People think Earth dies when the Sun becomes a red giant in five billion years — the more likely end is a billion years from now, when a star already 30 percent brighter than at birth pushes CO₂ too low for C3 plants, then a moist greenhouse strips the oceans of hydrogen and leaves a sterile Venus

The Sun is roughly 4.6 billion years old and about halfway through its main-sequence lifetime.

The popular conception holds that Earth’s destruction will come when the Sun becomes a red giant roughly 5,000,000,000 years from now, but Earth may become uninhabitable billions of years before that—not because of a sudden solar catastrophe, but because of the normal evolution of an ordinary star.



As the Sun ages, it gradually becomes brighter and more luminous.

Roughly 1,000,000,000 years from now, increased solar output could push Earth’s climate outside of the envelope that allows life to exist here. What does this look like?

The oceans evaporate. Water vapor increases.

Plant life dies.

And eventually, Earth transitions from the vibrant blue planet we know today into something closer to Venus—incapable of supporting life as we know it.

Getting Hotter as it Ages

The Sun is powered primarily by nuclear fusion in its core. Four hydrogen nuclei ultimately combine to form helium through the proton-proton chain, converting a small amount of mass into energy.

Every second, the Sun converts roughly 600 million metric tons of hydrogen into helium. Over billions of years, helium accumulates in the Sun’s core.

The mean molecular weight of the core increases, and the core slowly contracts under gravity.

Contraction raises the temperature and pressure.

The higher core temperature increases the fusion rate in surrounding hydrogen-rich material.

The result may seem counterintuitive: as the Sun exhausts its hydrogen, it becomes more luminous.

One might expect the Sun to grow dimmer as this process plays out, but that’s not the case.

The Sun today is estimated to be roughly 30 percent brighter than when it formed.

It should continue to become roughly ten percent more luminous per billion years during this phase.

Testing Earth

So far, the Earth has survived billions of years of increasing solar luminosity because our climate isn’t just passive.

The carbonate-silicate cycle acts as a geological thermostat over eons. So the Earth can adjust to the Sun’s changing levels of dimness or brightness.

But this mechanism has limits. Eventually, the Sun will become too bright to handle.

As the Sun brightens, the Earth needs less atmospheric CO₂ to maintain acceptable temperatures.

But eventually, CO₂ concentrations will become too low for much of Earth’s plant life, which would, of course, be catastrophic for all of us.

Plant Death

The Earth’s biosphere could be the first thing to fail. Photosynthetic organisms require carbon dioxide.

C3 plants (including most trees and many major crops) would struggle as atmospheric CO₂ declines.

More CO₂-efficient C4 plants could last longer.

But the loss of photosynthesis means declining plant biomass, oxygen production, and terrestrial food webs.

Complex animals, like humans, depend on those systems.

So the initial problem wouldn’t be the Sun getting hotter and the oceans boiling.

It would be more gradual, starting with the slow death of Earth’s plant systems.

Spiking CO₂

As the Sun brightens and temperatures rise, evaporation would increase.

This would lead to more atmospheric water vapor, which in turn would intensify the greenhouse effect and accelerate warming, restarting the cycle.

This would lead to something scientists call a moist greenhouse, where large amounts of water reach the upper atmosphere.

Today, because Earth’s upper atmosphere is relatively cold, it keeps water below, mitigating the moist greenhouse effect.

But with enough warming, more water would rise, where ultraviolet sunlight would break H₂O apart into hydrogen and oxygen.

Hydrogen, being so lightweight, could then escape into space, meaning Earth would lose the building block necessary to remake the oceans.

Over time, water loss would become permanent. Again, that would be a problem for life on Earth.

Five Billion Years

Of course, the one-billion-year estimate is speculative; it’s not an exact countdown.

Climate models produce different estimates based on dozens of finicky variables.

But either way, the Sun is dying. Five billion years from now, the Sun’s hydrogen core will be depleted, and it will leave the main sequence.

The core will contract while the outer layers expand, forcing the Sun to become a red giant. Mercury and Venus will be engulfed.

Earth could be engulfed, too.

Even if Earth weren’t engulfed, the surface would be completely destroyed. Not that it matters, because by then, Earth will have been sterile for billions of years.

About the Author: Harrison Kass

Harrison Kass is a writer and attorney focused on national security, technology, and political culture. His work has appeared in Tablet, City Journal, The Hill, The Spectator, and The Cipher Brief. He holds a JD from the University of Oregon and a master’s in Global & Joint Program Studies from NYU. More at harrisonkass.com.