the cause or causes of the piora oscillation are debated. a greenland ice core, gisp2, shows a sulfate spike and methane trough c. 3250 bce, suggesting an unusual occurrence — either a volcanic eruption or a meteor or an asteroid impact event. other authorities associate the piora oscillation with other comparable events, like the 8.2 kiloyear event, that recur in climate history, as part of a larger 1500-year climate cycle.