At that rate of decline, the field could vanish altogether in 1,500 to 2,000 years, said Jeremy Bloxham of Harvard University.

Hundreds of years could pass before a flip-flopped field returned to where it was 780,000 years ago. But scientists at a meeting of the American Geophysical Union cautioned that scenario is an unlikely one.

"The chances are it will not," Bloxham said. "Reversals are a rare event."

Instead, the weakening, measured since 1845, could represent little more than an "excursion," or lull, which can last for hundreds of years, said John Tarduno of the University of Rochester.

Such a lull could still have significant effects, especially in regions where the weakening is most pronounced.

Over the southern Atlantic Ocean, a continued weakening of the magnetic field has diminished the shielding effect it has locally in protecting the Earth from the natural radiation that bombards our planet from space, scientists said.

As a result, satellites in low-Earth orbit are left vulnerable to that radiation as they pass over the region, known as the South Atlantic anomaly.

Among the satellites that have fallen prey to the harmful effects was a Danish satellite designed, ironically, to measure the Earth's magnetic field, Bloxham said.

The weakening -- if coupled with a subsequently large influx of radiation in the form of protons streaming from the sun -- can also affect the chemistry of the atmosphere, said Charles Jackman of NASA's Goddard Space Flight Center.

That can lead to significant but temporary losses of atmospheric ozone, he said.