Definition
The north magnetic pole has migrated across the Canadian Arctic towards Siberia at rates that have exceeded 50 km per year in recent decades, and the field as a whole drifts and weakens gradually. This secular variation is why charts state variation for a specific epoch with an annual rate of change, and why global models such as the World Magnetic Model are revised every five years, in 2015, 2020 and 2025, with out of cycle updates when drift outpaces predictions.
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