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A dark gray and red sphere representing the scorched Earth lies against a black background to the right of a red circular object representing the Sun.
Conjectured illustration of the scorched Earth after the Sun has entered the red giant phase, seven billion years from now.[1]

The future of the Earth will be determined by a variety of factors, including increases in the luminosity of the Sun, loss of heat energy from the Earth's core, perturbations by the other bodies in the Solar System and the biochemistry at the Earth's surface. Milankovitch theory predicts the planet will continue to undergo glaciation cycles because of eccentricity, axial tilt, and precession of the Earth's orbit. As part of the ongoing supercontinent cycle, plate tectonics will likely result in a supercontinent in 250 million–350 million years. Some time in the next 1.5 billion–4.5 billion years, the obliquity of the Earth may begin to undergo chaotic variations, with changes in the axial tilt of up to 90°.

One billion to two billion years in the future, the steady increase in solar radiation caused by the helium buildup at the core of the Sun will result in the loss of the oceans and the cessation of continental drift. In four billion years from now, the increase in the Earth's surface temperature will cause a runaway greenhouse effect. By that point, most if not all the life on the surface will be extinct. The most likely ultimate 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 to cross the planet's orbit.

Contents

Human influence

Humans now play a key role in the biosphere, with the large human population dominating many of Earth's ecosystems.[2] This has resulted in a widespread, ongoing 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.[3] At current rates, about 30% of species are at risk of extinction in the next hundred years.[4] The Holocene extinction event is the result of habitat destruction, the widespread distribution of invasive species, and hunting and climate change.[5][6] 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.[7] The concentration of carbon dioxide in the atmosphere has increased by close to 30% since the start of the Industrial Revolution.[2]

The consequences of a persistent biotic crisis have been predicted to last for at least five million years.[8] 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 also 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. However, no new species of existing large vertebrates are likely to arise and food chains will likely be shorter.[9][10]

Orbit and rotation

The gravitational perturbations of the other planets in the Solar System combine to modify the orbit of the Earth and the orientation of its spin axis. These changes can influence the planetary climate.[11][12][13][14]

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Glaciation

Historically, there have been cyclical periods of glaciation in which ice sheets covered the higher latitudes of the continents. The Milankovitch theory predicts that glaciation occurs because of astronomical factors in combination with climate feedback mechanisms and plate tectonics. The primary astronomical drivers are a higher than normal orbital eccentricity, a low axial tilt (or obliquity), and the alignment of summer solstice with the aphelion.[12] 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.[15][16] 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.[17]

At present the Earth is in an interglacial period, which would normally be expected to end in about 25,000 years.[14] The current rate of increased carbon dioxide release into the atmosphere by humans may delay the onset of the next period of glaciation until at least 50,000–130,000 years from now. However, a global warming period of finite duration (based on the assumption that fossil fuel use will cease by the year 2200) will likely only impact the glaciation cycle for about 5,000 years. Thus, a brief period of global warming induced through a few centuries worth of greenhouse gas emission would only have a limited impact in the long term.[12]

Obliquity

A small gray circle at the top represents the Moon. A green circle centered in a blue ellipse represents the Earth and its oceans. A curved arrow shows the direction of the Earth's rotation, resulting in the long axis of the ellipse being slightly out of alignment with the Moon.
The rotational offset of the tidal bulge exerts a net torque on the Moon, boosting it while slowing the Earth's rotation.

The tidal acceleration of the Moon slows the rotation rate of the Earth and increases the Earth-Moon distance. Other effects that can dissipate the Earth's rotational energy are friction between the core and mantle, tides in the atmosphere, convection in the mantle, and climate changes that can increase or decrease the ice load at the poles. 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.[18]

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.[19] This stability is achieved because the Moon increases the precession rate of the Earth's spin axis, thereby avoiding resonances between the precession of the spin and precession frequencies of the ascending node of the planet's orbit.[20] (That is, the precession motion of the ecliptic.) However, as the semimajor axis of the Moon's orbit continues to increase in the future, this stabilizing effect will diminish. At some point perturbation effects will likely 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 within about 1.5–4.5 billion years, although the exact time is unknown.[21]

A high obliquity would likely result in dramatic changes in the climate and may destroy the planet's habitability.[13] When the axial tilt of the Earth reaches 54°, the equator will receive less radiation from the Sun than the poles. The planet could remain at an obiliquity of 60° to 90° for periods as long as 10 million years.[22]

Plate tectonics

An irregular green shape against a blue background represents Pangaea.
Pangaea was the last supercontinent to form before the present.

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 generation within the planet and the presence of a hydrosphere. With the loss of either of these, continental drift will come to a halt.[23] 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.[24]

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.[25] 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.[26][27]

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, causing a supercontinent to form around Antarctica.[28]

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 a point of stability after 65% of the current ocean mass has been subducted.[29]

Introversion

Christopher Scotese and his colleagues have mapped out the predicted motions several hundred million years into the future as part of the Paleomap Project.[25] 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. To the south, 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 metres (300 ft). The inland flooding of the continents will result in climate changes.[25]

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 while 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. The continent of Antarctica will reverse direction and return to the South Pole, building up a new ice cap.[30]

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 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.[31][32] 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.[33]

In an extroversion model, the closure of the Pacific Ocean would be complete by about 350 million years.[34] 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.[35] 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 K every 100 million years, which is the minimum lifetime of past supercontinents. As a consequence, volcanic activity may increase.[27][34]

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.[36] The rate of surface weathering can rise, resulting in an increase in the rate that organic material is buried. Supercontinents can cause a drop in global temperatures and an increase in atmospheric oxygen. These changes can result in more rapid biological evolution as new niches emerge. This, in turn, can affect the climate, further lowering temperatures.[37]

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.[38] The planet may then experience a warming period, as occurred during the Cretaceous period.[37]

Solar evolution

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 fusion process results in a steady buildup of helium. 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.[39]

At present, nearly half the hydrogen at the core has been consumed, with the remainder consisting primarily of helium. To compensate for the steadily decreasing number of hydrogen atoms per unit mass, the core temperature of the Sun has gradually increased through a rise in pressure. This has caused the remaining hydrogen to undergo fusion at a more rapid rate, thereby generating the energy needed to maintain the equilibrium. The result has been a steady increase in the energy output of the Sun. This increase can be approximated by the formula:

L(t)\ =\ \left[ 1 + \frac{2}{5} \left( 1 - \frac{t}{t_{Sun}} \right) \right]^{-1} L_{Sun}

where t is a time period less than or equal to the present time tSun, L(t) is the luminosity at time t, and LSun is the current solar luminosity.[39]

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, increasing by 1% every 110 million years.[40] 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 4.8 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 increase in luminosity reaches 121% of the present value. This marks the end of the Sun's main sequence lifetime, and thereafter it will evolve into a red giant.[1]

Climate impact

As the global temperature of the Earth climbs because of the rising luminosity of the Sun, the rate of weathering of silicate minerals will increase. This in turn will decrease the level of carbon dioxide in the atmosphere. Within the next 600 million years from the present, the concentration of CO2 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.[41] However, C4 carbon fixation can continue at much lower concentrations, down to above 10 parts per million. Thus plants using C4 photosynthesis will be able to survive until about a billion years from now.[42][43][44] Currently, C4 plants represent about 5% of Earth's plant biomass and 1% of its known plant species.[45] For example, about 50% of all grass species (Poaceae) use the C4 photosynthetic pathway,[46] as do many species in the herbaceous family Amaranthaceae.[47]

Some microbes are capable of photosynthesis at concentrations of CO2 of a few parts per million, so these life forms would likely disappear only because of rising temperatures and the loss of the biosphere.[42]

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. Initially, they believe that some insects, lizards, birds and small mammals may persist, along with sea life. Without oxygen replenishment by plant life, however, the animals would likely 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. Much of the surface would become a barren desert and life would primarily be found in the oceans.[48]

Once the solar luminosity is 10% higher than its current value, the average global surface temperature reaches 320 K (47 °C). The atmosphere will become a humid greenhouse leading to a runaway evaporation of the oceans.[49] 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 ultraviolet radiation, allowing hydrogen to escape the atmosphere. The net result would be a loss of the world's sea water in about 1.1 billion years from the present.[50][51]

Light brown clouds wrap around a planet, as seen from space.
The atmosphere of Venus is in a "supergreenhouse" state.

Still, there will continue to be some reservoirs at the surface as water is steadily released from the deep crust and mantle.[52] Some water may be retained at the poles and there may be occasional rainstorms, but for the most part the planet would be a dry desert. What happens next depends on the level of tectonic activity. The release of carbon dioxide by volcanic eruption may eventually cause the atmosphere to enter a "supergreenhouse" state like that of the planet Venus. However, without surface water, plate tectonics would likely come to a halt and most of the carbonates would remain securely buried.[53]

The loss of the oceans could be delayed until two billion years in the future if the total 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 has shown that at least 100 kilopascals (1 bar) of nitrogen has been removed from the atmosphere over the past four billion years; 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. However, beyond that point, the amount of water in the lower atmosphere will have risen to 40% and the runaway moist greenhouse will commence.[54]

When the luminosity from the Sun reaches 40% of its current value four billion years from now, a runaway greenhouse effect will take place.[50] The atmosphere will heat up and the surface temperature will rise.[51] However, most of the atmosphere will be retained until the Sun has entered the red giant stage.[55]

Red giant stage

A large red disk represents the Sun. An inset box shows the current Sun as a yellow dot.
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 at the core to burning hydrogen around a shell, the core will start to contract and the outer envelope will expand. The total luminosity will steadily increase over the next billion years until it reaches 2,730 times the Sun's current luminosity at the age of 12.167 billion years. During this phase the Sun will undergo 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 the Earth will increase to at most 150% of its current value.[40]

The most rapid part of the Sun's expansion into a red giant occurs during the final stages, when the Sun is about 12 billion years old. It is likely to expand to swallow both Mercury and Venus, reaching a maximum radius of 1.2 astronomical units (180 Gm). The Earth will interact tidally with the Sun's outer atmosphere, which would serve to decrease the orbital radius. Drag from the chromosphere of the Sun would also reduce the Earth's orbit. These effects will act to counterbalance the mass loss by the Sun, and the Earth will most likely be engulfed by the sun.[40]

See also

References

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