Future of the Earth
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Conjectured illustration of the scorched Earth after the Sun has entered the
red giant phase, 7 billion years from now.
[1]
The biological and geological
future of the Earth can be
extrapolated based upon the estimated effects of several long-term
influences. These include the chemistry at the Earth's surface, the rate
of cooling 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 in this extrapolation is the ongoing influence of technology introduced by humans, such as
geoengineering,
[2] which could cause significant changes to the planet.
[3][4] The current
biotic crisis[5] 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.
[8][9]
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 with diameters of 5–10 km (3.1–6.2 mi) or more, and the possibility of a massive stellar explosion, called a
supernova, within a 100-
light-year radius of the Sun, called a
Near-Earth supernova. Other large-scale geological events are more predictable. If the long-term effects of
global warming are disregarded,
Milankovitch 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
eccentricity,
axial tilt, and
precession of the Earth's orbit.
[10] As part of the ongoing
supercontinent cycle,
plate tectonics will probably result in a
supercontinent
in 250–350 million years. Some time in the next 1.5–4.5 billion years,
the axial tilt of the Earth may begin to undergo chaotic variations,
with changes in the axial tilt of up to 90°.
During the next four billion years, the luminosity of the
Sun will steadily increase, resulting in a rise in the
solar radiation reaching the Earth. This will cause a higher rate of
weathering of
silicate minerals, which will cause a decrease in the level of
carbon dioxide in the atmosphere. In about 600 million years, the level of
CO
2 will fall below the level needed to sustain
C3 carbon fixation photosynthesis used by trees. Some plants use the
C4 carbon fixation method, allowing them to persist at
CO
2 concentrations as low as 10 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 the food chain on Earth.
[citation needed]
In about 1.1 billion years, the solar luminosity will be 10% higher
than at present. This will cause the atmosphere to become a "moist
greenhouse", resulting in a runaway evaporation of the oceans. As a
likely consequence,
plate tectonics will come to an end.
[11] Following this event, the planet's
magnetic dynamo may come to an end, 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 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 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 to cross the planet's current orbit.
Human influence
Humans now play a key role in the
biosphere, with the large
human population dominating many of Earth's
ecosystems.
[3] 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. At current rates, about 30% of species are at risk of extinction in the next hundred years.
[14] The Holocene extinction event is the result of
habitat destruction, the widespread distribution of
invasive species, hunting, and
climate change.
[16]
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.
[4] The concentration of
carbon dioxide in the atmosphere has increased by close to 30% since the start of the
Industrial Revolution.
[3]
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 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 probably be shortened.
[5][17]
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 humanity pose to itself include the
misuse of nanotechnology, a
nuclear holocaust, warfare with a programmed
superintelligence, a
genetically engineered disease, or perhaps 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 also be the possibility of an infestation by an
extraterrestrial lifeform.
[18] The actual odds of these scenarios are difficult if not impossible to deduce.
[8][9]
Should the human race 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.
[9]
Random events
As the Sun orbits the
Milky Way, randomly moving stars may approach close enough to have a disruptive influence on the Solar System.
[19] 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.
[20]
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 at an average of once every 45 million years.
[21] The mean time for the Sun to
collide with another star in the solar neighborhood is approximately
3 × 1013 years, which is much longer than the estimated age of the Milky Way galaxy, at
1–2 × 1010 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 release from the impact of an
asteroid or comet with a diameter of 5–10 km (3.1–6.2 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, which lowers land
temperatures by about 15 °C (27 °F) within a week and halts
photosynthesis
for several months. 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
5–6 mass extinctions and 20–30 lower severity events. This matches the
geologic record of significant extinctions during the
Phanerozoic Eon. Such events can be expected to continue into the future.
[23]
A
supernova is a cataclysmic explosion of a star. Within the Milky Way galaxy, supernova explosions occur on average once every 40 years.
[24]
During the history of the Earth, multiple such events have likely
occurred within a distance of 100 light years. Explosions inside this
distance can contaminate the planet with
radioisotopes and possibly impact the biosphere.
[25] 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 radiation from the Sun. An increase in
UV-B radiation of only 10–30% is sufficient to cause a significant impact to 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.
[27]
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 millions 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.
[28][29]
During the same interval, the odds that the Earth will be scattered out
of the Solar System by a passing star are on the order of one part in
10
5. In such a scenario, the oceans would freeze solid within
several million years, leaving only a few pockets of liquid water about
14 km (8.7 mi) underground. There is a remote chance that the 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 one chance in three million.
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.
[10][31]
Glaciation
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
summer solstice with the
aphelion.
[10]
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.
[35][36] 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.
[37]
The 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 of
carbon dioxide release into the
atmosphere by humans 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 through a few centuries worth of
greenhouse gas emission would only have a limited impact in the long term.
[10]
Obliquity
The rotational offset of the tidal bulge exerts a net
torque on the Moon, boosting it while slowing the Earth's rotation. This image is 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.
[38]
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.
[39] 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.
[40] (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 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 within about 1.5–4.5 billion years, although the
exact time is unknown.
[41]
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 reaches 54°, the equator will receive
less radiation from the Sun than the poles. The planet could remain at
an obliquity of 60° to 90° for periods as long as 10 million years.
[42]
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 magnitude 9 events 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.
[9]
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.
[29] 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.
[9]
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 generation within the planet and the presence of a
hydrosphere. With the loss of either of these, continental drift will come to a halt.
[43] 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.
[44]
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.
[46][47]
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, causing a
supercontinent composed of Africa, Eurasia, Australia, Antarctica and South America to form around
Antarctica.
[48]
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.
[49]
Introversion
A rough approximation of
Pangaea Ultima, one of the three 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. 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 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 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.
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.
[53]
In an extroversion model, the closure of the Pacific Ocean would be complete in about 350 million years.
[54] 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.
[55] 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 (30–100 °C) every 100 million years,
which is the minimum lifetime of past supercontinents. As a
consequence,
volcanic activity may increase.
[47][54]
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, 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. 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.
Solidification of the outer core
The iron-rich core region of the Earth is divided into a 1,220 km (760 mi) radius solid
inner core and a 3,480 km (2,160 mi) radius liquid
outer core.
[59] 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.
[61]
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.
[62] Nearly all of the energy needed to power the dynamo is being supplied by this process of inner core formation.
[63]
The growth of the inner core may be expected to consume most of the
outer core by some 3–4 billion years from now, resulting in a nearly
solid 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
comes to an end, the interior will cool less efficiently, which may end
the growth of the inner core. 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.
[66]
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
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.
[67]
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 decrease, 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.
[67]
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.
[69]
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 pass through the
subgiant stage and
evolve into a
red giant.
[1]
Climate impact
Once the core hydrogen is exhausted in 5.4 billion years, the Sun will expand into a
subgiant
phase and slowly double in size over about half a billion years. It
will then expand more rapidly over about half a billion years until it
is over two hundred times larger than today. With the increased surface
area of the Sun, the amount of energy emitted will increase. Unless
Earth's orbital radius increases significantly, the global temperature
of the Earth will climb because of the rising luminosity of the Sun.
However, geoscientist
James Kasting says the oceans may evaporate much earlier, in about a billion years.
[70]
The rate of
weathering of
silicate minerals will increase. This in turn will decrease the level of
carbon dioxide in the atmosphere, as these weathering processes convert carbon dioxide into
carbonate materials. Within the next 600 million years from the present, the concentration of
CO
2 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,
[71] the last living trees being evergreen
conifers.
[72] However,
C4 carbon fixation can continue at much lower concentrations, down to above 10 parts per million. Thus plants using C
4
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.
[73][74][75] Currently, C
4 plants represent about 5% of Earth's plant biomass and 1% of its known plant species.
[76] For example, about 50% of all grass species (
Poaceae) use the C
4 photosynthetic
pathway, as do many species in the herbaceous family
Amaranthaceae.
[78]
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 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. Some microbes are capable of photosynthesis at concentrations of
CO
2 of a few parts per million, so these life forms would
probably disappear only because of rising temperatures and the loss of
the biosphere.
[73]
Plants—and, by extension, animals—could survive longer by evolving other strategies such as requiring less
CO
2 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
below).
[72]
The loss of plant life will also result in the eventual loss of
oxygen as well as ozone due to chemical reactions in the atmosphere and
volcanic eruptions, meaning less attenuation of
DNA-damaging
ultraviolet radiation,
[72] as well as the death of animals; the first animals to disappear would be large
mammals and fish, followed by small mammals, fish and birds,
amphibians,
reptiles, and finally
invertebrates.
[80]
In their work
The Life and Death of Planet Earth, authors
Peter D. Ward and
Donald Brownlee
have also argued that some form of animal life may continue even after
most of the Earth's plant life has disappeared. Initially, they expect
that some insects, lizards, birds and small mammals may persist, along
with sea life. Without oxygen replenishment by plant life, however, they
believe that the 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 animal life
will inevitably be driven back toward the poles, and possibly even
underground. Much of the surface would become a barren desert and life
would primarily be found in the oceans;
however, due also to a decrease of the amount or organic matter coming
to the oceans from the land as well as oxygen in the water,
[72]
life would disappear there too following a similar path to that on
Earth's surface with invertebrates being the last living animals,
[80] and of them those that do not depend on living plants such as
termites or those near
hydrothermal vents such as
worms of the genus
Riftia[72] As a result of these processes, multi-cellular lifeforms may be extinct in about 800 million years, and
eukaryotes in 1.3 billion years from now, leaving only the
prokaryotes.
[81]
Loss of oceans
By 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.
[82]
Once the solar luminosity is 10% higher than its current value, the
average global surface temperature will rise to 320 K (47 °C). The
atmosphere will become a "moist greenhouse" leading to a
runaway evaporation of the oceans.
[83] 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 by about 1.1 billion years from
the present.
[85][86]
The atmosphere of Venus is in a "supergreenhouse" state.
In this ocean-free era, there will continue to be reservoirs at the
surface as water is steadily released from the deep crust and mantle,
[49] where it is estimated 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 dry desert with
large
dunefields covering its equator, resembling how
Saturn's largest moon
Titan looks today.
[11] Even in these arid conditions, the planet may retain some microbial and possibly even multi-cellular life. Most of these microbes will be
halophiles. However, the increasingly extreme conditions will likely lead to the extinction of the prokaryotes between 1.6 billion years
[81] and 2.8 billion years
[80] from now, with the last of them living in residual ponds of water at high
latitudes and heights or in caverns with trapped ice; underground life, however, could last longer.
[80]
What happens next depends on the level of tectonic activity. A steady
release of carbon dioxide by volcanic eruption could eventually cause
the atmosphere to enter a "supergreenhouse" state like that of the
planet
Venus.
But without surface water, plate tectonics would probably come to a
halt and most of the carbonates would remain securely buried
[11] until the Sun became a red giant and its increased luminosity heated the rock to the point of releasing the carbon dioxide.
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 (0.99
atm)
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, unless most of
Earth's surface water has been lost by that time, in which case the
planet will stay in the same conditions until the starting of the red
giant phase, the amount of water in the lower atmosphere will have risen to 40% and the runaway moist greenhouse will commence
[88] when the luminosity from the Sun reaches 35–40% more than its current value, 3–4 billion years from now.
[85] The atmosphere will heat up and the surface temperature will rise sufficiently to melt surface rock.
[86] However, most of the atmosphere will be retained until the Sun has entered the red giant stage.
[89]
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 at its core to burning
hydrogen around its shell, 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 the Sun's
current
luminosity
at the age of 12.167 billion years. During this phase 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 the Earth will increase to at most 150% of its
current value.
[69]
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 (180,000,000
km).
The Earth will interact tidally with the Sun's outer atmosphere, which
would serve to decrease Earth's orbital radius. Drag from the
chromosphere of the Sun would also reduce the
Earth's orbit.
These effects will act to counterbalance the effect of mass loss by the
Sun, and the Earth will most likely be engulfed by the Sun.
[69] The
ablation and
vaporization caused by its fall on a decaying trajectory towards the Sun will remove Earth's
crust and
mantle, then finally destroy it after at most 200 years.
[90] Earth's sole legacy will be a very slight increase (0.01%) of the solar
metallicity.
[91]
Before this happens, most of Earth's atmosphere will have been lost
to space and its surface will consist of a magma ocean with floating
continents of metals and metal oxides as well as
icebergs of
refractory materials, with its surface temperature reaching more than 2,400 K (2,130 °C).
[92]
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 the Earth's
Roche limit.
This means that tidal interaction with the Earth would break apart the
Moon, turning it into a ring system. Most of the orbiting ring will then
begin to decay, and the debris will impact the Earth. Hence, even if
the Earth is not swallowed up by the Sun, the planet may be left
moonless.
[93]
Another scenario is that Earth somehow survives being engulfed to the
Sun, but that the ablation and vaporization mentioned above will strip
both its crust and mantle leaving just its
core.
[94]
Post red-giant stage
The
Ring 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.
Any planets that survive this process will continue to orbit this body,
but will receive little thermal radiation and become frigid bodies.
Over time intervals of around every 30 trillion years, the Sun will
undergo a close encounter with another star. As a consequence, the
orbits of their planets can become disrupted, potentially ejecting them
from the system entirely.
[95]
Should the Earth have survived these calamities, over a time of about 10
20 (100 quintillion) years, the planet's orbit will steadily decay through emission of
gravitational radiation, before finally being ripped apart by
tidal forces at a distance of around 1-6 radii from the Sun.
