (20 kilometers) above the Arctic recorded the maximum depletion, according to reports.

The loss of ozone in the Arctic is, according to NASA, due to the same process as in the Antarctic - when extremely cold conditions trigger reactions that convert atmospheric chlorine, drawn from man-made chemicals, into forms that destroy ozone. However, the generally warmer stratospheric conditions there limit the area affected, as well as the time frame during which the chemical reactions occur, resulting in far less ozone loss in most years in the Arctic than in the Antarctic.

The findings of the study, conducted by a team of scientists across 19 institutions in nine countries, including Canada and the United States, were published on Sunday, in the journal Nature.

The scientists also found that at some altitudes the cold period in the Arctic lasted more than 30 days longer than in any previously studied Arctic winter; a factor that could also explain the unprecedented ozone loss.

Why [all this] occurred will take years of detailed study. It was continuously cold from December through April, and that has never happened before in the Arctic. said Michelle Santee, from NASA's Jet Propulsion Laboratory, part of the group that monitored the hole from space using satellites.

The difference from previous winters is that temperatures were low enough to produce ozone-destroying forms of chlorine for a much longer time. This implies that if winter Arctic stratospheric temperatures drop just slightly in the future, for example as a result of climate change, then severe Arctic ozone loss may occur more frequently, she added.

Meanwhile, the 2011 Arctic ozone loss occurred over an area considerably smaller than that of the Antarctic ozone holes because the Arctic polar vortex, a persistent large-scale cyclone within which the ozone loss takes place, was about 40 percent smaller than a typical Antarctic vortex.

However, this ability to quantify polar ozone loss and associated processes can be reduced with NASA's Aura and CALIPSO spacecraft reach the end of their operational lifetimes, the NASA said, adding that it is imperative that this capability is maintained. 

Meanwhile, chlorine-based compounds continue to circulate in the upper atmosphere; a fact that indicates the ozone layer has some way to go before it is restored to a healthy state.