4. Third rock from the Sun - restless Earth

Basic data

Physical properties of the Earth

Mass 5.974 x 1024 kilograms
Mean radius 6.371 x 106 meters
Mean mass density 5.515 kilograms per cubic meter
Rotation period 23 hours 56 minutes 04 seconds = 8.6164 x 104 seconds
Orbital period 1 year = 365.24 days = 3.1557 x 107 seconds
Mean distance from Sun 1.49598 x 1011 meters = 1.000 AU
Age 4.55 x 109 years
Atmosphere 71 percent nitrogen, 28 percent oxygen
Surface pressure 1.013 bars at sea level
Surface temperature 288 to 293 degrees kelvin
Magnetic field strength 0.305 x 10-4 tesla at the equator
Magnetic dipole moment 7.91 x 1015 tesla meters cubed

Journey to the center of the Earth

Looking inside the Earth's hidden interior

Fig. .. 

The internal structure of the Earth can be mapped with the help of earthquake waves. The Greek word for earthquake is seismos, meaning "to quake or tremor". Today, earthquake waves are often called seismic waves, and the study of earthquakes is known as seismology.

The Earth is layered inside like a peach. Its deeper layers are more dense, and they are separated from one another in sharp transitions. There are three major parts: (1) the rocky crust, (2) a mantle of hot, plastic rock, and (3) the dense core. They are the skin, pulp, and pit of the Earth, so to speak. The core has a liquid outer component and a solid inner one.

Fig. .. 

Most earthquakes occur just beneath the Earth's surface, when massive blocks of rock grind, lurch and slide against one another. The reverberations resemble ripples spreading out from a disturbance on the surface of a pond. These waves move in all directions and their arrivals at various places on the Earth can be detected by seismometers. By comparing the arrival times at several seismic observatories, geologists can pinpoint the origin of the waves, the focus, and trace their motions through the Earth.

Fig. .. 

By careful mapping of the patterns of many earthquakes that travel to different depths, seismologists have peeled away the Earth's outer layers and looked at various levels within it. It is similar to using an ultrasonic scanner to map out the shape of an unborn infant in a mother's womb, and somewhat like using Computed Axial Tomography (CAT) scans to derive clear views of the insides of living bodies from the numerous readings of X-rays that cross through the body from different directions.

Two cores

The Earth's core reaches about half way to the surface, implying a volume that is one-eighth that of the entire Earth. If the mass density of the Earth were uniform, the core would have an equal share, one-eighth, of the mass of the Earth, but its actual mass is nearly three times greater. So the core's mass density is very high, and this points to iron as the most likely material. Iron has been identified as the main ingredient of the core because it is the most abundant heavy element in the Sun and in some meteorites, and also because laboratory measurements show that the densities and seismic-wave velocities of the core are more closely matched by iron than any other element.

Examination of earthquake waves has shown that there are two cores, an inner, crystalline solid core and an outer fluid one. The two cores are very different is size. The solid inner core has a radius of about 1.22 million meters, which is slightly smaller than the Moon whose radius is 1.74 million meters. The outer fluid core is about 3.48 million meters in radius, or 55 percent of the Earth's radius.

Fig. .. 

The Earth's inner core is a solid lump of iron suspended at the center of the much larger, fluid outer core, something like a golf ball levitated in the middle of a fish bowl. The outer core is itself curtained behind about 3 million meters of solid rock. So, the solid heart of iron is difficult to observe, but faint seismic vibrations in the ground have been used to look at it.

The seismic vibrations that pierce the inner core move through it at different speeds that depend on their direction, faster on polar north-south paths than equatorial east-west ones. This directional dependence of seismic-wave velocities is explained by the crystalline structure of the inner core. The crystals give the solid inner core a texture with a preferred orientation, like the grain in wood. By lining up along the Earth's spin axis, iron crystals make the inner core stiffer along this axis, thus making sound waves travel faster in this direction.

Recordings of weak earthquake rumbles, that have traveled through the central core of the Earth, indicate that it spins faster than the outer Earth, but that they both rotate in the same direction. The fast lane for seismic waves is tipped slightly with respect to the Earth's north-south axis, and it moves around it. This shift in orientation means that the crystalline globe at the center of the Earth is turning slowly within its solid rocky and liquid metal enclosure. It is spinning with respect to the Earth's surface at between 0.2 and 0.3 degrees per year, completing one lap in between 1,200 and 1,800 years.

Remodeling the Earth's surface

Continents, oceans and ocean floors

Fig. .. 

There are two major types of terrain on Earth - the high, dry continents and the low, wet floor of the ocean. Between them, and partially surrounding many continents, is a narrow strip of shallow ocean called the continental shelf. Today, the oceans cover 71 percent of the Earth's surface, and the world's continents amount only to scattered and isolated masses surrounded by water.

Ongoing erosion will wear down the world's highest mountains in just a few hundred million years, which is just a fraction of the Earth's age of 4.6 billion years. If the planet was a perfectly smooth sphere, the oceans would cover the entire globe to a depth of 2.8 thousand meters. So, we can tell right away that high, dry land must be continuously recreated and pushed up out of the water.

Continental drift

Fig. .. 

The idea that continents have not always been fixed in their present positions was suggested more than three centuries ago, in 1596 by the Dutch map maker Abraham Ortelius (1527-1598) in his work Thesaurus Geographicus. However, the theory of moving continents was not developed into a thorough scientific hypothesis until the early 20th century, by the German meteorologist Alfred Wegener (1880-1930) in his influential and controversial book Die Entstehung der Kontinente und Ozeane, or The Origin of Continents and Oceans. Wegener noticed that the outlines of the continents themselves exhibit a number of remarkable symmetries. For example, the eastern edge of South America would fit snugly into the western edge of Africa, a remarkable fit noticed by Ortelius. In fact, much of the east and west shores of the Atlantic are as well matched as the shores of a river.

Fig. .. 

Wegener based his concept of continental drift not only on the similar shapes of the present continental edges, but also on the striking match of certain rocks and geologic formations, fossil creatures, and ancient climates along the borders of continents on opposite sides of the ocean. He concluded that all of the continents were once a part of single land mass that fragmented and drifted apart. If spacecraft had existed back then, their camera eyes would have seen one large continent and a single ocean surrounding it. This hypothetical super-continent is called Pangaea, a Greek word meaning "all lands" and pronounced pan-gee-ah.

Sea-floor spreading

The bottom of the ocean is not flat. It contains underwater mountains and valleys that are as grand as those on any continent. Although we cannot see these features in the inky darkness of the deep sea, we can use sound waves to reach down and touch them. Their distance is determined by recording the time it takes for electrically-generated sound signals, called pings, to travel from a ship to the floor and back.

Fig. .. 

Nowadays, the United States Navy detects enemy submarines or ships with sonar, an acronym for sound navigation and ranging, transmitting a continuous rain of pulsed sound waves and using the same echo technique to measure distance. Many modern ships, including warships and some commercial fishing boats are also equipped with sonar to aid in navigation. Navy ships and research vessels equipped with sonar can now map a two-thousand meter swath at the bottom of the ocean in a single ping of the sonar. Gravitational data, obtained from satellites that bounce radio beams off the sea surface, complement the sonar data, and they together result in highly detailed maps of the entire ocean floor.

Fig. .. 

The global mid-ocean ridge is a gigantic network of underwater mountain ranges. The submerged mountains stand higher than the greatest peaks on land, and meander for more than 75 million meters, creating the longest mountain chain on Earth. It is long enough to accommodate the total length of the Alps, Andes, Himalayas and Rockies. The mid-ocean ridge winds around the Earth, girdling the globe like the stitched seams of a baseball, not in simple lines but in offset segments, and when the undersea mountains reach the surface they can form islands, like Iceland and its new neighboring island Surtsey, named after the Icelandic god of fire, Surtur.

Even more remarkable are the deep canyons, collectively known as the Great Global Rift, that run along the mid-ocean ridges, splitting them as though they had been sliced with a giant's knife. Hot magma emerges from beneath the sea floor, and oozes into the canyons of the Great Global Rift, filling them with lava. As the lava cools in the ocean water, it expands and pushes the ocean crust away form the ridge. More lava then fills the widening crack, creating new sea floor that moves laterally away from the ridge on both sides, with bilateral symmetry.

As it migrates away from the hot rift of its beginning, the new ocean floor grows colder and denser, subsiding to greater depths as it ages. After traveling across the Earth, in conveyer belt fashion for many millions of years, the older heavier floor bends and descends back into the Earth, often at the edges of continents, creating a deep ocean trench in the underlying rock. Such trenches are found all around the edges of the Pacific Ocean, and they can sink as far below sea level as the tallest mountains rise above it. The over-all concept is known as sea-floor spreading.

Fig. .. 

Perhaps the most decisive evidence for sea-floor spreading was the discovery of regular magnetic-field patterns in the ocean floor. Magnetic detectors towed behind ships and carried in aircraft could measure very small differences in the Earth's magnetic field from place to place, known as magnetic anomalies. Positive magnetic anomalies are places where the magnetic field is stronger than expected, and negative ones are weaker than anticipated. The pattern of magnetic anomalies was symmetrically placed, or mirrored, on each side of the mid-ocean ridge. The symmetric magnetic anomalies on the sea floor exactly match these polar reversals recorded on land.

By radioactive dating of volcanic rocks on land, it is possible to tell when they solidified and to build up a chronology of the magnetic changes. This chronology can then put dates on the reversals found in the sea floor, and from the distances traveled it is possible to compute the rate of sea-floor spreading, assuming that the floor has moved at a constant rate. The ocean floor moves away from the ridge at rates of 0.02 to 0.20 meters per year depending on the location, or just a little faster than your fingernails grow. When sustained for 200 million years, the spreading sea floor can push continents apart by between 4 million and 40 million meters - entirely adequate to explain the widths of the great oceans. At the measured rate, it took just 150 million years for a slight fracture in an ancient former continent to widen into today's Atlantic Ocean.

Plate tectonics

Fig. .. 

The rind of the Earth, its outer shell known as the lithosphere, is subdivided into a mosaic of large plates, million of meters across. They vaguely resemble the cracked pieces of an egg shell.

Six of the nine major plates are named for continents embedded in them: the North American, South American, Eurasian, African, Indo-Australian, and Antarctic Plates. The other three are almost entirely oceanic: the Pacific, Nazca, and Cocos Plates. Accompanying them are a host of smaller plates.

Driven by heat from below, the plates move with respect to one another, accounting for most of our world's familiar surface features and phenomena, such as mountains, earthquakes and ocean basins. The continents are implanted within the moving plates, and continental drift is a consequence of the motion of plates carried along by the sea-floor spreading. So the moving plates carry the continents with them, on an endless journey with nowhere special to go.

The rigid plates are in continual, relentless movement, and they deform at their boundaries. Like drops of olive oil gliding across a warm frying pan, the continents sometimes collide and coalesce, sometimes slide and rub against each other, and at other times break up and scatter. The transformations produced by these interactive motions are known as plate tectonics, from the Greek word tectonic for "carpenter or building". They are forever reconstructing the face of the Earth.

Fig. .. 

Radio interferometer measurements indicate that the Pacific Plate is migrating with a northwestward velocity of 0.048 meters per year, carrying Los Angeles northward and producing earthquakes along the edge of the plate. At this rate, Los Angeles will be a suburb of San Francisco in 10 million years. The interferometer observations also indicate that the Atlantic Ocean is widening by 0.017 meters per year, so it was 8.7 meters narrower when Columbus crossed it in 1492.


An earthquake is a trembling or shaking of the ground caused by a sudden release of energy stored in the rocks below the Earth's surface. The devastating tremors and after shocks can ravage large sections of the land, flattening entire cities, awakening dormant volcanoes and creating new ones, draining lakes and causing floods, avalanches and fires.

In addition to diverging plates, that are moving apart at a mid-ocean ridge, and convergent plates, that are heading toward a collision, there is a third type of plate boundary known as a transform fault. It is a place where plates move past one another, neither toward or away, and this is where earthquakes can occur. When the two plates meet along a transform fault, they "transform" their encounter into a slipping, sliding horizontal motion, and a sudden lurch in this motion can produce an earthquake.

The two plates on each side of a transform fault bump, crush, grind, rub and slide against each other, without creating or destroying crust, like two high-speed cars sideswiping each other, but in slow motion. A famous and visible example is the San Andreas Fault in California that marks the meeting of the Pacific Plate with the North American Plate.

The Earth's internal heat engine

Fig. .. 

What pushes the tectonic plates across the globe? Heat, bottled up deep inside the Earth, produces internal currents that move the plates and propel the drifting continents. This heat is left over from the time of the Earth's formation, within the liquid outer core, and augmented by the continued radioactive decay of elements such as uranium and thorium. As the internal heat tries to escape, it maintains a ceaseless, wheeling, churning and roiling motion, called convection, that turns and rolls over very slowly. Convection occurs when molten rock becomes swollen by heat and rises through the cooler overlying material of lower pressure, like the currents in a pot of thick soup or oatmeal about to boil.

An impermanent face

Moving plates provide the tools for sculpting the Earth's surface and altering its landscape. They have profoundly changed the way we view the world. It's entire surface is continuously shifting about and changing in shape and form. High-standing belts of mountains and volcanoes are continuously being created when two plates converge along their borders, helping to hold the land above the sea.

Fig. .. 

As an ocean plate disappears into the trenches, great chains of towering volcanoes are created along the margins of continents. The descending slab of lithosphere causes underground rock to melt, and the magma generated rises buoyantly to widen the continents at their edges. The Andes are still growing higher in this way, as the floor of the Pacific Ocean plunges beneath the west coast of South America.

Eventually, a moving continent reaches an open trench and jams it shut, like trying to shove an eggplant down a garbage disposal. Continents are too light and thick to be subducted, and when they arrive at a trench the suture is closed up.

Fig. .. 

The violent collisions between continents have created the world's tallest mountains. When the continents meet, they buckle upward to form a range of mountains. Both land and oceanic sediment, built up over many millions of years, are tossed into the sky. The magnificent Himalayan range was formed this way, when the Indo-Australian plate, with India firmly embedded, ran into the Eurasian plate, like a head-on collision of two cars. Slowly, the Himalayas shot up as India rammed into Asia, carrying the fossilized remains of ancient sea-creatures with them. Today the plate that carries India continues to slide beneath the Eurasian plate, widening the Indian Ocean and pushing the mighty Himalayas upward.

The edges of plates are not the only place that land rises above the sea, for some oceanic islands are located millions of meters from the nearest plate boundaries. Chains or strings of such isolated islands are attributed to hot spots. The hot spots are rising plumes of magma, liquid or molten rock, anchored far beneath the ocean and deep within the mantle, even as far down as the core-mantle boundary. The relatively small, long-lasting and exceptionally hot regions provide a persistent source of magma capable of penetrating the mantle and piercing an overriding lithosphere plate, like a fixed blowtorch might melt holes in a steel plate moving by. As the plate glides slowly overhead at the rate of a few meters every century, it can leave a trail of islands that have risen out of the sea.

Fig. ..a  Fig. ..b 

The Hawaiian islands were formed in such a way, as the Pacific Plate moved over a deep, stationary hot spot at the slow rate of 0.13 meters per year. Stretching to the north and west of the big island of Hawaii, they form a string of smaller islands, including Oahu and Midway, and submerged volcanoes, or seamounts, about 6 million meters long. Every one of these islands and seamounts was formed in the exact place where Hawaii now stands. The plume pushed the first Hawaiian island up above the ocean surface in this location about 70 million years ago.

Kiluea, the world's largest active volcano, is still rumbling because Hawaii has yet to move completely off the hot spot. At the same time, the underwater volcano, Loihi, is being formed as the Pacific Plate moves steadily on, continuing its relentless journey over the hot spot. In about fifty thousand years Loihi should grow high enough to form the next Hawaiian island.

Fig. .. 

If a plate carrying a continent comes to rest over a hot spot, the heat and pressure from the upwelling magma will weaken and stretch the overlying continental crust. And when the crust is stretched beyond its limits, cracks or rifts will form in it. The magma rises and squeezes through the widening cracks, forming volcanoes. If the upwelling is short-lived, the result is merely a rift scar, such as the Rhine Valley. If it persists, the rift widens and a continent can literally be split in two. In time, the gap reaches a coastline, permitting sea water to flow in and a new ocean is created.

Fig. ..  Fig. ..b 

Hot spots are now tearing Africa apart at its seams. A Great Rift Valley stretches from Ethiopia to Tanzania; as it widens the continent will break apart and the sea will eventually enter. The African and Arabian Plates have already pulled apart in another location, forming the Red Sea and the Gulf of Aden. They are developing into an ocean that may eventually rival the Atlantic Ocean in size. At the same time, the Mediterranean Sea is narrowing as Africa moves toward Europe.

The Earth's changing atmosphere

Our Sun-layered atmosphere

Our thin atmosphere is pulled close to the Earth by its gravity, and suspended above the ground by molecular motion. The atmosphere near the ground is compacted to its greatest density and pressure by the weight of the overlying air. At greater heights there is less air pushing down from above, so the compression is less and the density and pressure of the air falls off into the near vacuum of space.

Fig. .. 

Not only does the atmospheric pressure decrease as we go upward, the temperature of the air also changes. It decreases steadily with increasing height in the lowest region of our atmosphere, called the troposphere from the Greek tropo for "turning". The temperature falls at higher altitudes because the air expands in the lower pressure and becomes cooler. But the temperature is not a simple fall-off with height. It falls and rises in two full cycles as we move off into space. The temperature increases are produced by the Sun's invisible radiation.

When absorbed in our air, the invisible short-wavelength radiation from the Sun transfers its energy to the atoms and molecules there, causing the temperature to rise. There is, for example, a gradual increase in temperature just above the troposphere, within the next atmospheric layer named the stratosphere. This layer is located between 10 thousand and 50 thousand meters above the Earth's surface. Its name is coined from the words "stratum" and "sphere". The Sun’s invisible ultraviolet radiation is largely absorbed in the stratosphere, where it warms the gas and helps make ozone.

The vanishing ozone

The mesosphere, from the Greek meso for "intermediate", lies just above the stratosphere. The temperature declines rapidly with increasing height in the mesosphere, reaching the lowest levels in the entire atmosphere. The main reason for the decreasing temperatures is the falling ozone concentration and decreased absorption of solar ultraviolet.

The temperature then begins to rise again with altitude in the ionosphere, a permanent, spherical shell of ions and electrons, reaching temperatures that are hotter than the ground. The ionosphere is created and heated by absorbing the extreme ultraviolet and X-ray portions of the Sun’s energy. This radiation tears electrons off the atoms and molecules in the upper atmosphere, thereby creating ions and free electrons that are not attached to atoms.

Fig. .. 

Solar ultraviolet radiation supplies ozone to the stratosphere, at a rate that depends on the varying ultraviolet output of the Sun, and we have recently been punching holes in the ozone layer with chemicals used in our everyday lives. The threat of dangerous ultraviolet rays passing through the damaged ozone layer led to international agreement to ban the use of the destructive chemicals. However, because of their long lifetime and slow diffusion into the stratosphere, the ozone layer is not expected to regain full strength until well into the latter half of the 21st century. Further details are given in Ozone Depletion at the Human Impact part of this web site.

Heating by the greenhouse effect

Visible sunlight passes through our transparent atmosphere to warm the Earth’s land and oceans, and some of this heat is reradiated in infrared form. The longer infrared rays are less energetic than visible ones and do not slice through the atmosphere as easily as visible light. So our atmosphere absorbs some of the infrared heat radiation, and some of the trapped heat is reradiated downward to warm the planet’s surface and the air immediately above it. The warming by heat-trapping gases in the air is now commonly known as the “greenhouse effect”.

Right now, the warming influence is literally a matter of life and death. It keeps the average surface temperature of the planet at 288 degrees kelvin (15 degrees Celsius or 59 degrees Fahrenheit). Without this greenhouse effect, the average surface temperature would be 255 degrees kelvin (-18 degrees Celsius or 0 degrees Fahrenheit); a temperature so low that all water on Earth would freeze, the oceans would turn into ice and life, as we know it, would not exist.

The gases that absorb infrared heat radiation are minor ingredients of our atmosphere. The main ones are water vapor and carbon dioxide, with water vapor the most powerful heat trapping gas of the two.

Humans are pumping increasing amounts of carbon dioxide in the air

For hundreds of years, humans have been filling the sky with carbon dioxide. The invisible waste gas is dumped into the air by burning fossil fuels – coal, oil and natural gas. When these materials are burned, their carbon atoms, denoted C, enter the air and combine with oxygen atoms, O, or oxygen molecules, O2, to make carbon dioxide, abbreviated CO2.

Just a few decades ago, no one knew if any of the carbon dioxide stayed in the atmosphere or if it was all being absorbed in the oceans. Then in 1958 Charles D. Keeling (1928-) began measurements of its abundance in the clean air at the Mauna Loa Observatory in Hawaii. It is located at a remote high-altitude site in the midst of a barren lava field, far from cars and people that produce carbon dioxide and from nearby plants that might absorb it.

The sensitive measurements shown that the amount of carbon dioxide in the atmosphere increases and decreases in an annual cycle. Every spring plants bloom, sucking CO2 out of the air, and every fall CO2 is released back into the air as plants either decay or lose their leaves. The measurements had recorded the breathing of the plants all over the Northern Hemisphere.

Fig. .. 

But more importantly, Keeling’s measurements showed that humans are also changing the composition of the atmosphere. Superimposed on the annual fluctuations, there was a systematic increase over the entire period of observation, continuing nonstop since 1958. Year by year the total measured concentration of carbon dioxide grew, as inexorably as the expansion of the world’s population and human industry.

Since 1958, atmospheric concentrations of CO2 have increased from 315 parts per million (106), abbreviated 315 ppm, to 365 ppm at the turn of the century. Studies of ice deposits in Antarctica indicate that the amount of CO2 has been increasing at an exponential rate ever since the beginning of the industrial revolution in the mid-18th century. Concentrations of the gas averaged 280 ppm just before the industrial era. In the succeeding two and a half centuries, a mere blink in the eye of cosmic time, the atmospheric concentration of carbon dioxide has increased 31 percent.

The atmosphere now contains almost 800 billion tons of carbon dioxide. Humans continue to release about 7 billion tons of it each year. In other words, each person on Earth is, on average, dumping about a ton of carbon dioxide into the air every year, and there is no end in sight. (The world population in January 2002 was 6.202 billion, increasing at the rate of about 6 million people every month.)

Once added to the air, carbon dioxide spreads throughout the entire atmosphere. And it remains in the air for a long time, taking decades and even centuries to disappear. So future generations will have to contend with our present activities.

There are now many signs of a recent rise in global temperature, attributed at least in part to heat-trapping gases deposited in the atmosphere by human activity. Uncertain computer models forecast a wide range of possible consequences of global warming for a century from now, including catastrophic ones. Global warming predictions, natural and human-induced warming, likely consequences of global warming, the heated debates over global warming, and individual and political action to limit the emission of heat-trapping gases are given in Global Warming at the Human Impact part of this web site.

Using past records to separate the warming effects of the Sun and humans

The Sun is the driving force for all climate and weather on the Earth, including terrestrial winds and the seasons. Yet, our Sun is a variable star, and its changing luminosity can affect the Earth’s atmosphere and climate.

To most of us, the Sun looks like a perfect, white-hot globe, smooth and without a blemish. However, detailed scrutiny indicates that our star is not perfectly smooth, just as the texture of a beautiful face increases when viewed close up. Magnetism protrudes to darken the skin of the Sun in Earth-sized spots detected on the visual disk.

Fig. .. 

The magnetized atmosphere in, around and above groups of sunspot is called a solar active region. The sunspots come in pairs of opposite magnetic polarity, or direction, and they are connected by magnetic loops that rise above the visible solar disk. These loops of magnetism contain the hot, million-degree atmosphere of the Sun, which emits intense radiation at X-ray wavelengths. So, active regions shine brightly in X-rays, illuminating the thin magnetic loops that stitch the solar atmosphere together.

The total number of spots on the Sun varies periodically, from a maximum to a minimum and back to a maximum in about 11 years. The number of active regions, with their bipolar spots and the magnetic loops that join them, varies in step with the 11-year sunspot cycle, peaking at the sunspot maximum. The sunspot cycle is therefore also known as the solar cycle of magnetic activity.

Fig. .. 

Because active regions emit intense ultraviolet and X-ray radiation, the Sun is brightest at these wavelengths during the peak of the magnetic activity cycle and dimmest at cycle minimum. At the minimum of the 11-year cycle, the active regions are largely absent and the strength of the ultraviolet and X-ray emission of the Sun is greatly reduced. The ultraviolet emission doubles from activity minimum to activity maximum, while the Sun's X-ray emission increases by a factor of 100.

Our lives depend on the Sun's continued presence and steady output. It illuminates our days, warms our world, and makes life possible. Yet, as reliable as the Sun seems, it is an inconstant companion, with a luminosity that varies in tandem with the Sun's 11-year magnetic activity cycle.

The total amount of the Sun’s life-sustaining energy is called the “solar constant”, perhaps because no variations could be detected in it for a very long time. Until the early 1980s, it was not known if the Sun's visible light was anything but rock-steady because no variations could be reliably detected from the ground. The required measurement precision could not be attained here on Earth because of the changing amount of sunlight absorbed and scattered by our atmosphere.

Fig. .. 

Stable detectors placed aboard satellites above the Earth’s atmosphere have been precisely monitoring the Sun’s total irradiance of the Earth since 1978, providing conclusive evidence for small variations in the solar constant. It is almost always changing, in amounts of up to a few tenths of a percent and on time scales from 1 second to 20 years, and probably longer. This inconstant behavior can be traced to changing magnetic fields in the solar atmosphere.

Fig. .. 

Historical records indicate that the changing Sun has indeed been drastically altering the climate for thousands of years. The Little Ice Age (1400-1800), for example, overlaps the Spörer Minimum (1420-1500) and Maunder Minimum (1645-1715) in solar activity, when sunspots virtually disappeared from the face of the Sun. During the Little Ice Age, alpine glaciers expanded, the river Thames, England and the canals of Venice, Italy, regularly froze over, and painters depicted unusually harsh winters in Europe.

Humans seem to have taken over the climate in more recent times. The Earth is now hotter than it has been any time during the previous 1,000 years, and heat-trapping gases produced by our industrial societies are probably responsible.

Fig. .. 

Ice and fire

During the past one million years the Earth has undergone a series of warm and cold periods. During the cold periods, called ice ages, huge ice sheets build up on the continents and in the polar seas. The growing layer of continental ice flows towards the equator, scouring and covering large areas of land. Then the climate warms and the ice retreats.

Cores extracted from the glacial ice in Greenland and Antarctica have provided a natural archive of the Earth’s past climate over the past 420 thousand years. They strongly support the idea that changes in the Earth’s orbit and spin axis cause variations in the intensity and distribution of sunlight arriving at Earth, which in turn initiate natural climate changes and trigger the ebb and flow of glacial ice.

Fig. .. 

Air trapped in the polar ice cores indicates that the Antarctica air-temperature changes are associated with varying concentrations of atmospheric carbon dioxide and methane. The temperatures go up whenever the levels of carbon dioxide and methane do, and they decrease together as well. Scientists cannot however, yet agree whether the increase in greenhouse gases preceded or followed the rising temperatures.

On the other hand, the current level of greenhouse gases, recently deposited in our atmosphere by humans, far surpasses any natural fluctuation of these substances recorded during past ice ages. So we really don't know for certain what is going to happen to the environment in the next few thousand years.

Fig. .. 

But then, in about 7 billion years from now, the Sun will balloon into a red giant star with a dramatic increase in size and a powerful rise in luminosity. The giant Sun will be 2,300 times brighter than it is now, resulting in a substantial rise in temperatures throughout the solar system. It will become hot enough to melt the Earth’s surface. The frozen moons of the outer planets might then spring to life as the Sun melts their ice into seas of liquid water. The only imaginable escape would then be interplanetary migration to distant moons or planets with a warm, pleasant climate.

Space weather

Fig. ..c  Fig. .. 

There is danger blowing in the Sun's winds. Energetic charged particles and magnetic fields are being hurled into interplanetary space by explosions on the Sun, producing gusts and squalls in the solar wind that can wipe out unprotected astronauts and destroy satellites. Down here on the ground, we are shielded from their direct onslaught by the Earth’s atmosphere and magnetic fields, but out in deep space there is no place to hide.

Powerful explosions on the violent Sun come in two main varieties, known as solar flares and coronal mass ejections, or CMEs for short. Both kinds of solar activity are powered by the Sun's magnetic energy, and they both vary in step with the Sun's 11-year cycle of magnetic activity. Solar flares and CMEs are more frequent and tend to be more powerful during the maximum in the activity cycle.

Solar flares are brief, catastrophic outbursts that flood the solar system with intense radiation and high-speed electrons and protons. In just a few minutes they can release an explosive energy of up to 1025 Joule, equivalent to 20 million 100 megaton terrestrial nuclear bombs, raising the temperature of Earth-sized regions on the Sun to tens of millions of degrees. The other type of solar explosive activity, the CMEs, expand away from the Sun at speeds of hundreds of thousands of meters per second, becoming larger than the Sun and removing up to fifty billion tons, or 5 x 1013 kilograms, of the Sun's atmosphere.

Energetic protons hurled out from intense solar flares are especially hazardous. They endanger any astronaut caught in space without adequate protection. The high-speed solar protons could even kill an unprotected astronaut that ventures into space. The electrically-charged particles follow a narrow, curved path once they leave the Sun, guided by the spiral structure of the interplanetary magnetic field. Solar astronomers therefore keep careful watch over the Sun during space missions, to warn of possible activity occurring at just the right place on the Sun, at one end of a spiral magnetic field line that connects the flaring region to wherever astronauts happen to be. They can then avoid making repairs to their space stations, and curtail any strolls on the Moon or Mars, instead moving inside storm shelters.

Our technological society has become increasingly vulnerable to explosions on the Sun. They emit energetic particles, intense radiation, powerful magnetic fields and strong shocks that can have enormous practical implications when directed toward Earth. The solar emissions can disrupt navigation and communication systems, pose significant hazards to humans in space, destroy Earth-orbiting satellites, and create power surges that can black out entire cities. Recognizing our vulnerability, national centers and defense agencies continuously monitor the Sun from ground and space to forecast threatening activity. An example is the Space Environment Center, abbreviated SEC, of the United States National Oceanic and Atmospheric Administration. It collects and distributes space weather data, using satellites and ground-based telescopes to monitor the Sun and interplanetary space.

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What everyone wants to know is how strong the storm is and when it is going to hit us. Like winter storms on Earth, some of the effects can be predicted days in advance. A CME arrives at the Earth one to four days after leaving the Sun, and solar astronomers can watch solar explosions happen. Solar flares are another matter. As soon as you can see a solar flare on the Sun, its radiation and fastest particles have already reached us, taking just 8 minutes to travel from the Sun to Earth. Dangerous, but less energetic particles, might take an hour to get here. The ultimate goal of space-weather forecasters is to predict when the Sun is about to unleash its pent-up energy, before a solar flare or CME occurs. One promising technique is to watch to see when the magnetism has become twisted into a stressed situation, for it may then be about to explode.

Summary Diagram

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