The north magnetic pole’s rapid and inexorable shift requires the early release of a new model of Earth’s magnetic field for military and civilian navigation around the Arctic. The partial federal government shutdown will delay the release of this model from January 15 until at least January 30, despite the U.S. military’s request for the update ahead of schedule.

No one is quite sure why the change has accelerated nor why the field has surged in recent times, according to Nature.

The previous model of the magnetic field was released in 2015 and updates have been slated for 5-year intervals. However, the changes are massive enough for military and civilian navigation—mostly in the Arctic Ocean—that the latest revision had to appear in 2019. A surge in 2016 happened just after the last model was set, making the update more critical four years out.

This update doesn’t affect GPS receivers, which don’t rely on the magnetic north pole. A receiver instead picks up signals from multiple satellites for which the exact position in orbit is known, and use trilateration (the intersection of their signals) to determine a location. However, satellite orbits are optimized for reception in the most-populated parts of the world, and other factors reduce GPS accuracy and reception in the Arctic.

The magnetic north pole never been a fixed point, but that northern apex, towards which compass magnets point by their nature, has accelerated its movement over the last 40 years from movements recorded in early decades. (The geographic north pole and the magnetic north pole have no particular relationship: the former is fixed in place by cartographers.)

First measured in 1831 in the Canadian Arctic, the pole moved slowly northward. By the 1990s, the change accelerated from about 10 miles a year to 30 miles a year. That’s led to a total shift of 600 miles over 150 years. It’s now in the Arctic Ocean and heading for Siberia.

The Earth’s liquid core contributes most towards the planet’s magnetic field. As it churns and flows, the field changes around the world, as well as the “top” that identifies the north pole.