Unmoved mover

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Unmoved_mover

The unmoved mover (Ancient Greek: ὃ οὐ κινούμενον κινεῖ, romanized: ho ou kinoúmenon kineî, lit. 'that which moves without being moved') or prime mover (Latin: primum movens) is a concept advanced by Aristotle as a primary cause (or first uncaused cause) or "mover" of all the motion in the universe. As is implicit in the name, the unmoved mover moves other things, but is not itself moved by any prior action. In Book 12 (Greek: Λ) of his Metaphysics, Aristotle describes the unmoved mover as being perfectly beautiful, indivisible, and contemplating only the perfect contemplation: self-contemplation. He equates this concept also with the active intellect. This Aristotelian concept had its roots in cosmological speculations of the earliest Greek pre-Socratic philosophers and became highly influential and widely drawn upon in medieval philosophy and theology. St. Thomas Aquinas, for example, elaborated on the unmoved mover in the Quinque viae.

First philosophy

Aristotle argues, in Book 8 of the Physics and Book 12 of the Metaphysics, "that there must be an immortal, unchanging being, ultimately responsible for all wholeness and orderliness in the sensible world".

In the Physics (VIII 4–6) Aristotle finds "surprising difficulties" explaining even commonplace change, and in support of his approach of explanation by four causes, he required "a fair bit of technical machinery". This "machinery" includes potentiality and actuality, hylomorphism, the theory of categories, and "an audacious and intriguing argument, that the bare existence of change requires the postulation of a first cause, an unmoved mover whose necessary existence underpins the ceaseless activity of the world of motion". Aristotle's "first philosophy", or Metaphysics ("after the Physics"), develops his peculiar theology of the prime mover, as πρῶτον κινοῦν ἀκίνητον: an independent divine eternal unchanging immaterial substance.

Celestial spheres

Aristotle adopted the geometrical model of Eudoxus of Cnidus, to provide a general explanation of the apparent wandering of the classical planets arising from uniform circular motions of celestial spheres. While the number of spheres in the model itself was subject to change (47 or 55), Aristotle's account of aether, and of potentiality and actuality, required an individual unmoved mover for each sphere.

Final cause and efficient cause

Simplicius argues that the first unmoved mover is a cause not only in the sense of being a final cause—which everyone in his day, as in ours, would accept—but also in the sense of being an efficient cause (1360. 24ff.), and his master Ammonius wrote a whole book defending the thesis (ibid. 1363. 8–10). Simplicius's arguments include citations of Plato's views in the Timaeus—evidence not relevant to the debate unless one happens to believe in the essential harmony of Plato and Aristotle—and inferences from approving remarks which Aristotle makes about the role of Nous in Anaxagoras, which require a good deal of reading between the lines. But he does point out rightly that the unmoved mover fits the definition of an efficient cause—"whence the first source of change or rest" (Phys. II. 3, 194b29–30; Simpl. 1361. 12ff.). The examples which Aristotle adduces do not obviously suggest an application to the first unmoved mover, and it is at least possible that Aristotle originated his fourfold distinction without reference to such an entity. But the real question is whether, given his definition of the efficient cause, it includes the unmoved mover willy-nilly. One curious fact remains: that Aristotle never acknowledges the alleged fact that the unmoved mover is an efficient cause (a problem of which Simplicius is well aware: 1363. 12–14)...

— D. W. Graham, Physics

Despite their apparent function in the celestial model, the unmoved movers were a final cause, not an efficient cause for the movement of the spheres; they were solely a constant inspiration, and even if taken for an efficient cause precisely due to being a final cause, the nature of the explanation is purely teleological.

Aristotle's theology

The unmoved movers, if they were anywhere, were said to fill the outer void, beyond the sphere of fixed stars:

It is clear then that there is neither place, nor void, nor time, outside the heaven. Hence whatever is there, is of such a nature as not to occupy any place, nor does time age it; nor is there any change in any of the things which lie beyond the outermost motion; they continue through their entire duration unalterable and unmodified, living the best and most self sufficient of lives… From [the fulfilment of the whole heaven] derive the being and life which other things, some more or less articulately but other feebly, enjoy.

— Aristotle, De Caelo, I.9, 279 a17–30

The unmoved movers are, themselves, immaterial substance (separate and individual beings), having neither parts nor magnitude. As such, it would be physically impossible for them to move material objects of any size by pushing, pulling, or collision. Because matter is, for Aristotle, a substratum in which a potential to change can be actualized, any and all potentiality must be actualized in a being that is eternal but it must not be still, because continuous activity is essential for all forms of life. This immaterial form of activity must be intellectual in nature and it cannot be contingent upon sensory perception if it is to remain uniform; therefore, eternal substance must think only of thinking itself and exist outside the starry sphere, where even the notion of place is undefined for Aristotle. Their influence on lesser beings is purely the result of an "aspiration or desire", and each aetheric celestial sphere emulates one of the unmoved movers, as best it can, by uniform circular motion. The first heaven, the outmost sphere of fixed stars, is moved by a desire to emulate the prime mover (first cause), in relation to whom, the subordinate movers suffer an accidental dependency.

Many of Aristotle's contemporaries complained that oblivious, powerless gods are unsatisfactory. Nonetheless, it was a life which Aristotle enthusiastically endorsed as one most enviable and perfect, the unembellished basis of theology. As the whole of nature depends on the inspiration of the eternal unmoved movers, Aristotle was concerned to establish the metaphysical necessity of the perpetual motions of the heavens. It is through the seasonal action of the Sun upon the terrestrial spheres, that the cycles of generation and corruption give rise to all natural motion as efficient cause. The intellect, nous, "or whatever else it be that is thought to rule and lead us by nature, and to have cognizance of what is noble and divine" is the highest activity, according to Aristotle (contemplation or speculative thinking, theōríā). It is also the most sustainable, pleasant, self-sufficient activity; something which is aimed at for its own sake. (In contrast to politics and warfare, it does not involve doing things we'd rather not do, but rather something we do at our leisure.) This aim is not strictly human: to achieve it means to live in accordance not with mortal thoughts, but something immortal and divine which is within humans. According to Aristotle, contemplation is the only type of happy activity which it would not be ridiculous to imagine the gods having. In Aristotle's psychology and biology, the intellect is the soul (see also eudaimonia).

According to Giovanni Reale, the first Unmoved Mover is a living, thinking and personal God who "possesses the theoretical knowledge alone or in the highest degree...knows not only Himself, but all things in their causes and first principles."

First cause

In Book VIII of his Physics, Aristotle examines the notions of change or motion, and attempts to show by a challenging argument, that the mere supposition of a 'before' and an 'after', requires a first principle. He argues that in the beginning, if the cosmos had come to be, its first motion would lack an antecedent state; and, as Parmenides said, "nothing comes from nothing". The cosmological argument, later attributed to Aristotle, thereby draws the conclusion that God exists. However, if the cosmos had a beginning, Aristotle argued, it would require an efficient first cause, a notion that Aristotle took to demonstrate a critical flaw.

But it is a wrong assumption to suppose universally that we have an adequate first principle in virtue of the fact that something always is so … Thus Democritus reduces the causes that explain nature to the fact that things happened in the past in the same way as they happen now: but he does not think fit to seek for a first principle to explain this 'always' … Let this conclude what we have to say in support of our contention that there never was a time when there was not motion, and never will be a time when there will not be motion.

— Physics VIII, 2

The purpose of Aristotle's cosmological argument, that at least one eternal unmoved mover must exist, is to support everyday change.

Of things that exist, substances are the first. But if substances can, then all things can perish... and yet, time and change cannot. Now, the only continuous change is that of place, and the only continuous change of place is circular motion. Therefore, there must be an eternal circular motion and this is confirmed by the fixed stars which are moved by the eternal actual substance that's purely actual.

In Aristotle's estimation, an explanation without the temporal actuality and potentiality of an infinite locomotive chain is required for an eternal cosmos with neither beginning nor end: an unmoved eternal substance for whom the Primum Mobile turns diurnally and whereby all terrestrial cycles are driven: day and night, the seasons of the year, the transformation of the elements, and the nature of plants and animals.

Substance and change

Aristotle begins by describing substance, of which he says there are three types: the sensible, which is subdivided into the perishable, which belongs to physics, and the eternal, which belongs to "another science". He notes that sensible substance is changeable and that there are several types of change, including quality and quantity, generation and destruction, increase and diminution, alteration, and motion. Change occurs when one given state becomes something contrary to it: that is to say, what exists potentially comes to exist actually (see potentiality and actuality). Therefore, "a thing [can come to be], incidentally, out of that which is not, [and] also all things come to be out of that which is, but is potentially, and is not actually." That by which something is changed is the mover, that which is changed is the matter, and that into which it is changed is the form.

Substance is necessarily composed of different elements. The proof for this is that there are things which are different from each other and that all things are composed of elements. Since elements combine to form composite substances, and because these substances differ from each other, there must be different elements: in other words, "b or a cannot be the same as ba".

Number of movers

Near the end of Metaphysics, Book Λ, Aristotle introduces a surprising question, asking "whether we have to suppose one such [mover] or more than one, and if the latter, how many". Aristotle concludes that the number of all the movers equals the number of separate movements, and we can determine these by considering the mathematical science most akin to philosophy, i.e., astronomy. Although the mathematicians differ on the number of movements, Aristotle considers that the number of celestial spheres would be 47 or 55. Nonetheless, he concludes his Metaphysics, Book Λ, with a quotation from the Iliad: "The rule of many is not good; one ruler let there be."

Influence

See also: Ontological argument, Apophatic theology, and Hellenistic philosophy and Christianity

John Burnet (1892) noted

The Neoplatonists were quite justified in regarding themselves as the spiritual heirs of Pythagoras; and, in their hands, philosophy ceased to exist as such, and became theology. And this tendency was at work all along; hardly a single Greek philosopher was wholly uninfluenced by it. Perhaps Aristotle might seem to be an exception; but it is probable that, if we still possessed a few such "exoteric" works as the Protreptikos in their entirety, we should find that the enthusiastic words in which he speaks of the "blessed life" in the Metaphysics and in the Ethics (Nicomachean Ethics) were less isolated outbursts of feeling than they appear now. In later days, Apollonios of Tyana showed in practice what this sort of thing must ultimately lead to. The theurgy and thaumaturgy of the late Greek schools were only the fruit of the seed sown by the generation which immediately preceded the Persian War.

Aristotle's principles of being (see section above) influenced Anselm's view of God, whom he called "that than which nothing greater can be conceived." Anselm thought that God did not feel emotions such as anger or love, but appeared to do so through our imperfect understanding. The incongruity of judging "being" against something that might not exist, may have led Anselm to his famous ontological argument for God's existence.

Many medieval philosophers made use of the idea of approaching a knowledge of God through negative attributes. For example, we should not say that God exists in the usual sense of the term; all we can safely say is that God is not nonexistent. We should not say that God is wise; but, we can say that God is not ignorant (i.e. in some way God has some properties of knowledge). We should not say that God is One; but, we can state that there is no multiplicity in God's being.

Aristotelian theological concepts were accepted by many later Jewish, Islamic, and Christian philosophers. Key Jewish philosophers included Samuel Ibn Tibbon, Maimonides, and Gersonides, among many others. Their views of God are considered mainstream by many Jews of all denominations even today. Preeminent among Islamic philosophers who were influenced by Aristotelian theology are Avicenna and Averroes. In Christian theology, the key philosopher influenced by Aristotle was undoubtedly Thomas Aquinas. There had been earlier Aristotelian influences within Christianity (notably Anselm), but Aquinas (who, incidentally, found his Aristotelian influence via Avicenna, Averroes, and Maimonides) incorporated extensive Aristotelian ideas throughout his own theology. Through Aquinas and the Scholastic Christian theology of which he was a significant part, Aristotle became "academic theology's great authority in the course of the thirteenth century" and exerted an influence upon Christian theology that become both widespread and deeply embedded. However, notable Christian theologians rejected Aristotelian theological influence, especially the first generation of Christian Reformers and most notably Martin Luther. In subsequent Protestant theology, Aristotelian thought quickly reemerged in Protestant scholasticism.

Future of Earth

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Future_of_Earth 

 

Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, about 5–7 billion years from now

The biological and geological future of Earth can be extrapolated based on the estimated effects of several long-term influences. These include the chemistry at Earth's surface, the cooling rate of the planet's interior, the gravitational interactions with other objects in the Solar System, and a steady increase in the Sun's luminosity. An uncertain factor is the pervasive influence of technology introduced by humans, such as climate engineering, which could cause significant changes to the planet. For example, the current Holocene extinction is being caused by technology, and the effects may last for up to five million years. In turn, technology may result in the extinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.

Over time intervals of hundreds of millions of years, random celestial events pose a global risk to the biosphere, which can result in mass extinctions. These include impacts by comets or asteroids and the possibility of a near-Earth supernova—a massive stellar explosion within a 100-light-year (31-parsec) radius of the Sun. Other large-scale geological events are more predictable. Milankovitch's theory predicts that the planet will continue to undergo glacial periods at least until the Quaternary glaciation comes to an end. These periods are caused by the variations in eccentricity, axial tilt, and precession of Earth's orbit. As part of the ongoing supercontinent cycle, plate tectonics will probably result in a supercontinent in 250–350 million years. Sometime in the next 1.5–4.5 billion years, Earth's axial tilt may begin to undergo chaotic variations, with changes in the axial tilt of up to 90°.

The luminosity of the Sun will steadily increase, resulting in a rise in the solar radiation reaching Earth, resulting in a higher rate of weathering of silicate minerals, affecting the carbonate–silicate cycle, which will cause a decrease in the level of carbon dioxide in the atmosphere. In about 600 million years from now, the level of carbon dioxide will fall below the level needed to sustain C₃ carbon fixation photosynthesis used by trees. Some plants use the C₄ carbon fixation method to persist at carbon dioxide concentrations as low as ten parts per million. However, the long-term trend is for plant life to die off altogether. The extinction of plants will be the demise of almost all animal life since plants are the base of much of the animal food chain on Earth.

In about one billion years, the solar luminosity will be 10% higher, causing the atmosphere to become a "moist greenhouse", resulting in a runaway evaporation of the oceans. As a likely consequence, plate tectonics and the entire carbon cycle will end. Following this event, in about 2–3 billion years, the planet's magnetic dynamo may cease, causing the magnetosphere to decay and leading to an accelerated loss of volatiles from the outer atmosphere. Four billion years from now, the increase in Earth's surface temperature will cause a runaway greenhouse effect, creating conditions more extreme than present-day Venus and heating Earth's surface enough to melt it. By that point, all life on Earth will be extinct. Finally, the most probable fate of the planet is absorption by the Sun in about 7.5 billion years, after the star has entered the red giant phase and expanded beyond the planet's current orbit.

Human influence

Main articles: Anthropocene, Conservation biology, Climate change, Nuclear warfare, and Human impact on the environment

 

Horne foundry copper smelter in Rouyn-Noranda, Canada

Humans play a key role in the biosphere, with the large human population dominating many of Earth's ecosystems. This has resulted in a widespread, ongoing mass extinction of other species during the present geological epoch, now known as the Holocene extinction. The large-scale loss of species caused by human influence since the 1950s has been called a biotic crisis, with an estimated 10% of the total species lost as of 2007. At current rates, about 30% of species are at risk of extinction in the next hundred years. The Holocene extinction event is the result of habitat destruction, the widespread distribution of invasive species, poaching, and climate change. In the present day, human activity has had a significant impact on the surface of the planet. More than a third of the land surface has been modified by human actions, and humans use about 20% of global primary production. The concentration of carbon dioxide in the atmosphere has increased by close to 50% since the start of the Industrial Revolution.

The consequences of a persistent biotic crisis have been predicted to last for at least five million years. It could result in a decline in biodiversity and homogenization of biotas, accompanied by a proliferation of species that are opportunistic, such as pests and weeds. Novel species may emerge; in particular taxa that prosper in human-dominated ecosystems may rapidly diversify into many new species. Microbes are likely to benefit from the increase in nutrient-enriched environmental niches. No new species of existing large vertebrates are likely to arise and food chains will probably be shortened.

Anti-nuclear weapons protest march in Oxford, 1980

There are multiple scenarios for known risks that can have a global impact on the planet. From the perspective of humanity, these can be subdivided into survivable risks and terminal risks. Risks that humans pose to themselves include climate change, the misuse of nanotechnology, a nuclear holocaust, warfare with a programmed superintelligence, a genetically engineered disease, or a disaster caused by a physics experiment. Similarly, several natural events may pose a doomsday threat, including a highly virulent disease, the impact of an asteroid or comet, runaway greenhouse effect, and resource depletion. There may be the possibility of an infestation by an extraterrestrial lifeform. The actual odds of these scenarios occurring are difficult if not impossible to deduce.

Should the human species become extinct, then the various features assembled by humanity will begin to decay. The largest structures have an estimated decay half-life of about 1,000 years. The last surviving structures would most likely be open-pit mines, large landfills, major highways, wide canal cuts, and earth-fill flank dams. A few massive stone monuments like the pyramids at the Giza Necropolis or the sculptures at Mount Rushmore may still survive in some form after a million years.

Cataclysmic astronomical events

The Barringer Meteorite Crater in Flagstaff, Arizona, showing evidence of the impact of celestial objects upon Earth

As the Sun orbits the Milky Way, wandering stars may approach close enough to have a disruptive influence on the Solar System. A close stellar encounter may cause a significant reduction in the perihelion distances of comets in the Oort cloud—a spherical region of icy bodies orbiting within half a light-year of the Sun. Such an encounter can trigger a 40-fold increase in the number of comets reaching the inner Solar System. Impacts from these comets can trigger a mass extinction of life on Earth. These disruptive encounters occur an average of once every 45 million years. The mean time for the Sun to collide with another star in the solar neighborhood is approximately 30 trillion (3×10¹³) years, which is much longer than the estimated age of the Universe, at approximately 13.8 billion years. This can be taken as an indication of the low likelihood of such an event occurring during the lifetime of the Earth.

The energy released from the impact of an asteroid or comet with a diameter of 5–10 km (3–6 mi) or larger is sufficient to create a global environmental disaster and cause a statistically significant increase in the number of species extinctions. Among the deleterious effects resulting from a major impact event is a cloud of fine dust ejecta blanketing the planet, blocking some direct sunlight from reaching the Earth's surface thus lowering land temperatures by about 15 °C (27 °F) within a week and halting photosynthesis for several months (similar to a nuclear winter). The mean time between major impacts is estimated to be at least 100 million years. During the last 540 million years, simulations demonstrated that such an impact rate is sufficient to cause five or six mass extinctions and 20 to 30 lower severity events. This matches the geologic record of significant extinctions during the Phanerozoic Eon. Such events can be expected to continue.

A supernova is a cataclysmic explosion of a star. Within the Milky Way galaxy, supernova explosions occur on average once every 40 years. During the history of Earth, multiple such events have likely occurred within a distance of 100 light-years; known as a near-Earth supernova. Explosions inside this distance can contaminate the planet with radioisotopes and possibly impact the biosphere. Gamma rays emitted by a supernova react with nitrogen in the atmosphere, producing nitrous oxides. These molecules cause a depletion of the ozone layer that protects the surface from ultraviolet (UV) radiation from the Sun. An increase in UV-B radiation of only 10–30% is sufficient to cause a significant impact on life; particularly to the phytoplankton that form the base of the oceanic food chain. A supernova explosion at a distance of 26 light-years will reduce the ozone column density by half. On average, a supernova explosion occurs within 32 light-years once every few hundred million years, resulting in a depletion of the ozone layer lasting several centuries. Over the next two billion years, there will be about 20 supernova explosions and one gamma ray burst that will have a significant impact on the planet's biosphere.

The incremental effect of gravitational perturbations between the planets causes the inner Solar System as a whole to behave chaotically over long time periods. This does not significantly affect the stability of the Solar System over intervals of a few million years or less, but over billions of years, the orbits of the planets become unpredictable. Computer simulations of the Solar System's evolution over the next five billion years suggest that there is a small (less than 1%) chance that a collision could occur between Earth and either Mercury, Venus, or Mars. During the same interval, the odds that Earth will be scattered out of the Solar System by a passing star are on the order of 1 in 100,000 (0.001%). In such a scenario, the oceans would freeze solid within several million years, leaving only a few pockets of liquid water about 14 km (9 mi) underground. There is a remote chance that Earth will instead be captured by a passing binary star system, allowing the planet's biosphere to remain intact. The odds of this happening are about 1 in 3 million.

Orbit and rotation

The gravitational perturbations of the other planets in the Solar System combine to modify the orbit of Earth and the orientation of its rotation axis. These changes can influence the planetary climate. Despite such interactions, highly accurate simulations show that overall, Earth's orbit is likely to remain dynamically stable for billions of years into the future. In all 1,600 simulations, the planet's semimajor axis, eccentricity, and inclination remained nearly constant.

Glaciation

An artist's impression of ice age Earth at glacial maximum.

Historically, there have been cyclical ice ages in which glacial sheets periodically covered the higher latitudes of the continents. Ice ages may occur because of changes in ocean circulation and continentality induced by plate tectonics. The Milankovitch theory predicts that glacial periods occur during ice ages because of astronomical factors in combination with climate feedback mechanisms. The primary astronomical drivers are a higher than normal orbital eccentricity, a low axial tilt (or obliquity), and the alignment of the northern hemisphere's summer solstice with the aphelion. Each of these effects occur cyclically. For example, the eccentricity changes over time cycles of about 100,000 and 400,000 years, with the value ranging from less than 0.01 up to 0.05. This is equivalent to a change of the semiminor axis of the planet's orbit from 99.95% of the semimajor axis to 99.88%, respectively.

Earth is passing through an ice age known as the quaternary glaciation, and is presently in the Holocene interglacial period. This period would normally be expected to end in about 25,000 years. However, the increased rate at which humans release carbon dioxide into the atmosphere may delay the onset of the next glacial period until at least 50,000–130,000 years from now. On the other hand, a global warming period of finite duration (based on the assumption that fossil fuel use will cease by the year 2200) will probably only impact the glacial period for about 5,000 years. Thus, a brief period of global warming induced by a few centuries' worth of greenhouse gas emission would only have a limited impact in the long term.

Obliquity

The rotational offset of the tidal bulge exerts a net torque on the Moon, boosting it while slowing the Earth's rotation (not to scale).

The tidal acceleration of the Moon slows the rotation rate of the Earth and increases the Earth-Moon distance. Friction effects—between the core and mantle and between the atmosphere and surface—can dissipate the Earth's rotational energy. These combined effects are expected to increase the length of the day by more than 1.5 hours over the next 250 million years, and to increase the obliquity by about a half degree. The distance to the Moon will increase by about 1.5 Earth radii during the same period.

Based on computer models, the presence of the Moon appears to stabilize the obliquity of the Earth, which may help the planet to avoid dramatic climate changes. This stability is achieved because the Moon increases the precession rate of the Earth's rotation axis, thereby avoiding resonances between the precession of the rotation and precession of the planet's orbital plane (that is, the precession motion of the ecliptic). However, as the semimajor axis of the Moon's orbit continues to increase, this stabilizing effect will diminish. At some point, perturbation effects will probably cause chaotic variations in the obliquity of the Earth, and the axial tilt may change by angles as high as 90° from the plane of the orbit. This is expected to occur between 1.5 and 4.5 billion years from now.

A high obliquity would probably result in dramatic changes in the climate and may destroy the planet's habitability. When the axial tilt of the Earth exceeds 54°, the yearly insolation at the equator is less than that at the poles. The planet could remain at an obliquity of 60° to 90° for periods as long as 10 million years.

Geodynamics

Pangaea was the last supercontinent to form before the present.

Tectonics-based events will continue to occur well into the future and the surface will be steadily reshaped by tectonic uplift, extrusions, and erosion. Mount Vesuvius can be expected to erupt about 40 times over the next 1,000 years. During the same period, about five to seven earthquakes of magnitude 8 or greater should occur along the San Andreas Fault, while about 50 events of magnitude 9 may be expected worldwide. Mauna Loa should experience about 200 eruptions over the next 1,000 years, and the Old Faithful Geyser will likely cease to operate. The Niagara Falls will continue to retreat upstream, reaching Buffalo in about 30,000–50,000 years.

In 10,000 years, the post-glacial rebound of the Baltic Sea will have reduced the depth by about 90 m (300 ft). The Hudson Bay will decrease in depth by 100 m over the same period. After 100,000 years, the island of Hawaii will have shifted about 9 km (5.6 mi) to the northwest. The planet may be entering another glacial period by this time.

Continental drift

Further information: Continental drift

The theory of plate tectonics demonstrates that the continents of the Earth are moving across the surface at the rate of a few centimeters per year. This is expected to continue, causing the plates to relocate and collide. Continental drift is facilitated by two factors: the energy generated within the planet and the presence of a hydrosphere. With the loss of either of these, continental drift will come to a halt. The production of heat through radiogenic processes is sufficient to maintain mantle convection and plate subduction for at least the next 1.1 billion years.

At present, the continents of North and South America are moving westward from Africa and Europe. Researchers have produced several scenarios about how this will continue in the future. These geodynamic models can be distinguished by the subduction flux, whereby the oceanic crust moves under a continent. In the introversion model, the younger, interior, Atlantic Ocean becomes preferentially subducted and the current migration of North and South America is reversed. In the extroversion model, the older, exterior, Pacific Ocean remains preferentially subducted and North and South America migrate toward eastern Asia.

As the understanding of geodynamics improves, these models will be subject to revision. In 2008, for example, a computer simulation was used to predict that a reorganization of the mantle convection will occur over the next 100 million years, creating a new supercontinent composed of Africa, Eurasia, Australia, Antarctica and South America to form around Antarctica.

Regardless of the outcome of the continental migration, the continued subduction process causes water to be transported to the mantle. After a billion years from the present, a geophysical model gives an estimate that 27% of the current ocean mass will have been subducted. If this process were to continue unmodified into the future, the subduction and release would reach an equilibrium after 65% of the current ocean mass has been subducted.

Introversion

A rough approximation of Pangaea Ultima, one of the four models for a future supercontinent

Christopher Scotese and his colleagues have mapped out the predicted motions several hundred million years into the future as part of the Paleomap Project. In their scenario, 50 million years from now the Mediterranean Sea may vanish, and the collision between Europe and Africa will create a long mountain range extending to the current location of the Persian Gulf. Australia will merge with Indonesia, and Baja California will slide northward along the coast. New subduction zones may appear off the eastern coast of North and South America, and mountain chains will form along those coastlines. The migration of Antarctica to the north will cause all of its ice sheets to melt. This, along with the melting of the Greenland ice sheets, will raise the average ocean level by 90 m (300 ft). The inland flooding of the continents will result in climate changes.

As this scenario continues, by 100 million years from the present, the continental spreading will have reached its maximum extent and the continents will then begin to coalesce. In 250 million years, North America will collide with Africa. South America will wrap around the southern tip of Africa. The result will be the formation of a new supercontinent (sometimes called Pangaea Ultima), with the Pacific Ocean stretching across half the planet. Antarctica will reverse direction and return to the South Pole, building up a new ice cap.

Extroversion

The first scientist to extrapolate the current motions of the continents was Canadian geologist Paul F. Hoffman of Harvard University. In 1992, Hoffman predicted that the continents of North and South America would continue to advance across the Pacific Ocean, pivoting about Siberia until they begin to merge with Asia. He dubbed the resulting supercontinent, Amasia. Later, in the 1990s, Roy Livermore calculated a similar scenario. He predicted that Antarctica would start to migrate northward, and East Africa and Madagascar would move across the Indian Ocean to collide with Asia.

In an extroversion model, the closure of the Pacific Ocean would be complete in about 350 million years. This marks the completion of the current supercontinent cycle, wherein the continents split apart and then rejoin each other about every 400–500 million years. Once the supercontinent is built, plate tectonics may enter a period of inactivity as the rate of subduction drops by an order of magnitude. This period of stability could cause an increase in the mantle temperature at the rate of 30–100 °C (54–180 °F) every 100 million years, which is the minimum lifetime of past supercontinents. As a consequence, volcanic activity may increase.

Supercontinent

The formation of a supercontinent can dramatically affect the environment. The collision of plates will result in mountain building, thereby shifting weather patterns. Sea levels may fall because of increased glaciation. The rate of surface weathering can rise, increasing the rate at which organic material is buried. Supercontinents can cause a drop in global temperatures and an increase in atmospheric oxygen. This, in turn, can affect the climate, further lowering temperatures. All of these changes can result in more rapid biological evolution as new niches emerge.

The formation of a supercontinent insulates the mantle. The flow of heat will be concentrated, resulting in volcanism and the flooding of large areas with basalt. Rifts will form and the supercontinent will split up once more. The planet may then experience a warming period as occurred during the Cretaceous period, which marked the split-up of the previous Pangaea supercontinent.

Solidification of the outer core

The iron-rich core region of the Earth is divided into a 2,440 km (1,520 mi) diameter solid inner core and a 6,960 km (4,320 mi) diameter liquid outer core. The rotation of the Earth creates convective eddies in the outer core region that cause it to function as a dynamo. This generates a magnetosphere about the Earth that deflects particles from the solar wind, which prevents significant erosion of the atmosphere from sputtering. As heat from the core is transferred outward toward the mantle, the net trend is for the inner boundary of the liquid outer core region to freeze, thereby releasing thermal energy and causing the solid inner core to grow. This iron crystallization process has been ongoing for about a billion years. In the modern era, the radius of the inner core is expanding at an average rate of roughly 0.5 mm (0.02 in) per year, at the expense of the outer core. Nearly all of the energy needed to power the dynamo is being supplied by this process of inner core formation.

The inner core is expected to consume most or all of the outer core 3–4 billion years from now, resulting in an almost completely solidified core composed of iron and other heavy elements. The surviving liquid envelope will mainly consist of lighter elements that will undergo less mixing. Alternatively, if at some point plate tectonics cease, the interior will cool less efficiently, which would slow down or even stop the inner core's growth. In either case, this can result in the loss of the magnetic dynamo. Without a functioning dynamo, the magnetic field of the Earth will decay in a geologically short time period of roughly 10,000 years. The loss of the magnetosphere will cause an increase in erosion of light elements, particularly hydrogen, from the Earth's outer atmosphere into space, resulting in less favorable conditions for life.

Solar evolution

See also: Stellar evolution and Formation and evolution of the Solar System

The energy generation of the Sun is based upon thermonuclear fusion of hydrogen into helium. This occurs in the core region of the star using the proton–proton chain reaction process. Because there is no convection in the solar core, the helium concentration builds up in that region without being distributed throughout the star. The temperature at the core of the Sun is too low for nuclear fusion of helium atoms through the triple-alpha process, so these atoms do not contribute to the net energy generation that is needed to maintain hydrostatic equilibrium of the Sun.

At present, nearly half the hydrogen at the core has been consumed, with the remainder of the atoms consisting primarily of helium. As the number of hydrogen atoms per unit mass decreases, so too does their energy output provided through nuclear fusion. This results in a decrease in pressure support, which causes the core to contract until the increased density and temperature bring the core pressure into equilibrium with the layers above. The higher temperature causes the remaining hydrogen to undergo fusion at a more rapid rate, thereby generating the energy needed to maintain the equilibrium.

Evolution of the Sun's luminosity, radius and effective temperature compared to the present Sun. After Ribas (2010).

The result of this process has been a steady increase in the energy output of the Sun. When the Sun first became a main sequence star, it radiated only 70% of the current luminosity. The luminosity has increased in a nearly linear fashion to the present, rising by 1% every 110 million years. Likewise, in three billion years the Sun is expected to be 33% more luminous. The hydrogen fuel at the core will finally be exhausted in five billion years, when the Sun will be 67% more luminous than at present. Thereafter, the Sun will continue to burn hydrogen in a shell surrounding its core until the luminosity reaches 121% above the present value. This marks the end of the Sun's main-sequence lifetime, and thereafter it will pass through the subgiant stage and evolve into a red giant.

By this time, the collision of the Milky Way and Andromeda galaxies should be underway. Although this could result in the Solar System being ejected from the newly combined galaxy, it is considered unlikely to have any adverse effect on the Sun or its planets.

Climate impact

See also: Faint young Sun paradox and Medea hypothesis

The rate of weathering of silicate minerals will increase as rising temperatures speed up chemical processes. This in turn will decrease the level of carbon dioxide in the atmosphere, as reactions with silicate minerals convert carbon dioxide gas into solid carbonates. Within the next 600 million years from the present, the concentration of carbon dioxide will fall below the critical threshold needed to sustain C₃ photosynthesis: about 50 parts per million. At this point, trees and forests in their current forms will no longer be able to survive. This decline in plant life is likely to be a long-term decline rather than a sharp drop. It is likely that plant groups will die one by one well before the 50 parts per million level is reached. The first plants to disappear will be C₃ herbaceous plants, followed by deciduous forests, evergreen broad-leaf forests and finally evergreen conifers. However, C₄ carbon fixation can continue at much lower concentrations, down to above 10 parts per million. Thus plants using C₄ photosynthesis may be able to survive for at least 0.8 billion years and possibly as long as 1.2 billion years from now, after which rising temperatures will make the biosphere unsustainable. Researchers at Caltech have suggested that once C₃ plants die off, the lack of biological production of oxygen and nitrogen will cause a reduction in Earth's atmospheric pressure, which will counteract the rise in temperature, and allow enough carbon dioxide to persist for photosynthesis to continue. This would allow life to survive up to 2 billion years from now, at which point water would be the limiting factor. Currently, C₄ plants represent about 5% of Earth's plant biomass and 1% of its known plant species. For example, about 50% of all grass species (Poaceae) use the C₄ photosynthetic pathway, as do many species in the herbaceous family Amaranthaceae.

When the levels of carbon dioxide fall to the limit where photosynthesis is barely sustainable, the proportion of carbon dioxide in the atmosphere is expected to oscillate up and down. This will allow land vegetation to flourish each time the level of carbon dioxide rises due to tectonic activity and respiration from animal life. However, the long-term trend is for the plant life on land to die off altogether as most of the remaining carbon in the atmosphere becomes sequestered in the Earth. Plants—and, by extension, animals—could survive longer by evolving other strategies such as requiring less carbon dioxide for photosynthetic processes, becoming carnivorous, adapting to desiccation, or associating with fungi. These adaptations are likely to appear near the beginning of the moist greenhouse (see further).

The loss of higher plant life will result in the eventual loss of oxygen as well as ozone due to the respiration of animals, chemical reactions in the atmosphere, and volcanic eruptions. Modelling of the decline in oxygenation predicts that it may drop to 1% of the current atmospheric levels by one billion years from now. This decline will result in less attenuation of DNA-damaging UV, as well as the death of animals; the first animals to disappear would be large mammals, followed by small mammals, birds, amphibians and large fish, reptiles and small fish, and finally invertebrates.

Before this happens, it is expected that life would concentrate at refugia of lower temperature such as high elevations where less land surface area is available, thus restricting population sizes. Smaller animals would survive better than larger ones because of lesser oxygen requirements, while birds would fare better than mammals thanks to their ability to travel large distances looking for cooler temperatures. Based on oxygen half-life in the atmosphere, animal life would last at most 100 million years after the loss of higher plants. Some cyanobacteria and phytoplankton could outlive plants due to their tolerance for carbon dioxide levels as low as 1 ppm, and may survive for around the same time as animals before carbon dioxide becomes too depleted to support any form of photosynthesis.

In their work The Life and Death of Planet Earth, authors Peter D. Ward and Donald Brownlee have argued that some form of animal life may continue even after most of the Earth's plant life has disappeared. Ward and Brownlee use fossil evidence from the Burgess Shale in British Columbia, Canada, to determine the climate of the Cambrian Explosion, and use it to predict the climate of the future when rising global temperatures caused by a warming Sun and declining oxygen levels result in the final extinction of animal life. Initially, they expect that some insects, lizards, birds, and small mammals may persist, along with sea life. However, without oxygen replenishment by plant life, they believe that animals would probably die off from asphyxiation within a few million years. Even if sufficient oxygen were to remain in the atmosphere through the persistence of some form of photosynthesis, the steady rise in global temperature would result in a gradual loss of biodiversity.

As temperatures continue to rise, the last of animal life will be driven toward the poles, and possibly underground. They would become primarily active during the polar night, aestivating during the polar day due to the intense heat. Much of the surface would become a barren desert and life would primarily be found in the oceans. However, due to a decrease in the amount of organic matter entering the oceans from land as well as a decrease in dissolved oxygen, sea life would disappear too, following a similar path to that on Earth's surface. This process would start with the loss of freshwater species and conclude with invertebrates, particularly those that do not depend on living plants such as termites or those near hydrothermal vents such as worms of the genus Riftia. As a result of these processes, multicellular life forms may be extinct in about 800 million years, and eukaryotes in 1.3 billion years, leaving only the prokaryotes.

Loss of oceans

The atmosphere of Venus is in a "super-greenhouse" state. Earth in a few billion years could likely resemble present Venus.

One billion years from now, about 27% of the modern ocean will have been subducted into the mantle. If this process were allowed to continue uninterrupted, it would reach an equilibrium state where 65% of the current surface reservoir would remain at the surface. Once the solar luminosity is 10% higher than its current value, the average global surface temperature will rise to 320 K (47 °C; 116 °F). The atmosphere will become a "moist greenhouse" leading to a runaway evaporation of the oceans. At this point, models of the Earth's future environment demonstrate that the stratosphere would contain increasing levels of water. These water molecules will be broken down through photodissociation by solar UV, allowing hydrogen to escape the atmosphere. The net result would be a loss of the world's seawater by about 1.1 billion years from the present.

There will be two variations of this future warming feedback: the "moist greenhouse" where water vapor dominates the troposphere while water vapor starts to accumulate in the stratosphere (if the oceans evaporate very quickly), and the "runaway greenhouse" where water vapor becomes a dominant component of the atmosphere (if the oceans evaporate too slowly). In this ocean-free era, there will continue to be surface reservoirs as water is steadily released from the deep crust and mantle, where it is estimated that there is an amount of water equivalent to several times that currently present in the Earth's oceans. Some water may be retained at the poles and there may be occasional rainstorms, but for the most part, the planet would be a desert with large dunefields covering its equator, and a few salt flats on what was once the ocean floor, similar to the ones in the Atacama Desert in Chile.

With no water to serve as a lubricant, plate tectonics would very likely stop and the most visible signs of geological activity would be shield volcanoes located above mantle hotspots. In these arid conditions the planet may retain some microbial and possibly even multicellular life. Most of these microbes will be halophiles and life could find refuge in the atmosphere as has been proposed to have happened on Venus. However, the increasingly extreme conditions will likely lead to the extinction of the prokaryotes between 1.6 billion years and 2.8 billion years from now, with the last of them living in residual ponds of water at high latitudes and heights or in caverns with trapped ice. However, underground life could last longer.

What proceeds after this depends on the level of tectonic activity. A steady release of carbon dioxide by volcanic eruption could cause the atmosphere to enter a "super-greenhouse" state like that of the planet Venus. But, as stated above, without surface water, plate tectonics would probably come to a halt and most of the carbonates would remain securely buried until the Sun becomes a red giant and its increased luminosity heats the rock to the point of releasing the carbon dioxide. However, as pointed out by Peter Ward and Donald Brownlee in their book The Life and Death of Planet Earth, according to NASA Ames scientist Kevin Zahnle, it is highly possible that plate tectonics may stop long before the loss of the oceans, due to the gradual cooling of the Earth's core, which could happen in just 500 million years. This could potentially turn the Earth back into a waterworld, and even perhaps drowning all remaining land life.

The loss of the oceans could be delayed until 2 billion years in the future if the atmospheric pressure were to decline. A lower atmospheric pressure would reduce the greenhouse effect, thereby lowering the surface temperature. This could occur if natural processes were to remove the nitrogen from the atmosphere. Studies of organic sediments have shown that at least 100 kilopascals (0.99 atm) of nitrogen has been removed from the atmosphere over the past four billion years, which is enough to effectively double the current atmospheric pressure if it were to be released. This rate of removal would be sufficient to counter the effects of increasing solar luminosity for the next two billion years.

By 2.8 billion years from now, the surface temperature of the Earth will have reached 422 K (149 °C; 300 °F), even at the poles. At this point, any remaining life will be extinguished due to the extreme conditions. What happens beyond this depends on how much water is left on the surface. If all of the water on Earth has evaporated by this point already (via the "moist greenhouse" at ~1 Gyr from now), the planet will stay in the same conditions with a steady increase in the surface temperature until the Sun becomes a red giant. If not and there are still pockets of water left, and evaporates too slowly, then in about 3–4 billion years, once the amount of water vapor in the lower atmosphere rises to 40%, and the luminosity from the Sun reaches 35–40% more than its present-day value, a "runaway greenhouse" effect will ensue, causing the atmosphere to heat up and raising the surface temperature to around 1,600 K (1,330 °C; 2,420 °F). This is sufficient to melt the surface of the planet. However, most of the atmosphere will be retained until the Sun has entered the red giant stage.

With the extinction of life, 2.8 billion years from now it is expected that Earth's biosignatures will disappear, to be replaced by signatures caused by non-biological processes.

Red giant stage

The size of the current Sun (now in the main sequence) compared to its estimated size during its red giant phase

Once the Sun changes from burning hydrogen within its core to burning hydrogen in a shell around its core, the core will start to contract, and the outer envelope will expand. The total luminosity will steadily increase over the following billion years until it reaches 2,730 times its current luminosity at the age of 12.167 billion years. Most of Earth's atmosphere will be lost to space. Its surface will consist of a lava ocean with floating continents of metals and metal oxides and icebergs of refractory materials, with its surface temperature reaching more than 2,400 K (2,130 °C; 3,860 °F). The Sun will experience more rapid mass loss, with about 33% of its total mass shed with the solar wind. The loss of mass will mean that the orbits of the planets will expand. The orbital distance of Earth will increase to at most 150% of its current value (that is, 1.5 AU (220 million km; 140 million mi)).

The most rapid part of the Sun's expansion into a red giant will occur during the final stages, when the Sun will be about 12 billion years old. It is likely to expand to swallow both Mercury and Venus, reaching a maximum radius of 1.2 AU (180 million km; 110 million mi). Earth will interact tidally with the Sun's outer atmosphere, which would decrease Earth's orbital radius. Drag from the chromosphere of the Sun would reduce Earth's orbit. These effects will counterbalance the impact of mass loss by the Sun, and the Sun will likely engulf Earth in about 7.59 billion years.

The drag from the solar atmosphere may cause the orbit of the Moon to decay. Once the orbit of the Moon closes to a distance of 18,470 km (11,480 mi), it will cross Earth's Roche limit, meaning that tidal interaction with Earth would break apart the Moon, turning it into a ring system. Most of the orbiting rings will begin to decay, and the debris will impact Earth. Hence, even if the Sun does not swallow up Earth, the planet may be left moonless. Furthermore, the ablation and vaporization caused by its fall on a decaying trajectory towards the Sun may remove Earth's mantle, leaving just its core, which will finally be destroyed after at most 200 years. Earth's sole legacy will be a very slight increase (0.01%) of the solar metallicity following this event.

Beyond and ultimate fate

The Helix nebula, a planetary nebula similar to what the Sun will produce in 8 billion years

After fusing helium in its core to carbon, the Sun will begin to collapse again, evolving into a compact white dwarf star after ejecting its outer atmosphere as a planetary nebula. The predicted final mass is 54% of the present value, most likely consisting primarily of carbon and oxygen.

Currently, the Moon is moving away from Earth at a rate of 4 cm (1.6 inches) per year. In 50 billion years, if the Earth and Moon are not engulfed by the Sun, they will become tidelocked into a larger, stable orbit, with each showing only one face to the other. Thereafter, the tidal action of the Sun will extract angular momentum from the system, causing the orbit of the Moon to decay and the Earth's rotation to accelerate. In about 65 billion years, it is estimated that the Moon may end up colliding with the Earth, due to the remaining energy of the Earth–Moon system being sapped by the remnant Sun, causing the Moon to slowly move inwards toward the Earth.

On a time scale of 10¹⁹ (10 quintillion) years the remaining planets in the Solar System will be ejected from the system by violent relaxation. If Earth is not destroyed by the expanding red giant Sun and the Earth is not ejected from the Solar System by violent relaxation, the ultimate fate of the planet will be that it collides with the black dwarf Sun due to the decay of its orbit via gravitational radiation, in 10²⁰ (Short Scale: 100 quintillion, Long Scale: 100 trillion) years.