^"Depletion of the ozone ... has reached an unprecedented level over the Arctic this spring because of the continuing presence of ozone-depleting substances in the atmosphere and a very cold winter in the stratosphere," the WMO said in a statement.^

So the cold winter was also a causative factor. Of course, there are always variations and cycles of cold, heat, cold, etc. all over the globe; however, our weather is becoming more unpredictable and violent in its extremes. I hesitate to make an assumption on a subject on which I'm not an expert or even a semi-expert, but those violent extremes have to be affecting all facets of our planet's evolutionary cycle, including ozone depletion which was initially precipitated by hydrocarbon release into our atmosphere while we were all making ourselves more susceptible to cancer by using antiperspirants and other spray items. (Aluminum has been linked to breast cancer in some studies and aluminum is one of the ingredients in antiperspirants.)

I'll have to research if Ozone has varied to extremes over the millennia (if they have a way of measuring for a long historical period).