Cientistas prevêem repetição de fenómeno “Biblico”
If the trend continues, the field may collapse altogether and then reverse. Compasses would point south instead of north.
Not surprisingly, Hollywood has already seized on this new twist in the natural-disaster genre. Last year Tinseltown released The Core, a film in which the collapse of Earth’s magnetic field leads to massive electrical storms, blasts of solar radiation, and birds incapable of navigation.
Entertainment value aside, the portrayal wasn’t accurate, according to scientists who say the phenomenon of Earth’s fading magnetic field is no cause to worry.
When molten lava erupts onto the Earth’s crust and hardens, it preserves a snapshot of Earth’s polarity, much in the way that iron filings on a piece of cardboard align themselves to the field of a magnet held beneath it.
According to Earth’s geologic record, our planet’s magnetic field flips, on average, about once every 200,000 years. The time between reversals varies widely, however. The last time Earth’s magnetic field flipped was about 780,000 years ago.
“We hear the magnetic field today looks like it is decreasing and might reverse. What we don’t hear is it is on a time scale of thousands of years,” Glatzmaier said. “It’s nothing we’ll experience in our lifetime.”
But several generations from now, humans just may witness a reversal. By then, Glatzmaier said, scientists will better understand the process and be prepared to cope with the effects.
Scientists believe the magnetic field is generated deep inside the Earth where the heat of the planet’s solid inner core churns a liquid outer core of iron and nickel.
The solid inner core is thought to be a mass of iron about the size of the moon that is heated to several thousand degrees Fahrenheit. Heat radiated by this inner core builds up at its boundary with Earth’s liquid outer core, causing the fluid there to expand.
“When it expands it becomes a little less dense [and more] buoyant. So it starts to rise. That’s convection,” Glatzmaier said. “Hot fluid rises, then cools off and sinks again.”
The convection generates an electric current and, as a result, a magnetic field.
Additional currents are created as Earth cools. Some of the molten iron solidifies onto the inner core, releasing lighter material in the process. The rotation of the Earth also generates forces that curve the flow of fluid as it rises, twisting the magnetic field.
All of these currents constantly replenish the magnetic field, a maintenance process that prevents it from decaying.
Typically each newly generated field lines up in the direction of the existing magnetic field. But every now and again, some force will cause the new field to line up in the opposite direction. This process can lead to a net weakening of Earth’s magnetic field.
Over time a new field can continue to grow. This further weakens the original magnetic field. If the process continues, the two fields would eventually cancel each other out. Earth’s magnetic field would collapse and then, maybe, flip.
“But more likely than not what will happen is the original [field] will get stronger again and overwhelm the instability,” Glatzmaier said.
“The field has reversed many times in the past, and life didn’t stop,” said Gary Glatzmaier, an earth scientist and magnetic field expert at the University of California, Santa Cruz.
In the distant past we know from evidence found in major differences in directional dipole indicators found in world-wide prehistoric lava flows, that the planetary magnetic field has completely reversed its polarity on several occasions over the last 100 million years.
Unfortunately, these have at times coincided with major extinction events that saw the disappearance of many species that had been vulnerable to radiation over-dosages. And such happenings were usually preceded by geomagnetic weakening and polar wandering.
Glatzmaier is keeping an eye on our planet’s weakening magnetic field as he tries to learn more about how Earth’s geodynamo works. The geodynamo is the mechanism that creates our planet’s magnetic field, maintains it, and causes it to reverse.
Earth’s geodynamo creates a magnetic field that shields most of the inhabited parts of our planet from charged particles that come mostly from the sun. The field deflects the speeding particles toward Earth’s Poles.
Without our planet’s magnetic field, Earth would be subjected to more cosmic radiation. The increase could knock out power grids, scramble the communications systems on spacecraft, temporarily widen atmospheric ozone holes, and generate more aurora activity.
A number of Earth’s creatures, including some birds, turtles, and bees, rely on Earth’s magnetic field to navigate. The field is in constant flux, scientists say. But even without it, life on Earth will continue, researchers say.
“There are small fluctuations, which lead to nothing, and large ones, which we know from the geologic record are associated with reversals,” said Peter Olson, a geophysicist at Johns Hopkins University in Baltimore, Maryland.
Flashback: Will Compasses Point South?
William J. Broad
The New York Times
The 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.
NASA – Science News
Earth and space are about to come into contact in a way that’s new to human history. To make preparations, authorities in Washington DC are holding a meeting: The Space Weather Enterprise Forum at the National Press Club on June 8th.
Richard Fisher, head of NASA’s Heliophysics Division, explains what it’s all about:
“The sun is waking up from a deep slumber, and in the next few years we expect to see much higher levels of solar activity. At the same time, our technological society has developed an unprecedented sensitivity to solar storms. The intersection of these two issues is what we’re getting together to discuss.”
The National Academy of Sciences framed the problem two years ago in a landmark report entitled “Severe Space Weather Events – Societal and Economic Impacts.” It noted how people of the 21st-century rely on high-tech systems for the basics of daily life.
Physicists in France have linked subtle variations in the length of day with conditions in the Earth’s core – where the Earth’s magnetic field originates. The finding could improve our poor understanding of how the field is generated and why it changes in response to conditions deep within the Earth’s interior.
Molten iron flowing in the outer core generates the Earth’s geodynamo, leading to a planetary-scale magnetic field. Beyond this, though, geophysicists know very little for certain about the field, such as its strength in the core or why its orientation fluctuates regularly. Researchers do suspect, however, that field variations are strongly linked with changing conditions within the molten core.
As we cannot access the Earth’s core directly, researchers look to clues at the Earth’s surface. One intriguing suggestion is that changing conditions at the core could have an impact on angular momentum throughout the whole Earth system. The implication is that variation to the flow patterns in the core could have an impact on the Earth’s rotation, which could lead to slight variations in the length of a day.