Research Highlight: Heading Due South

The northern lights illuminate the night sky in Hawaii, wary travelers orient their compasses with the South Pole and birds no longer fly south for the winter. These aren’t scenes from the latest sci-fi thriller, but potential real-life consequences of a geomagnetic field reversal.

Earth’s magnetic field has been steadily weakening since 1845, when scientists began tracking it.  Its strength has waxed and waned throughout Earth’s history and sometimes, at its weakest moments, has reversed in polarity.  Reversals happen on average every 250,000 years, and the last full reversal occurred 780,000 years ago, which suggests the next one is overdue.

“The current field protects us from cosmic rays as well as shields our power and communications systems from bombardment by particles in the solar wind,” said Cathy Constable, director of the Earth Sciences section and professor of geophysics at Scripps Institution of Oceanography at UC San Diego.  “A severe decrease in the field could produce effects similar to a very bad magnetic storm and would have an adverse impact on the communication systems we have today.”

Without this protective shield, life on Earth would confront a much harsher environment.  The magnetic field shields the earth, and a network of telecommunications satellites, from a steady stream of particles flying off the sun. A weak field would leave Earth vulnerable to geomagnetic storms that could disrupt electrical grids and telecommunication satellites, widen the ozone holes in the atmosphere and cause more frequent and southerly displays of auroras, including the most famous, the northern lights.

Many animals such as birds, fishes, turtles, and bees also rely on the magnetic field to navigate.  A weak field or change in its direction could interrupt natural migration patterns to spawning and feeding grounds.  

The magnetic field’s mysterious origin and unpredictable reversals are intriguing to scientists, not just to science fiction enthusiasts. Constable and other scientists at Scripps are keeping tabs on the field in order to better understand what drives these fluctuations deep within the earth’s core.

Scripps scientists are in search of geomagnetic clues frozen in time. As lava erupts over the earth’s surface and cools forming rocks, the magnetic moments in particles partially align with the current state of the field. Researchers use this information, along with magnetic traces left behind in sedimentary rocks and ancient ceramics that have been fired, to create a global timeline that can fit the current field changes trend into their historical context.

In 2007, Lisa Tauxe, Scripps professor of geophysics and director of the Scripps Paleomagnetic Laboratory, trekked across Israel and Jordan in search of history. Tauxe and UCSD colleague Thomas Levy were collecting archeological artifacts from ancient copper mines that can be used to examine the magnetic field over the last 6,000 years.

Tauxe and members of her research team analyzed copper-mining slag, a by-product left behind from the melting of copper ore, collected from a variety of archeological sites in the magnetically neutral paleomagnetic lab at Scripps. The lab, which contains a small, windowless room packed with magnetometers and samples from around the world, is lined with special field-canceling wallpaper to prevent the present day magnetic field from influencing the analysis.

The study revealed that the field peaked in intensity about 2,000 to 3,000 years ago and has been weakening ever since.

Tauxe’s deposits were formed from fast-cooling melts used in copper mining operations in the southern Levant region, the modern day border region of Jordan and Israel. The new find has been a boon to geoscientists offering them a valuable new source of material to extract field strength information as well as a way to settle archaeological debates on the age of specific artifacts from this time period.

Lava expelled from volcanoes also holds valuable clues into the magnetic field’s ancient history.  In an effort to look at the ancient field from a global viewpoint, Scripps researchers including Constable, Tauxe, Hubert Staudigel and Catherine Johnson led an ambitious multi-institutional sampling effort that ended last year, which Constable refers to as “The Million Year Study.”

“The goal was to get as many samples of lava flows of a broad time series as globally as possible to map the magnetic field over the last few million years,” said Constable.

The field campaign took Scripps researchers from pole to pole, from McMurdo Sound in Antarctica, Costa Rica, the Azores to Spitsbergen, collecting lava sources that date back several million years.

“It turns out to be a very challenging project because it’s nearly impossible to accurately date lava that is a million years old,” said Constable.

Besides trying to estimate the strength and direction of the magnetic field over the last million years, the study could help researchers identify what drives the magnetic field.

Scientists believe that motions in the earth’s liquid outer core are the source of the magnetic field. The main energy source for the geodynamo, as most earth scientists refer to it, arises from heat released when iron solidifies to make the solid inner core and from buoyancy effects because some lighter material in the liquid outer core doesn’t freeze along with the iron that makes up the inner core.

“It’s a generator that is not made of mechanical parts but of liquid moving around,” explains Constable.

Tectonic plates that make up the earth’s crust are subducted back into the earth and some at least are thought to reach the core-mantle boundary.  The subducted plates are cooler than the surrounding rock at that depth in the mantle at the boundary. Some scientist believe that these temperature variations influence the way material moves around in the hot iron core and plays a major role in the magnetic field changes fluctuations we see at Earth’s surface.

Constable used the information from global field studies by Tauxe, herself and others to create the latest generation of models to explain the field. Current models only calculate the field’s strength and do not account for changes in direction, which according to Constable doesn’t tell the whole story.

Earth’s magnetic north and the geographic North Pole are only rarely lined up perfectly.  This deviation, which results in a slight misdirection for the present field if compass navigators forget to take this into account, would be a major problem during times when the field strength is low as is expected during a reversal.

When Constable’s models are lined up next to the current field model, the downward trend appears much less significant, suggesting the present-day field is still above average compared to the last several million years. Constable developed the latest set of models to provide a better understanding of the past field, but  some researchers are already interested in  more accurate models that they believe may one-day be used to forecast future fluctuations and reversals.

There are major challenges to forecasting reversals, but the model has already contributed to one important historical debate.  A historian has used it to track Christopher Columbus voyages to the New World and back via the Azores, and believes it can help determine the exact island where he first made landfall in his search for the New World.  Scholars plugged his compass position into the magnetic field model and were able to show that most evidence points to Plana Cays in the Southern Bahamas as his landing point on his first voyage.

Scientists acknowledge that, at this point, trying to predict the field’s next move is like trying to predict the stock market’s future.  So for now scientists, like stockbrokers, have to consider the field as an average over time.

“If it continues like this, the field’s intensity will be zero in the next five hundred years,” said Lisa Tauxe, which leads many people to suspect that a full reversal is imminent. The evidence, however, hasn’t quite convinced Tauxe or Constable.

Tauxe is confident enough that a flip won’t happen this time that she has initiated a bet with a colleague—albeit a bet that she acknowledges may take hundreds of years to settle.

According to Constable, “It’s going to flip, it’s just a matter of time.”  But like Tauxe, she doesn’t believe it’s heading toward a full reversal right now. They suggest that this is just a brief fluctuation, geologically speaking, and the field will strengthen again before the next reversal, as has happened repeatedly in the past.

Scientists continue to piece together the magnetic field’s elusive origins and speculate its future direction. Will scientists ever be able to predict a future reversal?

“Never is a very strong word,” said Constable, but she acknowledges there is much to understand in order to reach the point of forecasting a future reversal.

Global models, like Constable’s, are the best hope to fully understand the erratic behavior of the earth’s magnetic field.  Until scientists are able to pinpoint the exact trigger of a full reversal, the field’s next move will remain a mystery.