The collapse of the Earth’s magnetic field, which both guards the planet and guides many of its creatures, appears to have started in earnest about 150 years ago. The field’s strength has waned 10 to 15 percent, and the deterioration has accelerated of late, increasing debate over whether it portends a reversal of the lines of magnetic force that normally envelop the Earth.

During a reversal, the main field weakens, almost vanishes, then reappears with opposite polarity. Afterward, compass needles that normally point north would point south, and during the thousands of years of transition, much in the heavens and Earth would go askew.

A reversal could knock out power grids, hurt astronauts and satellites, widen atmospheric ozone holes, send polar auroras flashing to the equator and confuse birds, fish and migratory animals that rely on the steadiness of the magnetic field as a navigation aid. But experts said the repercussions would fall short of catastrophic, despite a few proclamations of doom and sketchy evidence of past links between field reversals and species extinctions.

Although a total flip may be hundreds or thousands of years away, the rapid decline in magnetic strength is already damaging satellites.

Last month, the European Space Agency approved the world’s largest effort at tracking the field’s shifts. A trio of new satellites, called Swarm, are to monitor the collapsing field with far greater precision than before and help scientists forecast its prospective state.

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