Scorched or Frozen: What is the Earth’s Ultimate Fate?

The effects of global warming on our environment are becoming increasingly evident. Human-induced or otherwise, climate change is happening, and throughout Earth’s history, the planet has undergone a number of cooling and warming periods.

There was a time, 650 million years ago, when the Earth may have been entirely frozen over; go back 4 billion years and you would find that our planet’s surface was still partially molten. We’ll never seen such drastic changes in our lifetime, but on a geologic timescale, the Earth’s climate has been a rollercoaster of extremes, and the ride is far from over.

Between the various cooling and warming factors, what is the ultimate fate of our planet? Will it become a frozen desert like Mars, a sweltering boiler room like Venus, or will it forever remain a cosmic oasis?

How fast has the Earth been cooling since formation?

When the Earth formed, it was a ball of molten lava. Since then, it has been cooling and solidifying over time. This means the Earth’s temperature should continue to decline, right?

It is estimated that 47 terawatts of heat flow from the Earth’s interior to its surface, warming the planet. But not all of this heat is left over from when our planet formed – roughly half of it is self-generated. While the Earth isn’t the massive fusion reactor that our Sun is, it does produce its own heat by the radioactive decay of elements.

Further, the Sun’s heat delivers 173,000 terawatts of power to the surface of our planet, meaning the Earth’s leftover heat of formation contributes little more than 0.01% of the total heat energy that warms the surface. What this means is that, yes, the Earth has been cooling, but that cooling is negligible. The only factor that really matters is how much heat we’re receiving from the Sun.

Is the Earth moving further away from the Sun?

The further away a planet is from the Sun, the less heat energy it receives. We see that Venus has a surface temperature of 462 °C, while Mars is an icy −63 °C. If the Earth moves further away from the Sun, won’t it eventually freeze?

Every star has a region around it called the habitable zone, or Goldilocks zone – a region where the surface temperatures of an Earth-like planet would be neither too hot nor too cold for water to exist in liquid form. Current estimates of our solar system’s habitable zone place it at 0.99 AU to 1.688 AU – an AU, or Astronomical Unit, is a unit of distance equal to the average distance between the Earth and the Sun. Earth sits right at the inner edge of the habitable zone. If its distance from the Sun were to decrease by just 2%, it would boil, and if it increased by a whopping 70%, it would freeze.

So are we getting closer or further? We observe that the Moon is getting further and further away from the Earth over time due to tidal forces slowing down the Earth’s rotation and speeding up the Moon’s orbit. A similar – but much less powerful – effect is occurring between the Sun and the Earth. However, even generous estimates put the rate of distancing at 15 centimeters per year. That means that in 5 billion years, the Earth will have moved 0.005 AU further away from the Sun – nowhere close to exiting the other side of the habitable zone.

But while the Earth won’t move out of the habitable zone, the habitable zone itself may move until the Earth is no longer within it. If the Sun gets hotter, the habitable zone will move further out; if it cools, the zone will retreat further in.

Is the Sun getting hotter or colder?

The Sun is halfway through its lifespan. If it’s burning through fuel, that must mean it’s shrinking and getting colder, right?

While it may seem counterintuitive, the Sun is actually burning brighter and brighter with time. The Sun is basically a fusion reactor that produces energy by converting hydrogen into helium. As it loses more and more hydrogen, less fusion energy is output.

It’s sounding like the Sun should be getting colder, but that’s where the twist comes in. Less energy means the atoms exert less pressure against the forces of gravity and the weight of the atoms atop them. This causes volume to decrease, which results in an increase in density and, consequently, temperature, as the remaining atoms are squeezed together more tightly. The higher temperature accelerates the rate of fusion, generating more energy. The result? The Sun’s brightness has been increasing by 1% every 110 million years.

Since the Sun is getting hotter, it’s just a matter of time before the Earth slips out out of the habitable zone. How much time? Astronomers use a simple formula to estimate the center of the habitable zone: √(L)×1.34 AU, where 1.34 is the current center of our habitable zone and L is the ratio of a star’s luminosity relative to the Sun’s current luminosity.

Of course, this is a simplification, so pinning down a precise time would be inexact. But what we do see is that, when the Sun will be 10% brighter than it is now – in 1.1 billion years – the center of the habitable zone will be 1.41 AU and the inner boundary will be 1.06 AU, making the Earth a steamy, uninhabitable greenhouse.

Will CO₂ levels continue to rise?

Is the Earth then fated to become a runaway greenhouse like Venus, in which the greenhouse effect becomes so strong that liquid water can no longer exist on its surface?

The Earth’s carbon cycle is a very complex system with numerous sources and sinks. Photosynthesis is the sink everyone is familiar with – plants clean our air by “breathing in” CO₂ and expelling oxygen – but another sink is the weathering of rocks. Similarly to how rust forms on metal by reacting with water and oxygen, rocks react with water and carbon dioxide. While this weathering isn’t a huge carbon sink at present, as temperatures rise, so too will the rate of weathering.

Plantlife depends on carbon dioxide to survive, and just like with a predator-prey relationship, there has historically been a balance between the amount of CO₂ in the atmosphere and the amount of plantlife on Earth. If polar bear populations rise too high, seal populations will decline, leading to a decline in the polar bear population, until balance is once again achieved. But if you introduce a new predator into the system, there’s no guarantee that the polar bear population will rebound. Similarly, it’s predicted that in about 600 million years, weathering will have decreased the CO₂ levels below a point at which most plantlife can survive any longer.

With less CO₂ in the atmosphere, shouldn’t the greenhouse effect settle down and temperatures drop? Actually, water vapor presently accounts for the largest percentage of the greenhouse effect. By one billion years from now, the increasing brightness of the Sun will have raised global surface temperatures to about 47 °C, resulting in more water vapor in the atmosphere, a stronger greenhouse effect, and a feedback loop that will boil away our oceans by about 1.1 billion years from now. Earth will become an uninhabitable, runaway greenhouse.

What will happen when the Earth loses its magnetic field?

If the Earth’s magnetic field protects the atmosphere from being stripped away by the solar wind, then wouldn’t the greenhouse effect stop once the Earth loses its magnetosphere?

Mars’ magnetic field died out four billion years ago. While the planet presently lies in the Sun’s habitable zone, the reason its surface is so cold is because its atmosphere is too thin for any appreciable greenhouse effect to keep it warm. It is believed that Mars had a thicker atmosphere in the past, and the loss of its magnetosphere is at least partially responsible for the loss of its atmosphere.

A planet’s magnetosphere is generated by the motion of a liquid core layer. Once that liquid cools into a solid, it stops generating a magnetic field. Mars, being smaller than the Earth, cooled much faster. It’ll take another 3-4 billion years for the Earth’s liquid outer core to solidify.

Does that mean that temperatures will then cool as the Earth’s atmosphere is stripped away? Not quite. It turns out that a planet’s mass is far more important to retaining its atmosphere than its magnetic field. That’s why Venus, which is about as massive as the Earth, has a dense atmosphere despite its lack of a magnetosphere.

What will happen to the Earth when the Sun becomes a Red Giant?

Red is a colder color than yellow, so won’t the Sun generate less heat once it becomes a red giant, cooling the Earth?

The Sun’s current surface temperature is about 5505 °C, whereas red giants have temperatures of under 4800 °C. But while it is true that red giants are cooler, they are also significantly larger.

When the Sun enters the final stages of its life in 6.5 billion years, it will swell up into a red giant, swallowing Mercury and Venus and reaching a radius of 1.2 AU. By that time, the Earth will have drifted to at most 1.5 AU due to the Sun’s mass loss, meaning it will be significantly closer to the surface of the Sun than it is today – and possibly even within the Sun.

Optimistic scenarios involve the Earth’s atmosphere being stripped away and surface temperatures reaching more than 2,130 °C, bathing our planet in a magma ocean. The grimmest outcome sees drag forces from the Sun’s atmosphere slow down the Earth, decreasing its orbital distance and sucking it deeper and deeper into the Sun until it completely vaporizes like a small meteor in the Earth’s atmosphere.

Ultimately, no matter which way you look at it, there is only one vision for the Earth’s distant future.

Global warming: it is our destiny.