Will Compasses Point South?
By WILLIAM J. BROAD
Published: July 13, 2004The 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.
''We want to get some idea of how this would evolve in the near future, just like people trying to predict the weather,'' said Dr. Gauthier Hulot, a French geophysicist working on the satellite plan. ''I'm personally quite convinced we should be able to work out the first predictions by the end of the mission.''
The discipline is one of a number -- like high-energy physics and aspects of space science -- where Europeans have recently come from behind to seize the initiative, dismaying some American experts.
No matter what the new findings, the public has no reason to panic, scientists say. Even if a flip is imminent, it might take 2,000 years to mature. The last one took place 780,000 years ago, when Homo erectus was still learning how to make stone tools.
Some experts suggest a reversal is overdue. ''The fact that it's dropping so rapidly gives you pause,'' said Dr. John A. Tarduno, a professor of geophysics at the University of Rochester. ''It looks like things we see in computer models of a reversal.''
In an interview, Dr. Tarduno put the odds of an impending flip at more likely than not, adding that some of his colleagues were placing informal bets on the possibility but realized they would probably be long gone by the time the picture clarified.
Deep inside the Earth, the magnetic field arises as the fluid core oozes with hot currents of molten iron and this mechanical energy gets converted into electromagnetism. It is known as the geodynamo. In a car's generator, the same principle turns mechanical energy into electricity.
No one knows precisely why the field periodically reverses, but scientists say the responsibility probably lies with changes in the turbulent flows of molten iron, which they envision as similar to the churning gases that make up the clouds of Jupiter.
In theory, a reversal could have major effects because over the ages many aspects of nature and society have come to rely on the field's steadiness.
When baby loggerhead turtles embark on an 8,000-mile trek around the Atlantic, they use invisible magnetic clues to check their bearings. So do salmon and whales, honeybees and homing pigeons, frogs and Zambian mole rats, scientists have found.
On a planetary scale, the magnetic field helps shield the Earth from solar winds and storms of deadly particles. Its so-called magnetosphere extends out 37,000 miles from Earth's sunlit side and much farther behind the planet, forming a cometlike tail.
Among other things, the field's collapse, scientists say, could let in bursts of radiation, causing a variety of disruptions.
Dr. Charles H. Jackman, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., has worked with European colleagues on a computer model that mimics the repercussions. A weak field, they reported in December, could let solar storms pummel the atmosphere with enough radiation to destroy significant amounts of the ozone that protects the Earth from harmful ultraviolet light.
Ultraviolet radiation, the short, invisible rays from the sun, can harm some life forms, depress crop yields and raise cancer rates, causing skin cancer and cataracts in humans. Dr. Jackman said that the ozone damage from any one solar storm could heal naturally in two to three years but that the protective layer would stay vulnerable to new bursts of radiation as long as the Earth's magnetic field remained weak.
''It would be significant'' in terms of planetary repercussions, he said in an interview, ''but not catastrophic.'' High levels of ultraviolet radiation would spread down from polar regions as far south as Florida.
Like many of the Earth's invisible rhythms, the field reversals are typically slow, taking anywhere from 5,000 to 7,000 years to complete.
Strong evidence of their reality first emerged in the 1950's and 1960's when scientists towing magnetic sensors behind ships found that the rocky seabed exhibited odd stripes of magnetization.
It turned out that continuous flows of seabed lava became alternately magnetized over the ages as the polarities of the Earth's field switched one way, then the other. The seabed acted like a huge tape recorder, and the same proved true of the layered deposits of old volcanoes on land.
How did the rocky memories form? Molten lava proved to hold tiny mineral grains that acted like innumerable compasses, or miniature magnets, freely aligning themselves with the contemporary field. But as the lava cooled, the tiny compasses froze in place, immobile even if the field shifted. Experts called it paleomagnetism and found that the tiny compasses were often made of magnetite, a naturally magnetic mineral.
Paleomagnetic studies showed that the Earth's field reversed every half million years or so, but in a fairly random way and with early patterns more chaotic. During the age of dinosaurs, for instance, no flips occurred for roughly 35 million years.
As scientists began to understand the importance of reversals in the planet's history, they examined the fossil record for evidence of damage to life. In 1971, Dr. James D. Hays of the Lamont-Doherty Earth Observatory of Columbia University noted a strong correlation between recent flips and species extinctions of tiny marine creatures known as radiolarians. ''The evidence,'' he wrote in The Bulletin of the Geological Society of America, ''is strongly suggestive that magnetic reversals either directly or indirectly exert a selective force.''
But no consensus ever formed on how the flips might have doomed some creatures and spared others, and some experts faulted the correlations as statistically insignificant.
Meanwhile, starting in the late 1970's, scientists began to find wide evidence that many animals relied on the Earth's magnetic field for navigation. Dr. Joseph L. Kirschvink of the California Institute of Technology discovered such reliance in bees, pigeons, bacteria, salmon, whales and newts, among other animals. The magnetic sense, he found, usually relies on tiny crystals of magnetite -- the same mineral that gets immobilized in cooling lava.
Investigators looking into the origin of the reversals got new clues in 1995 when scientists at the Los Alamos National Laboratory and the University of California at Los Angeles succeeded in making the first computer simulation of the geodynamo in action, including field reversals.
Dr. Gary A. Glatzmaier, who was one of the Los Alamos scientists, said it showed that the Earth's solid inner core resisted the flipping because the field there could not change as rapidly as it did in the fluid outer core. ''The reversal starts with a small region that gets larger,'' he said in an interview. ''Most of the time they die away, but other times they continue to grow.'' To date, the simulations of millions of years have produced more than a dozen flips.
The current collapse drew wide scientific attention on April 11, 2002, when Nature, the British journal, published a major paper that detailed its growing weakness. Dr. Hulot and colleagues at the Institut de Physique du Globe de Paris, where he works, as well as the Danish Space Research Institute, called the large drop remarkable.
They found it by comparing readings made in 1979 and 1980 by the American Magsat satellite with measurements by the Danish Oersted satellite, launched in 1999 and still operating. In particular, Dr. Hulot and his team discovered a north polar region and a spot below South Africa where the magnetism is growing extremely weak.
The finding drew wide attention because the magnetic anomalies seemed consistent with what the computer simulations identified as the possible beginnings of a flip.
''We postulate,'' Dr. Hulot and his co-authors wrote, that the new evidence reflects how ''the geodynamo operates before reversing.''
In an interview, he said that the field's southern spot was 30 percent weaker than elsewhere and that some satellites passing over it had already suffered electronic malfunctions when highly charged particles from the sun were able to penetrate the weakened magnetic shield.
In March 2003, ''The Core,'' a Hollywood film, gave a wildly exaggerated portrayal of what would happen if the field vanished. People with pacemakers fall dead. Pigeons fly into people and windows. And the planet, a scientist warns, will fry in a year.
Dr. Tarduno said that practical effects on things like satellites and the ozone layer would be the same no matter whether the field reversed or simply weakened and bounced back. A major collapse of the Earth's magnetic shield, he added, could let speeding particles penetrate deeper into the atmosphere to widely knock out power grids, as solar storms do occasionally.
The consensus among biologists seems to be that the reversals are slow enough, and the Earth's creatures resilient enough, that most would learn to adapt. They note the lack of correlations in the fossil record between flips and mass extinctions.
Dr. Kenneth J. Lohmann, a biologist at the University of North Carolina who has pioneered magnetic navigation studies in loggerhead turtles, said if the field became weak enough ''there would be problems for the turtles.'' His research suggests they use it not only for a general sense of direction but as a precise map of their location.
To better understand the current collapse, the European Space Agency plans to launch three satellites in 2009. The spacecraft, flying in polar orbits a few hundred miles up, are to map its intricacies until perhaps 2015.
Dr. Hulot said scientists would combine the satellite data with computer simulations to make not only distant forecasts but possible warnings of current hazards. Among the possible solutions would be to increase satellite shielding.
''It will be interesting to see what's going to happen in that South Atlantic anomaly,'' he said. ''If you want to keep satellites flying, you want to know if the situation is going to deteriorate.''
