But this technique, dubbed paleomagnetism, has an error margin of 10 percent.
"A 10 percent error in field strength is bigger than the change we've seen in the past 200 or 300 years," Gubbins said.
To more accurately measure older samples of the field's rate of decline, Gubbins and his colleagues used new data from historic sources, including ships' logs.
The ships' logs provide information on declinationthe direction compasses pointand, beginning around 1700, inclinationthe angle a magnetic needle makes relative to the horizon.
(Lesson plan: How to Navigate by Compass.)
Using statistics to combine intensity measurments starting in 1590 with the data from the ships' logs allowed the team to reduce the inaccuracies, Gubbins explained.
What they determined is that the field was relatively stable for about 250 years.
Gary Glatzmaier is an earth scientist at the University of California, Santa Cruz. He models the processes deep inside the Earth that create the magnetic field and lead to reversals over hundreds of thousands of years.
He says the results of the new study confirm his own ideas based on his models that the magnetic-field strength doesn't change at a constant rate but is always erraticincreasing, decreasing, or staying flat for varying lengths of time.
"If [the field's activity] stayed constant, that would be worth noting," he said.
When Gubbins and his colleagues analyzed their data, they found that the most recent decline can be explained by patches of reverse magnetic field that have been growing in and migrating around the Southern Hemisphere since about 1800.
"It does look like the patches first formed toward the end of the 18th century, when Captain Cook was busy sorting out navigation and measuring magnetic field all over the world," Gubbins said.
Captain James Cook was an English explorer famous for his voyages to the Pacific Ocean (Oceania map) and for mapping Australia's east coast, the Hawaiian Islands, Newfoundland, and New Zealand.
Glatzmaier agrees that the patches of reverse magnetic field are responsible for the measured decrease in field intensity over the past two centuries.
He added, however, that the patches are not the cause of the weakening magnetic field but rather a "manifest of what is happening deep below in the core. The field is changing because of the dynamo well below the surface."
The dynamo is the geologic process that creates the magnetic field, maintains it, and causes it to reverse.
Scientists believe the dynamo occurs where heat from the solid inner core churns the liquid outer core of nickel and iron.
Since the last field reversal about 800,000 years ago, the field has tried but failed to reverse somewhere between 10 and 20 times, Gubbins says.
"So the field [intensity] is continually zigzagging all the time," he said.
Studying the most recent change in intensity, he adds, will provide a new window on the physical processes of how the magnetic field reverses.
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