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Future geography

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Also see: Geology

Geography is, so to speak, the stage on which life acts, directly shaping the fortunes of species and clades. For any project in speculative biology, the geography of a world needs to be carefully considered, and so does the way it changes in time (planets conductive to life will most probably have tectonic activity to change the position and nature of continents). A particular case occurs in future evolution, as the geography of Earth in the next millions of years directly follows from what it's like now; to a certain measure, it's possible to predict it.

Future ice ages

The Earth is currently (since 2.5 million years ago) passing through the Quaternary Ice Age, an unusual phase of its history where a significant amount of water is locked as polar ice, with a global average temperature below 15°C. During glaciations, temperature drops even further, and glaciers can extend up to 45 degrees of latitude, halfway between the Poles and the Equator.

The last glaciation ended 12,000 years ago, and now Earth experiences an interglacial period of the ice age. According to a theory, glaciations are triggered by the Milankovitch cycles, variations of Earth's orbit over hundreds of thousands of years, which would locate the start of the next glaciation about 15 000-25 000 years hence. Anyway, the amount of carbon dioxide released into the atmosphere by human activities, if not removed, might delay it until 50 000-130 000 years hence. The end of that glaciation will probably come 100-200 000 years after its beginning.

World18000

Modern Earth would look like this during a glaciation. (C) C. R. Scotese.

Next ice ages are less predictable, as they can depend from the position of continents: a land mass located on a pole (such as Gondwana in the late Carboniferous or Antarctica from the late Paleogene) can trigger an ice age by allowing glaciers to spread further, as land sustains more ice than water.

Besides glaciations, the mechanics of Earth's orbit have another important consequence: as the Moon takes angular momentum away from the Earth, the distance between the two bodies increases, and rotation slows down. This effect makes the day 36 minutes longer each 100 million years, or an hour every 170 million years.

Continental drift

50 mya map1

Earth 50 Ma in the future. (C) C. R. Scotese.

World150

Earth 150 Ma in the future. (C) C. R. Scotese.

More information about historical and future continental drift here.

The current known direction and speed of movement of the tectonic plates allows for a cautious forecast of what Earth will look like in several million years.

In the next 50 million years, for example, we're almost certai that the Mediterranean will close as Africa moves northward (and will be replaced by a huge mountain range similar to Himalaya, extended from Portugal to Arabia); Australia will merge with Indonesia; California will break up from North America, sliding north, while other mountain ranges will grow on the eastern coast of the Americas due to subduction; Africa will fissure along the Great Rift Valley, producing a large Madagascar-like island in the Indian Ocean that Dixon called "Lemuria".

In a subsequent phase, Antarctica will move away from the South Pole, losing its glaciers; this will increase the temperature and, along with the ice melting in Greenland, will raise the sea-level of 90 metres, flooding lowland areas such as deltas and floodplains. Earth might return to the warm and moist climate of Cretaceous, with roughly equal temperatures on all the planet, and the new forests (especially on a newly tropical Antarctica) will greatly increase the amount of oxygen in the atmosphere. At this point, the Atlantic Ocean will reach its greatest width; high temperatures and fragmentation will bring biodiversity to a peak. On the other hand, such a situation could stop ocean circulation, causing anoxia in deep waters and blocking the carbon cycle. It's also suspected an alteration of tectonics that woyld cause catastrophic basaltic floods.

The next supercontinent

Also see: Pangea 2
Pangea utilma

Pangaea Ultima, one of the models of the future supercontinent. (C) C. R. Scotese.

and here.

Roughly every 500 million years the motion of continents brings them to collide and merge in a single supercontinent that includes the great majority of land area on the planet. Since the moment of maximum cohesion of the last one was 250 million years ago, we can predict that the next one will start forming 200 Ma in the future, and reach maximum cohesion 250 Ma in the future.

There are three models of this next supercontinent:

  • Amasia: This model was developed by Chris Hartnady in 1992, from the assumption that the Atlantic Ocean will keep growing ("extroversion scenery"). As the Americas swing clockwise, the coast of South America will collide with Australia, closing the Pacific Ocean. Amasia is the mass resulting from the fusion of Eurasia and America, with Africa lying along southern Asia and Antarctica isolated at south (immobile, as its' not close to any subduction zone).
  • Novopangaea: In the late 1990s, Roy Livermore added to Hartnady's model a new rift between the Indian Ocean and the North Atlantic, causing Africa to move eastwards, colliding with Southeast Asia and Australia; Antarctica moves northwards and wedges itself between Asia and South America, creating a vast Himalaya-like plateau. For the rest, the result is very similar to Amasia.
  • Pangaea Ultima or Pangaea Proxima: According to Christopher Scotese (author of the maps shown in this page), the Atlantic expansion will start to invert 100 Ma in the future, bringing the Old World and the New World close again. Strong subduction on the western coast will create a massive mountain range, similar to the Andes, on the eastern coast of the Americas, that will be further increased by the collision with Europe and Africa. Antarctica and Australia will once again move southwards; a closed inner sea might form from the remains of the Indian Ocean.

The consequences of a supercontinent on climate and life are quite extreme. The inland is so far from the sea that barely any rain reaches it, creating a vast desert; biodiversity on the coasts decreases due to competition and amalgamation, while the lesser circulation in the ocean produces a strong anoxia even in surface water. The coasts could see monsoons and hurricanes due to the large temperature difference between the ocean and the hot inland, while the reduced subduction could cause the warming of the mantle and thus increased volcanic activity. This sort of environment is explored in TFIW's last segment and in a chapter of Life and Death of Planet Earth.

After Pangaea Proxima's breakup, it's probably impossible to predict the motion of continents. Radioactive decay in the core will power continental drift for about 1.1 billion years, enough for the breakup of Pangaea Proxima, the formation of another supercontinent and probably the breakup of this one (but see below for the conditions in this time period).

Solar evolution

Solar evolution

Development of the Sun's relative luminosity (red), radius (purple) and temperature (green).

Also see: The Life and Death of Planet Earth

As the Sun grows older, more and more hydrogen in its core is fused into helium, exhausting its "fuel", and thereby decreasing the radiation pressure outwards. As the core is thus crushed by gravity, its temperature rises and, paradoxically, causes the external layers to become larger and brighter. The current increase in luminosity is 1% every 110 million years.

According to Peter Ward, the incrase of solar energy will wreak havoc on Earth's ecosystems as the temperature grows higher and higher, by increasing weathering of silicate minerals and trapping carbon dioxide. Normally, this would reduce the greenhouse effect and lower the temperature again, but as the Sun becomes steadily brighter all the carbon dioxide is removed from the atmosphere. Plants whose photosynthesis employs C3 carbon fixation die put when the carbon dioxide falls under 50 ppm (600 Ma hence), while those with C4 carbon fixation, such as most grasses, survive down to 10 ppm (800-900 Ma hence). Algae and photosynthetic bacteria might survive slightly longer, but oxygen-breathing organisms won't survive for much time after the extinction of plants.

Time from today Solar luminosity Global temperature Day length Supercontinent cycle
+0 Ma 1.00 15°C 24 h Breakup
+250 Ma 1.02 25°C 25.5 h Supercontinent
+500 Ma 1.05 40°C 27 h Breakup
+750 Ma 1.07 50°C 28.5 h Supercontinent
+1000 Ma 1.09 70°C 30 h Breakup
+1250 Ma 1.12 > 100°C 32.5 h (c. drift stopped)

A billion years in the future, the continental drift will have all but stopped, as Earth's core will have dissipated its radioactive heat. With the continents motionless, subduction will trap water in the mantle, while the temperature will rise above 70°C, causing the oceans to evaporate. As water molecules are split by radiations, the hydrogen will leak in space, while the oxygen will form a thick layer around Earth, increasing the temperature by hundreds of degrees; while prokaryotes might still extist in extreme conditions, the last ones will probably die out 1300-1500 million years hence, as the last traces of water disappear.

Red giant Sun

Sun red giant

The Sun as a red giant, 7 billion years in the future.

Once the hydrogen in the Sun's core will be depleted, it will contract quickly, and the outer shell will expand. The solar wind will disperse about one third of its mass, pushing the planetary orbit outwards. The expansion will continue until 7-8 billion years in the future, when the Sun will be 12 billion years old and with a radius of 1-1.2 AU (an AU is the current average distance between the Sun and Earth).

While its mass is too small for it to explode as a supernova, the Sun will most likely engulf Mercury and Venus' orbits, while the Earth could survive if the solar wind pushes it far enough; however, its surface will be an ocean of molten magma with floating metal oxides, with a temperature above 2000°C. If the Earth and the Moon survive beyond this point, the Moon will cross its Roche limit, 18 470 km from Earth, and will breakup into a ring of debris.

After 8 billion years in the future, the Sun will start contracting and cooling down. The surface temperature of every planet in the Solar System will drop well below -200°C as the Sun becomes a small white dwarf, gradually becoming colder and dimmer over the next several billion years.

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