by Alok Jha
The angle of the Earth’s rotational axis is the main driver of how any energy that reaches our planet is stored and distributed. Known as the “obliquity,” it is measured as the angle between our planet’s axis of rotation and a line perpendicular to its orbital plane around the Sun.
In the early 20th century, the Serbian astronomer and mathematician Milutin Milankovitch proposed a link between the Earth’s climate and its slowly moving position and angle relative to the Sun. He worked out that the variations in the orbital characteristics (including eccentricity, precession and obliquity) taken together affected the amount of sunlight hitting the Earth’s surface and, over millions of years, the rise and fall of ice ages on our planet.
Over the course of around 41,000 years, Milankovitch calculated, obliquity oscillates naturally between 22.1 and 24.5 degrees; every 100,000 years, the eccentricity of the Earth’s orbit ranges from naught to five percent.
The Earth’s rotational axis is tilted with respect to the Sun, which means different parts of the globe are exposed to different amounts of sunlight through the year. This is the cause of the seasons and is a large part of how life on our planet survives.
As the Earth’s obliquity rises, the summers get warmer and the winters get cooler. This is because, as the planet tilts further toward the Sun, it receives more hours of sunlight and the light it gets will be at an angle nearer the vertical, which means that it will heat the surface more efficiently. Conversely, in winter, higher axial tilt reduces the energy hitting the surface.
At present, the Earth is tilted in the middle of this oscillation, at around 23.4 degrees, and its obliquity is decreasing. In comparison, the axial tilt of Venus is almost 180 degrees, because it rotates in the opposite direction to the other planets, with its north pole pointing in the direction we would call “down.” Uranus spins on its side relative to Earth, with an axial tilt of around 97 degrees.
Why does the axis change?
Scientists predict that if everything goes as predicted, the Earth’s obliquity will continue to decrease until it reaches a minimum value of around 22 degrees in around 10,000 years. During this time, summers will become cooler and winters will warm up. The effects can be dramatic.
When obliquity is at its lowest, for example, the higher latitudes of the Earth, and particularly the poles, get much less solar radiation, and conditions become more favorable to the formation of glaciers.
There is good evidence to suggest that shifts in the Earth’s axis and orbit around the Sun have had devastating effects on complex life forms throughout our planet’s history.
In 2006, Jan van Dam of Utrecht University examined the fossilized teeth of more than 100 types of rodents in Spain in an attempt to work out how often species of rats and mice there had risen and fallen in the period from 24.5 million to 2.5 million years ago. He found that species extinction happened in two distinct cycles, every 1 million and 2.5 million years.
These cycles correspond to times when two or more of the cycles calculated by Milankovitch peaked together, leaving the Earth much cooler than normal. “Pulses of turnover occur at minima of the 2.37 million-year eccentricity cycle and nodes of the 1.2-million-year obliquity cycle,” van Dam wrote in Nature. This “astronomical hypothesis” for species turnover provided a crucial “missing piece in the puzzle of mammal species and genus-level evolution.” The hypothesis also offered a “plausible explanation for the characteristic duration of 2.5 million years of the mean species lifespan in mammals, and may explain similar durations in other biological groups as well,” wrote van Dam.
Extreme changes in obliquity can also have dangerous consequences. In a 2003 study published in the International Journal of Astrobiology, Darren M. Williams and David Pollard of Pennsylvania State University took Milankovitch’s ideas further by modeling what would happen to life on Earth if the planet’s axis became tilted further than its natural limits. Life might not be destroyed completely, they concluded, but advanced civilizations such as ours would be in grave danger of being cooked. Any Earth-like planet with obliquity greater than 54 degrees would experience huge changes in climate, with “temperatures reaching 80–100°C over the largest middle- and high-latitude continents around the summer solstice,” they wrote.
* * *
IF OBLIQUITY INCREASES...
At 54 degrees obliquity, temperatures would be 80–100°C
* * *
The high-temperature extremes exhibited in most of their simulations would be problematic for “all but the simplest life forms on Earth today.” “Photosynthetic organisms would be challenged by the long periods of darkness that would affect nearly an entire hemisphere for months. Some of our planets might only be suitable then to a class of organisms known on Earth as extremophiles, which occupy the dark ocean bottom or deep underground and which can withstand temperatures approaching 400°C, provided they are near a source of water. Such organisms would easily withstand the temperature variations of extraordinary amplitude that we have simulated here.”
Some form of life could survive if the Earth’s axis shifted to an extreme obliquity, but it would not be able to live on the continents, where summers would be unbearably hot and water resources would presumably run dry. In this scenario, any human habitation would have to be abandoned.
Could it happen?
Aside from the natural oscillations, could the Earth’s axis be forced to change in other ways? Felix Landerer of NASA’s Jet Propulsion Laboratory has proposed that global warming has slowed our planet’s obliquity, as warming oceans and melting ice sheets shift the amount of physical material sitting on different parts of the Earth’s surface. He estimates that the rush of fresh water into the oceans from the melting Greenland ice sheet, for example, is causing the Earth’s axis to tilt by just over an inch per year.
Using a computer model, Landerer predicted that a doubling of carbon dioxide in the atmosphere by 2100 (which is only the most moderate of projections by scientists) would push more water on to the Earth’s shallower ocean shelves. This redistribution of mass would move the Earth’s northern pole by around half an inch per year in the direction of Alaska.
Earthquakes can also shift the Earth’s axis. The magnitude nine quake off the north-eastern coast of Japan in March 2011 moved the Earth’s axis by around 15 cm (six inches), according to meteorologist Bethan Harris of the University of Reading. It also moved the Japanese land mass by several feet, and this redistribution means that the Earth’s rate of rotation has increased (albeit by a tiny amount).
A more frightening scenario is if our Moon was somehow knocked out of orbit, given that our planet is effectively held upright (at low obliquity) by the strong gravitational influence of our satellite companion. “Earth’s axial tilt is stable with the Moon present for obliquities of less than 60 degrees,” wrote Williams and Pollard in their 2003 paper, which modeled extreme axial tilts on Earth-like planets. ‘Without the Moon, Earth’s obliquity would vary chaotically as a consequence of solar tides between 0 degrees and 90 degrees on timescales of less than 10 million years. This result [suggests] that the Moon is in some sense necessary for the existence of life on Earth because it stabilizes the spin axis at low obliquity and maintains climatic
Lethal Space Dust
* * *
Everything in our quiet corner of the galaxy seems far apart, slow-moving and, well, peaceful. But it is all illusion. Just because there have been no cosmic catastrophes in the mere blink of time that coincides with our lifespan, it hardly means that our galactic neighborhood is eternally safe.
* * *
This might sound strange. Yes there are rocks that hurtle around every so often, and many of them have hit the Earth over the course of its 4-billion-year history, but there is nothing too massive in our vicinity, no supermassive stars or weird black holes. Why the worry? To answer this, you need to think in cosmic time scales.
The Earth moves around the Sun, and the Sun is also moving, around the center of the galaxy.
During that 250-million-year orbit, traveling at 200 km/s (125 miles per second), our solar system passes through all manner of clouds of dust and rock, raising sharply the number of asteroids that bombard the planets. When we are passing through the densest regions of space, billions upon billions of lumps of rock rain down on the Earth and the other planets, for thousands of years at a time.
In the three billion years that there has been life on Earth, this periodic rain of dust and rock seems to have virtually wiped out all our planet’s species several times. In the history of the Earth, cycles of destruction are all too regular.
Regular extinctions, regular impacts
The fossil record shows that around every 30 million years, a large number of species on Earth become extinct. Over roughly the same period, the cycle of impacts from cosmic objects also seems to rise and fall. More asteroids, comets and other debris, which normally sit at a safe distance from the Earth, seem to become attracted to it during these periods. Among all the many inconsequential bits and pieces of dust and ice, there can be something massive enough to cause global problems—boulders a mile across that can cause tsunamis as they hit our surface and block out the Sun by throwing dust into the atmosphere. This cycle of devastation can be traced back for more than 250 million years, and includes the end of the Cretaceous period 65 million years ago, when the dinosaurs became extinct.
* * *
When we are passing through the densest regions of space, billions upon billions of lumps of rock rain down on the Earth and the other planets, for thousands of years at a time.
* * *
What causes this periodic bombardment? When scientists began examining the clues—including factoring in where the asteroids might be coming from and the reasons why they have appeared to be on collision courses with Earth so often throughout history—they came to a startling conclusion. The periodic extinctions were happening, it seemed, because the entire solar system was moving up and down through the plane of the Milky Way at the rate of one cycle every 30 million to 35 million years. During this process, whenever the Earth crossed the densest part of the galactic disc, it got in the way of lots of cosmic objects.
“The approximate 30 [million year] periodicity that appears to exist in the record of mass biological extinctions and terrestrial impact cratering has been interpreted [...] as evidence of a quasi-periodic bombardment of the Earth by comets that have been perturbed into Earth-crossing orbits by close gravitational encounters of the Solar System with massive interstellar clouds of gas and dust,” wrote Richard Stothers of the NASA Goddard Space Flight Center in a Nature paper in 1984. “In this model, the underlying clockwork is the Solar System’s vertical oscillation through the midplane of the galaxy, toward which most of the massive interstellar clouds are concentrated.”
This vertical oscillation brings the solar system into the path of danger, and also disturbs the huge cloud of dust and rock, called the Oort cloud, that peacefully envelops our solar system. This only contributes further to the catastrophic hail of rocks that smashes into the Earth every 30 million years or so.
The Oort cloud
Most of the asteroids and comets that come near the Earth today originate either in the belt of asteroids that orbits the Sun between Mars and Jupiter, or else in the region of space just beyond Pluto called the Kuiper belt. To most intents and purposes, this is the edge of the solar system, the furthest we have detected objects and known that they were influenced by the gravity of the Sun. But this is not the end of the story.
It might be common to think of the solar system as ending at the orbit of the most distant known planetary objects, such as Neptune and Pluto, but the Sun’s gravitational influence extends more than 3,000 times further, halfway to the nearest stars. “And that space is not empty—it is filled with a giant reservoir of comets, leftover material from the formation of the solar system,” says Paul Weissman, an astronomer who specializes in the dynamics of comets at NASA’s Jet Propulsion Laboratory in California. “That reservoir is called the Oort cloud.”
Weissman refers to the enormous, spherical Oort cloud as the Siberia of the solar system, a “vast, cold frontier filled with exiles of the Sun’s inner empire and only barely under the sway of the central authority. Typical noontime temperatures are a frigid 4°C above absolute zero, and neighboring comets are typically tens of millions of kilometers apart. The Sun, while still the brightest star in the sky, is only about as bright as Venus in the evening sky on Earth.”
The cloud was named after the Dutch astronomer Jan Oort, and its presence has been inferred, rather than directly observed, from its physical effects—a steady trickle of comets with very long periods (in other words they take a long time to complete one orbit around the Sun) that get into the inner solar system.
Our Sun’s gravitational influence spreads 3,000 times further than the orbits of Neptune and Pluto, causing rocks, ice and debris to be weakly attracted to our star. This enormous cloud of sparsely filled space is known as the Oort cloud.
In 1950, Oort showed that the comets in this vast cloud were so weakly bound to the Sun by gravity that a random passing star could easily change their orbits. Around a dozen stars pass within one parsec (just over 3.2 light years) of the Sun every million years or so, and this is enough to stir some of the comets into action. Oort described the cloud as a “garden, gently raked by stellar perturbations.”
Sometimes a star can travel right through the Oort cloud, which causes a violent shake-up of the comets there. Statistically a star is likely to pass within 10,000 astronomical units of the Sun every 36 million years, and within 3,000 astronomical units every 400 million years (an astronomical unit is the distance between the Earth and the Sun). These close encounters do not have any direct effects on the planets of the solar system, but their indirect effects via the comets can be devastating.
A study in 1981 showed that a close passage of a star could send a rain of comets toward the planets, raising the bombardment rates enough to cause mass extinctions on Earth. A few years later, Weissman calculated that the frequency of comets coming into the inner solar system during such an event could reach 300 times the normal rate and last up to 3 million years.
Kenneth Farley, a geochemist at the California Institute of Technology, found observational evidence for Weissman’s argument when, in 1998, he examined how much interplanetary dust had been collecting in the Earth’s ocean sediments throughout prehistory, a reflection of the number of comets passing by the planets. He found that the influx of comets spiked at the end of the Eocene epoch, around 36 million years ago, a time associated with a moderate biological extinction event. The influx decreased over the following 2 to 3 million years, just as the theoretical models predicted.
Another theory is that the Oort cloud is affected whenever the solar system passes through the galaxy’s spiral arms, which contain vast clouds of molecules and massive blue stars. These will also have a gravitational effect on the cloud of comets at the edge of the solar system. These birthplaces of stars and planetary systems can be between 100,000 and a million times more massive than the Sun. Getting close to such a big mass of material would rip comets out of the Oort cloud and throw them in all directions.
Are we at risk?
In an article for Scientific American in 1998, Weissman pondered whether the Earth was in any present danger from a shower of comets that had been dislodged from the Oort cloud by stars or molecular clouds. “Fortunately not,” he concluded. Using positions and velocities measured by satellites, he reconstructed the paths and motions of stars near the solar system. He found evidence that a star has passed close to the Sun in the past million years but that the next close passage will occur in 1.4 million years, a small red dwarf called Gliese 710, which will pass through the outer Oort cloud about 70,000 astronomical units from the Sun.
“At that distance, Gliese 710 might increase the frequency of comet passages through the inner solar system by 50%,” wrote Weissman. “A sprinkle
perhaps, but certainly no shower.”
And what about the massive molecular clouds that the solar system might encounter on its journey around the galaxy? These encounters, though violent, are infrequent, according to Weissman’s analysis, occurring only once every 300 million to 500 million years. Over the entire history of the solar system, molecular clouds have had about the same cumulative effect as all passing stars.
But it is worth thinking again about the length of time the Earth and, hopefully, humans will be around. Over cosmic timescales, even the most improbable things are bound to happen.
* * *
A close passage of a star could send a rain of comets toward the planets, raising the bombardment rates enough to cause mass extinctions on Earth.
* * *
Runaway Black Hole
* * *
Black holes have the ability to induce epic fear. We know they have unimaginable levels of power; we know they can tear apart stars and dust clouds billions of times bigger than the Earth or the Sun. If one happened to get close to our solar system, it would make short work of us before moving on.