by Alok Jha
Scientists reckon that the amount of H2S entering the atmosphere at the end of the Permian was 2,000 times more than that given off by volcanoes today, killing plants and animals. “Around the time of multiple mass extinctions, major volcanic events are known to have extruded thousands of square kilometers of lava onto the land or the seafloor. A by-product of this tremendous volcanic outpouring would have been enormous volumes of carbon dioxide and methane entering the atmosphere, which would have caused rapid global warming. During the latest Permian and Triassic as well as in the early Jurassic, middle Cretaceous and late Paleocene, among other periods, the carbon-isotope record confirms that CO2 concentrations skyrocketed immediately before the start of the extinctions and then stayed high for hundreds of thousands to a few million years,” wrote Ward.
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If the base of the food chain is destroyed, it is not long until the organisms higher up are in desperate straits as well.
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As if that was not bad enough, rising ocean temperatures meant that it was harder for oxygen to dissolve. This would mean there was even less of the gas in the water, ratcheting up the amount of H2S. According to Ward, “Oxygen-breathing ocean life would have been hit first and hardest, whereas the photosynthetic green and purple H2S-consuming bacteria would have been able to thrive at the surface of the anoxic ocean. As the H2S gas choked creatures on land and eroded the planet’s protective shield, virtually no form of life on the earth was safe.”
What are the possible sources of anoxia today?
There are already hundreds of “dead zones” in waters around the world, areas where H2S is battling with oxygen. Most notable are those off the east coast of the US, including some around Chesapeake Bay, the southern coasts of Japan and China, the northern Adriatic and the Scandinavian strait of Kattegat. A 2008 study, published in Science, counted 405 dead zones around the world, with the largest being in the Baltic Sea, where oxygen does not reach the bottom of the water for most of the year.
These zones are blamed on runoff of fertilizer from the land. This contains large amounts of nitrogen, which contributes to the blooms of algae in the water. When the algae die, they sink to the bottom of the ocean and are broken down by microbes that consume oxygen in the process. More algae leads to less oxygen in the water, killing other animals and plants, including fish and clams. Once the coast is clear and the situation is anoxic, that’s when the oxygen-hating microbes move in and pump up levels of H2S.
If anoxic conditions have led to mass extinctions before, could the same happen again?
“Although estimates of the rates at which carbon dioxide entered the atmosphere during each of the ancient extinctions are still uncertain, the ultimate levels at which the mass deaths took place are known,” wrote Ward. The so-called “thermal extinction” at the end of the Paleocene began when atmospheric CO2 was around 1,000 parts per million (ppm). At the end of the Triassic, it was just above 1,000 ppm.
Today CO2 is around 390 ppm, so we might seem a long way away from any catastrophe. But our levels of fossil-fuel consumption are still putting around 2 ppm into the atmosphere every year, and this rate is expected to accelerate to 3 ppm as more of the developing world burns oil and coal to power its development.
And it is not just CO2 that we need to worry about—after a certain amount of warming, huge frozen blocks of methane (some 10,000 gigatonnes of which are sitting on the sea floor) will start to melt and escape into the atmosphere. Methane is 25 times more potent as a greenhouse gas than CO2, and will accelerate the warming experienced by the Earth.
That means that by the middle or end of the next century, amounts of CO2 and other greenhouse gases could be approaching the levels required to warm the Earth to the same extent as during previous mass extinctions. The conditions that bring about the beginnings of ocean anoxia may then be in place. “How soon after that could there be a new greenhouse extinction?” asks Ward. “That is something our society should never find out.”
Geomagnetic Reversal
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Compasses have guided people for centuries, helping ships across oceans and travelers across deserts. A tiny sliver of metal that aligns itself to the Earth’s magnetic field—it’s so simple, so reliable. Well, it is reliable as long as the Earth’s magnetic field stays put.
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If the Earth’s magnetic poles started wandering around, compasses would become useless. If the poles did the unthinkable, and somehow shifted wholesale to the opposite ends of the planet, things would get very confusing indeed.
It is a good job that this does not happen. Well, not too often, anyway. Every few hundred thousand years, the Earth’s magnetic poles do indeed start to move around, and eventually switch positions. And getting lost during a reversal is not the only thing you would need to worry about.
The Earth’s magnetic field is much more than a way to orient life forms on its surface (though it is very useful for that purpose). It also projects out into space and provides a shield against harmful particles and radiation streaming in from our Sun. If this stuff were to reach the Earth’s surface in any great amounts, it would tear life apart. High-energy radiation can rip through DNA and cause irreparable damage to delicate biological cells. Never mind the havoc that this radiation would cause to the world’s electrical systems by overloading them and shutting them down, perhaps permanently.
If the Earth’s magnetic field was lost for long enough during a reversal, it could be a disaster.
How does the Earth get its poles?
Our planet is a bit like a plum, made of several soft layers of different thicknesses and densities surrounding a small, solid core. The land and oceans sit on the skin of the plum, a relatively thin solid layer of the Earth called the crust. Directly underneath the crust is a vast layer of molten rock, the mantle, which stretches to a depth of almost 3,200 km (2,000 miles). Below that is a region of iron-rich liquid, the outer core, which is constantly in motion around a hotter, solid core made from iron and nickel. The liquid and solid cores are the source of the Earth’s magnetic field.
The inner and outer core stay hot (at around 4,000 and 6,000°C respectively) partly due to the energy left over from the formation of the Earth billions of years ago, but mostly because of the energy released by decaying radioactive elements that are present in this broiling mass of material. The outer core keeps moving, at speeds of tens of kilometers per year, and as this metal fluid passes across existing magnetic field lines, electrical currents are induced within it. These in turn generate more magnetic field.
The Earth’s magnetic field is often characterized as if there was a huge bar magnet buried inside the planet, with a north and south pole at roughly the corresponding points on the globe and field lines emanating from both sides in the characteristic semicircular pattern familiar from school experiments. Though this is a useful shorthand, the real thing is far more complex and variable, with different intensities and directions at different points on the planet’s surface. And it varies over time.
Using mathematical models of the Earth’s magnetic field from the past few centuries, geologists have been able to track the pattern of subtle differences in different parts of the world. According to the British Geological Survey (BGS), regions of reversed magnetic flux at the core–mantle boundary have grown over time. “In these regions the compass points in the opposite direction, in or out of the core, compared to that of surrounding areas. It is the growth in area of such a reversed flux patch under the south Atlantic that is primarily responsible for the decay in the main dipolar field. This reverse patch is also responsible for the minimum in field strength called the South Atlantic Anomaly, centered over north-east Brazil. In this region energetic particles can approach Earth more closely, causing increased radiation risk to low Earth orbit satellites.”
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If this stuff were to reach the Earth’s surface in any great amounts, it would tear life apart.
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Why do the magnetic poles move?
Every so often, the interplay between the motion of the liquid outer core and the solid inner core in the Earth forces the locations of the poles to move, though no one knows what the exact trigger might be. A magnetic reversal happens when the north pole is transformed into a south pole, and vice versa, an event that last happened almost 780,000 years ago.
There are no complete records of the history of any reversal, so scientists’ calculations about how fast a reversal happens are based on evidence in rocks containing imprints of ancient magnetic fields as they were formed, in addition to mathematical simulations. The consensus among geologists seems to be that a full reversal can typically take several thousand years to complete, which is lightningfast by geological standards—though there is some evidence that they happen even faster than that. A study on 15-million-year-old rocks from Nevada, carried out by geologist Scott Bogue of Occidental College, found evidence that showed possible geomagnetic reversal in just four years.
The Earth’s magnetic field is a result of the interaction between the solid inner core and the liquid iron core. As these move around relative to one another, the metals move across existing lines of magnetic field, which generate electrical currents. These, in turn, generate more magnetic field.
During a reversal, the geometry of the magnetic field would be much more complex than it is now, and a compass might point in almost any direction, depending on location on the Earth and how the field happened to be changing. “One of the things that is interesting about reversals is that there is no apparent periodicity to their occurrence,” says the US Geological Survey (USGS). “Reversals are random events. They can happen as often as every 10,000 years or so, and as infrequently as every 50 million years or more.”
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EARTH’S MAGNETIC FIELD
Has reduced by 10–15%
over the last
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What would happen to life?
In the mid 1980s, scientists began noting coincidences in the timing of geomagnetic reversals and mass extinctions.
“There has been a flurry of papers in the past two years on reversals of the Earth’s magnetic field and their possible connection with extraterrestrial catastrophic events,” wrote J.A. Jacobs, an Earth scientist at Cambridge University, in a 1986 edition of Nature. “What has sparked this sudden interest is the reported periodicity of approximately 30 million years in the frequency of reversals, comet/asteroid impacts on the Earth and mass extinctions.”
A year earlier, a University of Chicago geoscientist, David Raup, had examined this seeming set of coincidences, arguing that a definitive link between extinction and magnetic reversal was hard to make. There was probably a connection, he said, between mass extinction and impacts from asteroids, and possibly a tenuous one between asteroid impacts and geomagnetic reversals. “It is thus not impossible that at least some reversals are caused by comet or asteroid impact and it is in this context that the relationship between periodic intensification of reversal activity and periodic extinction becomes important,” he wrote in Nature. “The discrepancies in analytical results mean that no unequivocal conclusion can be drawn; however there is a possibility that biological extinction, magnetic reversal and large-body impact are linked.”
So much for past extinctions, but would a reversal have negative effects on our modern society? Looking at it from first principles, it is safe to assume that if the magnetic field did disappear, the Earth’s surface would be bathed in harmful radiation, damaging us and our electronic equipment. But how bad would it be?
The USGS is not convinced that it would be so severe. “Reversals happen rather frequently, every million years or so, compared to the occurrence of mass extinctions, every hundred million years or so. In other words, many reversals and, in fact, most reversals, appear to be of no consequence for extinctions.”
The pattern of field lines emanating from the Earth, which buffets against the wind of particles coming from the Sun, is known as the magnetosphere. This does protect us from fast-moving charged particles streaming from the Sun, says the USGS, but so does the atmosphere. “It is not clear whether or not the radiation that would make it to the Earth’s surface during a polarity transition, when the magnetic field is relatively weak, is sufficient to affect evolution, either directly or indirectly, and cause extinctions, such as that of the dinosaurs. But it seems that the radiation is probably insufficient.”
The BGS says that even if the magnetosphere failed, the Earth’s atmosphere shields us from high-energy radiation as effectively as a “concrete layer some 13 feet thick.”
Would the loss of a magnetic field affect other animals? “Some animals, such as pigeons and whales, may use the Earth’s magnetic field for direction finding,” says the BGS. “Assuming that a reversal takes a number of thousand years, that is, over many generations of each species, each animal may well adapt to the changing magnetic environment, or develop different methods of navigation.”
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Reversals are random events. They can happen as often as every 10,000 years or so, and as infrequently as every 50 million years or more.
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Which doesn’t sound too bad at all. Add in the evidence that our non-human ancestors would have been around during the last big geomagnetic reversal and that they don’t appear to have suffered as a result, and it seems as though we should be safe from any major harm during a geomagnetic reversal.
It is worth adding a footnote about some things that our pre-human ancestors would not have had to worry about: they might not have suffered too many physical effects from previous geomagnetic reversals, but who knows how their society or lifestyle might have been affected if they were as dependent as we are on iPhones, electrical grids and satellites, all of which would be damaged irreparably by cosmic and solar radiation if the Earth’s magnetic field disappeared temporarily during a reversal. That, though, is a story for another chapter.
Superstorms
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You may have experienced big storms before, but nothing could prepare you for this. The winds, blowing at almost 1,100 km/h, flatten everything in their path. Cars, lorries and trains fly into the air, entire forests are ripped from the ground and streets of hurricane-proof buildings are torn from their foundations.
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The extent of the storm is unimaginable, covering an area equivalent to the entire continental US. As boulders are thrown around, coastal areas face tsunamis. When this is over, entire cities across many countries will be razed to the ground.
And that’s when the devastation goes global. Because all the while the winds were tearing chunks out of the ground, they were also busy destroying the Earth’s ozone layer. In time, with no protection from the harmful rays streaming from our Sun, the Earth will be sterilized of life. This is our world after a hypercane.
How big can a storm be?
A hurricane—or typhoon or cyclone, depending on where you are in the world—is the Earth’s way of dumping excess energy from the sea and distributing it around the world and back into space. They occur every year in the warm oceans of the world, including the Atlantic, Caribbean, Indian and western Pacific. In the North Atlantic, for example, the season for hurricanes starts in June and runs until the end of November, usually toward the end of that period, after the seas have been warmed by the Sun for many months.
In its normal state, the air above an ocean is not in thermal equilibrium with the underlying water. This is useful because it allows water to evaporate into the air and carry away some of the energy coming direct from the Sun. Small storms form all the time above tropical waters, thanks to the energy raining down from the Sun.
Hurricanes occur when the levels of energy get higher. If a few thunderstorms, for example, start to rotate around an area of low pressure above the sea, they reinforce each other and the system grows stronger. If the winds rise above 119 km/h (74 mph), the storm system can be called a hurricane
. Warm, moist air from the surface of the sea will feed power to the hurricane, and as long as it is above water, the system can continue to grow. Warm air at the center of the storm moves up and away from the surface, reinforcing the low pressure there and causing even more air to rush in from surrounding high-pressure areas.
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HURRICANES
Category 1: 153 km/h 1.5 m high
Category 5: over 250 km/h over 5.5 m high
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A category one hurricane has wind speeds between 119 and 153 km/h (74 and 95 mph) and damage at landfall is minimal. Storm surges as a result of the weather system can reach a few meters, causing flooding if it reaches land. Category five hurricanes, in contrast, can be devastating if they make landfall, with wind speeds above 250 km/h (155 mph) and storm surges greater than seven meters.
Typical hurricanes can last a week and move at up to 32 km/h (20 mph) across an ocean. Once it hits land, a storm system will tend to slow down as its source of energy drops and there is more friction. Hurricane Katrina, the most destructive storm to hit the US, made landfall as a category three storm, though it reached a higher category when it was still above the warm waters of the Gulf of Mexico.
Thanks to climate change, the number and intensity of hurricanes is increasing. In the past century, the surface temperature of the Atlantic has risen by 0.7°C, and a 2005 study published in Science showed that the number of category four or five hurricanes around the world had almost doubled over 35 years.