for National Geographic News
Earth's magnetic field is weakening in staggered steps, a new analysis of centuries-old ships logs suggests.
The finding could help scientists better understand the way Earth's magnetic poles reverse.
The planet's magnetic field flipsnorth becomes south and vice versaon average every 300,000 years. However, the actual time between reversals varies widely.
The field last flipped about 800,000 years ago, according to the geologic record.
Since 1840, when accurate measures of the intensity were first made, the field strength has declined by about 5 percent per century.
If this decline is continuous, the magnetic field could drop to zero and reverse sometime within the next 2,000 years.
But the field might not always be in steady decline, according to a new study appearing in tomorrow's issue of the journal Science. The data show that field strength was relatively stable between 1590 and 1840.
"It now looks as though it happens in steps rather than just one continuous fall," said David Gubbins, an earth scientist at the University of Leeds in the United Kingdom.
Records and Math
The magnetic field protects Earth from cosmic radiation. In its absence, scientists say, Earth would be subjected to more electrical storms that disrupt power grids and satellite communications (sun storm photos).
Humans and other animals would possibly be exposed to additional health hazards.
For time periods before 1840, when more accurate techniques came available, scientists have been able to measure the field's intensity by looking at the orientation of magnetic minerals in rocks and pottery shards.
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