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The biological and geological future ofEarth can beextrapolated based on the estimated effects of several long-term influences. These include thechemistry at Earth's surface, thecooling rate of the planet's interior,gravitational interactions with other objects in theSolar System, and a steady increase in theSun's luminosity. An uncertain factor is the influence of human technology such asclimate engineering,[2] which could cause significant changes to the planet.[3][4] For example, the currentHolocene extinction[5] is being caused by technology,[6] and the effects may last for up to five million years.[7] In turn, technology may result in theextinction of humanity, leaving the planet to gradually return to a slower evolutionary pace resulting solely from long-term natural processes.[8][9]
Over time intervals of hundreds of millions of years, random celestial events pose a global risk to thebiosphere, which can result inmass extinctions. These include impacts bycomets orasteroids and the possibility of anear-Earth supernova—a massivestellar 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 undergoglacial periods at least until theQuaternary glaciation comes to an end. These periods are caused by the variations ineccentricity,axial tilt, andprecession of Earth's rotation and orbit.[10] As part of the ongoingsupercontinent cycle,plate tectonics will probably create asupercontinent 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°.[11]
The luminosity of the Sun will steadily increase, causing a rise in thesolar radiation reaching Earth and resulting in a higher rate ofweathering ofsilicate minerals. This will affect thecarbonate–silicate cycle, which will reduce the level ofcarbon dioxide in the atmosphere. About 600 million years from now, the level of carbon dioxide will fall below the level needed to sustainC3 carbon fixation photosynthesis used by trees. Some plants use theC4 carbon fixation method to persist at carbon dioxide concentrations as low as ten parts per million. However, in the long term, plants will likely die off altogether. The extinction of plants would cause the demise of almost all animal life since plants are the base of much of the animalfood chain.[12][13]
In about one billion years, solar luminosity will be 10% higher, causing the atmosphere to become a "moist greenhouse", resulting in arunaway evaporation of the oceans. As a likely consequence, plate tectonics and the entirecarbon cycle will end.[14] Then, in about 2–3 billion years, the planet'smagnetic dynamo may cease, causing themagnetosphere to decay, leading to an accelerated loss ofvolatiles from the outer atmosphere. Four billion years from now, the increase in Earth's surface temperature will cause arunaway greenhouse effect, creating conditions more extreme than present-dayVenus and heating Earth's surface enough to melt it. By that point, all life on Earth will be extinct.[15][16] Finally, the planet will likely be absorbed by the Sun in about 7.5 billion years, after the star has entered thered giant phase and expanded beyond the planet's current orbit.[17]
Humans play a key role in thebiosphere, with the largehuman population dominating many of Earth'secosystems.[3][18] This has resulted in a widespread, ongoingmass extinction of otherspecies during the present geologicalepoch, now known as theHolocene extinction. The large-scale loss of species caused by human influence since the 1950s has been called abiotic crisis, with an estimated 10% of the total species lost as of 2007.[6] At current rates, about 30% of species are at risk ofextinction in the next hundred years.[19] The Holocene extinction event is the result ofhabitat destruction, the widespread distribution ofinvasive species, poaching, andclimate change.[20][21][22] 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 globalprimary production.[4] The concentration ofcarbon dioxide in the atmosphere has increased by close to 50% since the start of theIndustrial Revolution.[3][23]
The consequences of a persistent biotic crisis have been predicted to last for at least five million years.[7] It could result in a decline inbiodiversity and homogenization ofbiotas, accompanied by a proliferation of species that areopportunistic, such as pests and weeds. Novel species may emerge; in particulartaxa that prosper in human-dominated ecosystems may rapidly diversify into many new species.Microbes are likely to benefit from the increase in nutrient-enrichedenvironmental niches. No new species of existing largevertebrates are likely to arise andfood chains will probably be shortened.[5][24]
There aremultiple 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 andterminal risks. Risks that humans pose to themselves include climate change, themisuse of nanotechnology, anuclear holocaust, warfare with a programmedsuperintelligence, agenetically engineered disease, or a disaster caused by a physics experiment. Similarly, several natural events may pose adoomsday threat, including a highlyvirulent disease, theimpact of an asteroid or comet,runaway greenhouse effect, andresource depletion. There may be the possibility of an infestation by anextraterrestrial lifeform.[25] The actual odds of these scenarios occurring are difficult if not impossible to deduce.[8][9]
Should the human species become extinct, then the various features assembled by humanity will begin to decay. The largest structures have an estimated decayhalf-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 theGiza Necropolis or the sculptures atMount Rushmore may still survive in some form after a million years.[9][a]
As the Sun orbits theMilky Way, wandering stars such asGliese 710 may approach close enough to have a disruptive influence on theSolar System.[26] A close stellar encounter could cause a significant reduction in theperihelion distances of comets in theOort cloud—a hypothetical spherical region of icy bodies orbiting within half alight-year of the Sun.[27] Such an encounter could trigger a 40-fold increase in the number of comets reaching the inner Solar System. Impacts from these comets could trigger a mass extinction of life on Earth. These disruptive encounters are estimated to occur an average of once every 45 million years.[28] There is a 1% chance every billion years that a star will pass within100 AU of the Sun, potentially disrupting the Solar System.[29] The mean time for the Sun tocollide with another star in thesolar 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.[30] Based on results from theGaia telescope's second data release from April 2018, an estimated 694 stars will approach the Solar System to less than 5parsecs in the next 15 million years. Of these, 26 have a good probability to come within 1.0 parsec (3.3 light-years) and 7 within 0.5 parsecs (1.6 light-years).[31]
The energy released from the impact of anasteroid or comet with a diameter of 5–10 km (3–6 mi) or larger is sufficient to create a globalenvironmental disaster and cause astatistically 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 somedirect sunlight from reaching the Earth's surface thus lowering land temperatures by about 15 °C (27 °F) within a week and haltingphotosynthesis for several months (similar to anuclear 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 thePhanerozoic Eon. Such events can be expected to continue.[32]
Asupernova is a cataclysmic explosion of a star. Within the Milky Waygalaxy, supernova explosions occur on average once every 40 years.[33] During thehistory of Earth, multiple such events have likely occurred within a distance of 100 light-years; known as anear-Earth supernova. Explosions inside this distance can contaminate the planet withradioisotopes and possibly impact the biosphere.[34]Gamma rays emitted by a supernova react withnitrogen in the atmosphere, producingnitrous oxides. These molecules cause a depletion of theozone layer that protects the surface fromultraviolet (UV) radiation from the Sun. An increase inUV-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 oceanicfood 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.[35] Over the next two billion years, it is predicted that there will be about 20 supernova explosions and onegamma ray burst that will have a significant impact on the planet's biosphere.[36]
The incremental effect ofgravitational perturbations between the planets causes the inner Solar System as a whole to behavechaotically over long time periods. This does not significantly affect thestability 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 eitherMercury,Venus, orMars.[37][38] 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 passingbinary star system, allowing the planet's biosphere to remain intact. The odds of this happening are about 1 in 3 million.[39]
The gravitational perturbations of the other planets in the Solar System combine to modify theorbit of Earth and theorientation of its rotation axis. These changes can influence the planetary climate.[10][40][41][42] 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'ssemimajor axis,eccentricity, andinclination remained nearly constant.[43]
Historically, there have been cyclicalice ages in which glacial sheets periodically covered the higher latitudes of the continents. Ice ages may occur because of changes inocean circulation andcontinentality induced byplate tectonics.[44] TheMilankovitch theory predicts thatglacial periods occur during ice ages because of astronomical factors in combination with climate feedback mechanisms. The primary astronomical drivers are a higher than normalorbital eccentricity, a lowaxial tilt (or obliquity), and the alignment of the northern hemisphere'ssummer solstice with theaphelion. 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.[45][46] This is equivalent to a change of thesemiminor axis of the planet's orbit from 99.95% of thesemimajor axis to 99.88%, respectively.[47]
Earth is passing through an ice age known as theQuaternary glaciation, and is presently in theHoloceneinterglacial period. This period would normally be expected to end in about 25,000 years.[42] However, the increased rate at which humans release carbon dioxide into theatmosphere may delay the onset of the next glacial period until at least 50,000–130,000 years from now. On the other hand, aglobal warming period of finite duration (based on the assumption thatfossil 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 ofgreenhouse gas emission would only have a limited impact in the long term.[10]
Thetidal acceleration of theMoon slows the rotation rate of the Earth and increases theEarth-Moon distance. Friction effects—between thecore andmantle and between the atmosphere and surface—can dissipate the Earth's rotational energy. These combined effects are expected to increase thelength of the day by more than 1.5 hours over the next 250 million years, and to increase theobliquity by about a half degree. The distance to the Moon will increase by about 1.5 Earth radii during the same period.[48]
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.[49] This stability is achieved because the Moon increases theprecession 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 theecliptic).[50] 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.[11]
A high obliquity would probably result in dramatic changes in the climate and may destroy the planet'shabitability.[41] When the axial tilt of the Earth exceeds 54°, the yearlyinsolation 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.[51]
Tectonics-based events will continue to occur well into the future and the surface will be steadily reshaped bytectonic uplift,extrusions, anderosion.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 theSan 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 theOld Faithful Geyser will likely cease to operate. TheNiagara Falls will continue to retreat upstream, reachingBuffalo in about 30,000–50,000 years.[9]Supervolcano events are the most impactful geological hazards, generating over1,000 km3 of fragmented material and covering thousands of square kilometers with ash deposits. However, they are comparatively rare, occurring on average every 100,000 years.[52]
In 10,000 years, the post-glacial rebound of theBaltic Sea will have reduced the depth by about 90 m (300 ft). TheHudson Bay will decrease in depth by 100 m over the same period.[38] After 100,000 years, the island ofHawaii will have shifted about 9 km (5.6 mi) to the northwest. The planet may be entering another glacial period by this time.[9]
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 ahydrosphere. With the loss of either of these, continental drift will come to a halt.[53] The production ofheat through radiogenic processes is sufficient to maintainmantle convection and platesubduction for at least the next 1.1 billion years.[54]
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.[55] Thesegeodynamic models can be distinguished by the subduction flux, whereby theoceanic 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.[56][57]
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 newsupercontinent composed of Africa, Eurasia, Australia, Antarctica and South America to form around Antarctica.[58]
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.[59]
Christopher Scotese and his colleagues have mapped out the predicted motions several hundred million years into the future as part of thePaleomap Project.[55] 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, andBaja 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 ofits ice sheets to melt. This, along with the melting of theGreenland ice sheets, will raise the average ocean level by 90 m (300 ft). The inland flooding of the continents will result in climate changes.[55]
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 calledPangaea 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.[60]
The first scientist to extrapolate the current motions of the continents was Canadian geologistPaul 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 aboutSiberia until they begin to merge with Asia. He dubbed the resulting supercontinent,Amasia.[61][62] 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.[63]
In an extroversion model, the closure of the Pacific Ocean would be complete in about 350 million years.[64] This marks the completion of the currentsupercontinent cycle, wherein the continents split apart and then rejoin each other about every 400–500 million years.[65] Once the supercontinent is built, plate tectonics may enter a period of inactivity as the rate of subduction drops by anorder 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.[57][64]
The formation of a supercontinent can dramatically affect the environment. The collision of plates will result inmountain building, thereby shifting weather patterns.Sea levels may fall because of increased glaciation.[66] The rate of surfaceweathering 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 rapidbiological evolution as newniches emerge.[67]
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.[68] The planet may then experience a warming period as occurred during theCretaceous period,[67] which marked the split-up of the previousPangaea supercontinent.
The iron-rich core region of the Earth is divided into a 2,440 km (1,520 mi) diameter solidinner core and a 6,960 km (4,320 mi) diameter liquidouter core.[69] The rotation of the Earth creates convective eddies in the outer core region that cause it to function as adynamo.[70] This generates amagnetosphere about the Earth that deflects particles from thesolar wind, which prevents significant erosion of the atmosphere fromsputtering. 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 releasingthermal energy and causing the solid inner core to grow.[71] This ironcrystallization 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.[72] Nearly all of the energy needed to power the dynamo is being supplied by this process of inner core formation.[73]
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 otherheavy elements. The surviving liquid envelope will mainly consist of lighter elements that will undergo less mixing.[74] 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, themagnetic field of the Earth will decay in a geologically short time period of roughly 10,000 years.[75] The loss of themagnetosphere will cause an increase in erosion of light elements, particularlyhydrogen, from the Earth's outer atmosphere into space, resulting in less favorable conditions for life.[76]
The energy generation of the Sun is based uponthermonuclear fusion of hydrogen intohelium. This occurs in the core region of the star using theproton–proton chain reaction process. Because there is noconvection in thesolar 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 thetriple-alpha process, so these atoms do not contribute to the net energy generation that is needed to maintainhydrostatic equilibrium of the Sun.[77]
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.[77]
.svg)
The result of this process has been a steady increase in the energy output of the Sun. When the Sun first became amain sequence star, it radiated only 70% of the currentluminosity. The luminosity has increased in a nearly linear fashion to the present, rising by 1% every 110 million years.[17] 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 thesubgiant stage andevolve into ared giant.[1]
By this time, thecollision 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.[79][80]
The rate of weathering ofsilicate minerals will increase as rising temperatures speed chemical processes up.[81] This, in turn, will decrease the level of carbon dioxide in the atmosphere, as reactions with silicate minerals convert carbon dioxide gas into solidcarbonates.[81] Within the next 600 million years from the present, the concentration of carbon dioxide will fall below the critical threshold needed to sustainC3 photosynthesis: about 50 parts per million. At this point, trees and forests in their current forms will no longer be able to survive.[82] This decline in plant life is likely to be a long-term decline rather than a sharp drop. Plant groups will likely die one by one well before the 50 parts per million level is reached. The first plants to disappear will be C3herbaceous plants, followed bydeciduous forests,evergreen broad-leaf forests and finally evergreenconifers.[81] 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.[81][83][84][85] Researchers atCaltech 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 temperature rise, 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.[86]Currently, C4 plants represent about 5% of Earth's plantbiomass and 1% of its known plant species.[87] For example, about 50% of all grass species (Poaceae) use the C4 photosyntheticpathway,[88] as do many species in the herbaceous familyAmaranthaceae.[89]
When the carbon dioxide levels 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 totectonic activity andrespiration 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 becomessequestered in the Earth.[13] Plants—and, by extension, animals—could survive longer by evolving other strategies such as requiring less carbon dioxide for photosynthetic processes, becomingcarnivorous, adapting todesiccation, orassociating withfungi. These adaptations are likely to appear near the beginning of the moist greenhouse (seefurther).[81]
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. Modeling of the decline in oxygenation predicts that it may drop to 1% of the current atmospheric levels by one billion years from now.[90] This decline will result in lessattenuation ofDNA-damaging UV,[81] as well as the death of animals; the first animals to disappear would be largemammals, followed by small mammals, birds,amphibians and large fish,reptiles and small fish, and finallyinvertebrates.[12]
Before this happens, it is expected that life would concentrate atrefugia of lower temperatures 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's half-life in the atmosphere, animal life would last at most 100 million years after the loss of higher plants.[12] Somecyanobacteria andphytoplankton 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.[12]
In their workThe Life and Death of Planet Earth, authorsPeter D. Ward andDonald 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 theBurgess Shale inBritish Columbia, Canada, to determine the climate of theCambrian 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 withsea life; however, without oxygen replenishment by plant life, they believe that animals would probably die off fromasphyxiation 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 gradualloss of biodiversity.[13]
As temperatures rise, the last of animal life will be driven toward the poles, possibly underground. They would become primarily active during thepolar night,aestivating during thepolar day due to the intense heat. Much of the surface would become a barren desert and life would primarily be found in the oceans.[13] However, due to a decrease in the amount of organic matter entering the oceans from land as well as a decrease indissolved oxygen,[81] sea life would disappear too, following a similar path to that on Earth's surface. This process would start with the loss offreshwater species and conclude with invertebrates,[12] particularly those that do not depend on living plants such astermites or those nearhydrothermal vents such asworms of the genusRiftia.[81] As a result of these processes,multicellular life forms may be extinct in about 800 million years, andeukaryotes in 1.3 billion years, leaving only theprokaryotes.[91]
An alternate scenario that may occur is that, according to a 2024 study inThe Planetary Science Journal, assuming that weathering is only weakly correlated with rising temperatures, multicellular life may survive much longer, beyond 1 billion years for a while.[92] In this scenario, silicate weathering does not increase fast enough to deplete atmospheric carbon dioxide.[92] Such levels would only fall below the threshold for plants utilizing C3 photosynthesis leading to their extinction in 800 million years. However, carbon dioxide levels would remain sustainable for C4 photosynthesis, only declining below present levels in 1.1 billion years.[92] As such, the remaining plant life and subsequently animal life will survive for substantially longer. By 1.6 billion years from now, however, as the Earth's surface temperature rises to about 338 K (65 °C; 149 °F), the maximum tolerance temperature as documented for a symbiont ofDichanthelium lanuginosum, these processes will eventually force weathering to accelerate and will lead to carbon dioxide levels falling below the threshold for C4 plants. The end result would be carbon dioxide starvation, oxygen depletion, and then extinction of the remaining plant and animal life in 1.86 billion years.[92]
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 present day surface reservoir would remain at the surface.[59] Once the solar luminosity is 10% higher than its current value, the averageglobal surface temperature will rise to 320 K (47 °C; 116 °F). The atmosphere will become a "moist greenhouse" leading to arunawayevaporation of the oceans.[93][94] At this point, models of the Earth's future environment demonstrate that thestratosphere would contain increasing levels of water. These water molecules will be broken down throughphotodissociation by solar UV, allowing hydrogen toescape the atmosphere. The net result would be a loss of the world's seawater in about 1 to 1.5 billion years from the present, depending on the model.[95][96][97]
There will be one of two variations of this future warming feedback: the "moist greenhouse" where water vapor dominates thetroposphere 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 theatmosphere (if the oceans evaporate too slowly). In this ocean-free era, there would continue to be surface reservoirs as water is steadily released from the deep crust and mantle,[59] which could contain an amount of water equivalent to several times that present in the Earth's oceans.[59][98] 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 largedunefields covering its equator, and a fewsalt flats on what was once the ocean floor, similar to the ones in theAtacama Desert in Chile.[14]
With no water to serve as a lubricant, plate tectonics would likely stop and the most visible signs of geological activity would beshield volcanoes located above mantlehotspots.[94][81] In these arid conditions the planet may retain some microbial and possibly even multicellular life.[94] Most of these microbes will behalophiles and life could find refuge in the atmosphere ashas been proposed to have happened on Venus.[81] However, the increasingly extreme conditions will likely lead to the extinction of theprokaryotes between 1.6 billion years[91] and 2.8 billion years from now, with the last of them living in residual ponds of water at highlatitudes and heights or in caverns with trapped ice. However, underground life could last longer.[12]
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[14] until the Sun becomes a red giant and its increased luminosity heats the rock to the point of releasing the carbon dioxide.[98] However, as pointed out by Peter Ward and Donald Brownlee in their bookThe 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 water world, and even perhaps drowning all remaining land life.[99]
The loss of the oceans could be delayed until 2 billion years in the future if theatmospheric pressure were to decline. A lower atmospheric pressure would reduce thegreenhouse effect, thereby lowering the surface temperature. This could occur ifnatural processes were to remove the nitrogen from the atmosphere. Studies of organic sediments have shown that at least 100kilopascals (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.[86]
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 (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.[94] If not and there are still pockets of water left, and they evaporate 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,[95] a "runaway greenhouse" effect will ensue, causing the atmosphere to warm 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.[96][94] However, most of the atmosphere is expected to be retained until the Sun has entered the red giant stage.[100]
With the extinction of life, 2.8 billion years from now, it is expected that Earth'sbiosignatures will disappear, to be replaced by signatures caused by non-biological processes.[81]
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 currentluminosity at the age of 12.167 billion years. Most of Earth's atmosphere will be lost to space. Its surface will consist of alava ocean with floating continents of metals and metal oxides andicebergs ofrefractory materials, with its surface temperature reaching more than 2,400 K (2,130 °C; 3,860 °F).[101] The Sun will experience more rapid mass loss, with about 33% of its total mass shed with thesolar 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)).[17]
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 thechromosphere 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 from now.[17]
The drag from the solar atmosphere may cause theorbit 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'sRoche limit, meaning that tidal interaction with Earth would break apart the Moon, turning it into aring system. Most of the orbiting rings will begin to decay, and the debris will impact Earth. Hence, even if the Sun does not swallow the Earth, the planet may be left moonless.[102]
Theablation andvaporization caused by Earth's fall on a decaying trajectory towards the Sun may remove Earth's mantle, leaving just the core, which will finally be destroyed after at most 200 years.[103][104] Earth's sole legacy will be a very slight increase (0.01%) of the solarmetallicity following this event.[105]: IIC
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After fusing helium in its core tocarbon, the Sun will begin to collapse again,evolving into a compactwhite dwarf star after ejecting its outer atmosphere as aplanetary nebula. The predicted final mass is 54% of the present value, most likely consisting primarily of carbon and oxygen.[1]
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 becometidally locked into a larger, stable orbit, with each showing only one face to the other.[106][107][108] Thereafter, the tidal action of the Sun will extractangular momentum from the system, causing theorbit of the Moon to decay and the Earth's rotation to accelerate.[109] In about 65 billion years, it is estimated that the Moon may collide with the Earth, due to the remaining energy of theEarth–Moon system being sapped by the remnant Sun, causing the Moon to slowly move inwards toward the Earth.[110]
Beyond this point, the ultimate fate of the Earth (if it survives) depends on what happens. On a time scale of 1015 (1 quadrillion) years the remaining planets in the Solar System will be ejected from the system by close encounters with other stellar remnants, and Earth will continue to orbit through the galaxy for around 1019 (10 quintillion) years before it is ejected or falls into asupermassive black hole. If Earth is not ejected during a stellar encounter, thenits orbit will decay viagravitational radiation until it collides with the Sun in 1020 (100 quintillion) years.[111] Ifproton decay can occur and Earth is ejected to intergalactic space, then it will last around 1038 (100 undecillion) years before evaporating into radiation.[112]
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