This time of year combines cold stratospheric temperatures with the "polar dawn", as solar ultraviolet radiation begins to reach the polar stratosphere, where it causes photolytic destruction of human-made gases in the stratosphere, such as the CFCs, methyl bromide and nitrous oxide. ... Long-term predictions are uncertain since they involve assumptions about not only future ozone levels but also future variations in cloud cover, troposphere ozone and lower atmospheric pollution. ...

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