What would be the first thing which will render the Earth uninhabitable?

Question

There are quite a few things which can make an otherwise habitable planet uninhabitable, and some of these will eventually happen, sooner or later:

1. engulfed by its star when it becomes a red giant
2. destroyed by the nova or supernova of its star
3. loses its magnetosphere and the solar wind blows its atmosphere away
4. collides with a large enough object
5. gamma ray burst
6. maybe other astronomical phenomena?

some of these will inevitably happen to Earth in the future, but which one is to be expected to happen sooner? By uninhabitable I mean uninhabitable to currently known multicellular life.

For the sake of this question, assume that humans don't invent technology in the future which would stop this from happening, and also no extraterrestrial visitors, supernatural forces or similar effects intervene. Actually, let's assume that humans magically vanish today, just to not get distracted by the effects of human technology.

Answer 1

I'll try to put time scales on each of these events.

1. The Sun as a red giant - 5 to 6 billion years$^1$

   The Sun is currently on the main sequence, which means that it's a "full grown" star - think of it as being middle-aged. It's been on the main sequence for around 4 to 5 billion years, and in 5 to 6 billion years, it will leave the main sequence.

   Most yellow (i.e. spectral class G-ish) stars spend about 10 billion years on the main sequence. Then, they swell up as red giants. The Sun is just over halfway through the main sequence, so it has about 5 to 6 billion years left. After 5.4 billion years, it will grow. Here's an image showing how its radius (and other factors) will increase:

   

   The Sun won't swallow the inner planets up right away - see how it takes a while for it to grow - but it will happen eventually. It will take another billion years to reach a radius 200 times that of its current size. It will enter the AGB phase, where it will finally grow enough to swallow the inner planets. It's uncertain as to whether or not Earth will be destroyed, but it will be heated up so much that liquid water will disappear and the planet will be uninhabitable.

   NASA has an article on a Sun-like star swallowing up one of its inner planets, just as the Sun may do. The original paper can be found here.

2. Supernova/nova - Never

   There are two types of supernovae: Type II and Type I. Type II supernovae are characterized by hydrogen; Type I supernovae have no hydrogen. You've probably heard of Type II supernovae when you learned about supernovae, and they're the more well-known. They're the result of the collapse of the core of a massive star. However, the progenitor, has t be much larger than the Sun - usually at least 8 solar masses.

   Type I supernovae have no hydrogen. There are two main classes: Type Ia and Type Ib/c. The first category are the result of white dwarfs accreting mass from a companion star. The Sun has no binary companion, so even though it will become a white dwarf, it will never undergo this kind of supernova. Type Ib/c also result from core collapse, just like Type II. However, their progenitors are unstable, violent stars, typically Wolf-Rayet stars. Our Sun will fortunately never become one of those!

   A nova is small compared to a supernova. It, like Type Ia supernovae, may be the result of a white dwarf accreting matter from a companion. Once again, though, the Sun has no companion star to accrete matter from.

3. Magnetosphere loss - Never

   The Earth's magnetic field is very important, because it shields us from the solar wind. Without it, life on Earth probably couldn't exist! Its disappearance would not be good for us.

   Fortunately, it doesn't appear that that will ever happen.

   The Earth's magnetic field is believed to be caused by the movement of fluids in Earth's core. Wikipedia lists three conditions necessary for the existence of the magnetic field:

   1. An electrically conductive fluid medium

   2. Kinetic energy provided by planetary rotation

   3. An internal energy source to drive convective motions within the fluid.

   The fluid in the Earth's core will never go away, and so long as the planet keeps rotating, that kinetic energy will always be there. So will the Coriolis effect, which also helps circulate the fluid and control the magnetic field. The source of the energy that keeps the material hot is due in part to tidal forces between the Earth and other bodies, such as the Moon and the Sun.

4. Collision - Not for a long time

   The Earth has had one major collision throughout its life, and that spawned the moon. This idea is known as the giant impact hypothesis. When the solar system - and Earth - was young, the Sun had a disk of small pre-planetary bodies, known as protoplanets. There were more protoplanets than there are planets today, because many collided to form the planets.

   The hypothesized protoplanet, named Theia, is generally described as Mars-like. It didn't hit the Earth head-on, but rather hit it an a 45 degree angle. The result was that Theia was destroyed and some of Earth was ejected. Much of this material came back to Earth, but some stayed and formed the Moon.

   There aren't any more protoplanets, but there are still some large asteroids. The biggest is Ceres, which is tiny compared to Theia. Here's a picture of Earth, the Moon and Ceres (bottom left):

   

   We're not likely to see another giant impact again. The other planets all have stable orbits, and the Moon's actually receding from Earth due to tidal forces. Sure, asteroid impacts are a possibility, but the possibility of a doomsday asteroid is tiny. Even a body like the one that killed the dinosaurs wouldn't extinguish all life. The Jet Propulsion Laboratory has information on the likelihood of such a catastrophic event (not a high one!).

5. Gamma-ray burst - Probably never

   I wrote an answer on Astronomy about this a while ago. If I may, I'll quote from that answer:

   Okay, back to the topic at hand. Wikipedia, of course, has a short tidbit on the frequency of gamma-ray bursts effecting Earth:

   Estimating the exact rate at which GRBs occur is difficult, but for a galaxy of approximately the same size as the Milky Way, the expected rate (for long-duration GRBs) is about one burst every 100,000 to 1,000,000 years. Only a small percentage of these would be beamed towards Earth. Estimates of rate of occurrence of short-duration GRBs are even more uncertain because of the unknown degree of collimation, but are probably comparable.

   'A small percentage' isn't too specific. The BBC article that is cited in this passage says the following:

   Observations of deep space suggest that gamma ray-bursts are rare. They are thought to happen at the most every 10,000 years per galaxy, and at the least every million years per galaxy.

   However, the 'small percentage' is never elaborated on.

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$^1$An estimate

Earth will become uninhabitable long before the Sun swallows it up - if it is swallowed up - because its surface temperature will be so high life cannot exist. I wanted to see if I could figure out when that would be.

You can approximate the temperature of a planet by using the formula for effective temperature: $$T=\left(\frac{L(1-a)}{16 \pi \sigma D^2} \right)^{\frac{1}{4}}$$ where $L$ is the star's luminosity, $a$ is the planet's albedo $\sigma$ is the Stefan-Boltzmann constant, and $D$ is the distance from the planet to the star.

The current mean temperature of Earth is about 15 degrees Celsius (288 K). However, this is the average surface temperature. Fortunately, the equation given for surface temperature is also of the form $$T=C L^{\frac{1}{4}}$$ where $C$ is a constant. This means that $$T^4 \propto L$$ The boiling point of water is 373 K, which is 1.295 times the current surface temperature. If we plug this in, $$L= \propto (1.295)^{\frac{1}{4}} \approx. 2.81$$ So the luminosity of the Sun would have to be about 2.81 times the current luminosity of the Sun. This value isn't on the graph I gave at the top, though it appears to be about 7 billion years from now. Wikipedia gives this point as only 1 billion years from now, because of a greenhouse effect. So we've only got 1 billion years to live!

Answer 2

The end of multicellular life (and of life itself) has been the subject of many documentaries, many of which are based on scientific observations and associated assumptions of the past in the rocks and out in space. Some are outright bizarre imaginings of people. But, one important aspect is that we need to look to the past evidence on Earth to gauge the likelihood of an event occurring in the future - though, given the fact that we are still exploring the mechanisms behind these past events, it is hard to say which may happen next.

The first place to look are the mechanisms of past mass-extinctions on Earth. A few examples are below:

• An asteroid/comet impact - which is a widely accepted theory of the cause of the Cretaceous-Neogene extinction event that wiped out a significant proportion of life 65 million years ago, including most of the dinosaurs. This potentially could occur at anytime with little or no warning.

• Mass volcanism - such as the formation of the Siberian Traps, what is thought by many scientists to have caused the extensive extinction event (over 90% life forms extinct) at the end of the Permian period around 250 million years ago. The onset of these may be detected.

• Gamma ray burst - which some scientists hypothesise, through computer modelling, may caused a mass extinction at the end of the Ordovician period, around 440 million years ago. This is very conjectural, but may occur without warning.

• Nebula Winters, a relatively recent theory presented in the journal Gondwana Research, in that encounters with astronomical nebulae can be (and according to theory in the article, has been) be catastrophic for life on the planet. Including their theory that a starburst may have caused the Snowball Earth global glaciation events.

Some of these events can be sudden, without warning, whereas some can occur concurrently and their effects take some time - for example, the extinction event at the end of the Permian is theorised to have had multiple causes - volcanism, impact, methane clathrate release etc and the extinction occurred in pulses separated by thousands of years.

Some suggest that we are in a mass extinction now, as presented in the article 6th Mass Extinction? Humans Kill Species Faster Than They're Created, where human influences and manipulation may be replicating some aspects of past mass extinctions.

Answer 3

You've missed a number of calamities:

• Everything dies because all plant life dies from ever decreasing levels of CO₂ in the atmosphere.
  This is projected to happen within less than a billion years.

• Everything dies because the oceans boil away and the Earth becomes Venus 2.0.
  This is projected to happen within one or two billion years due to increased luminosity from our Sun.

• Everything dies because plate tectonics stops.
  This is projected to happen shortly after the oceans boil away.

These inevitable events will all happen well before our Sun becomes a red giant (which is another inevitable event). All of the other calamities presented in the opening post are only possibilities. Unless something intervenes to move the Earth away from the Sun and add CO₂ to the atmosphere, the end of life as we know it is an inevitability that will happen long before the Sun turns into a red giant. That something that staves off the end of life as we know it a billion or so years from now will be the doing of our children's children's ... children, if humanity ever lives that long.

Answer 4

According to a recent Scientific American article, in about a half-billion years the Sun will have expanded enough and the Earth's orbit decayed enough that Earth will no longer be in the "habitable zone". (Earth is already on the inner margin of that zone.)

(Of course, humans will destroy things far sooner than that.)

BTW: For folks interested in this it's worth it to look up that article: Planets More Habitable Than Earth May Be Common in Our Galaxy -- published in the last month or two. I just skimmed it -- didn't read in any detail, but it discussed the criteria of a habitable planet.

Also relevant: Earth's Days Are Numbered