Life Might Thrive on the Surface of Earth for an Extra Billion Years

The Sun is midway through its life of fusion. It’s about five billion years old, and though its life is far from over, it will undergo some pronounced changes as it ages. Over the next billion years, the Sun will continue to brighten.

That means things will change here on Earth.

As the Sun goes about its business fusing helium into hydrogen, the ratio of hydrogen to helium in its core changes. Over time, the core slowly becomes more enriched in helium. As helium accumulates in its core, the core’s density increases, meaning protons are more closely packed together. That creates a situation where the Sun can fuse hydrogen more efficiently. After a chain reaction of processes and cause and effect, the end result is that the Sun’s luminosity increases. The Sun’s luminosity has already increased by about 30% since its formation, and the brightening will continue.

Any increase in the Sun’s luminosity can have a pronounced effect on Earth. Environmental cycles like the carbon, nitrogen, and phosphorous cycles sustain Earth’s biosphere. As the Sun becomes brighter, it will affect these cycles, including the carbonate-silicate cycle, which moderates the accumulation of carbon dioxide (CO₂) in the planet’s atmosphere.

Scientists think that over the next billion years, the brightening Sun will disrupt this cycle, leading to declining CO₂ levels. Plants rely on CO₂ and the levels are expected to plummet, which means that complex land life would end in the next billion years.

It’s a bleak prognosis, but new research suggests it might not happen.

The new research is “Substantial extension of the lifetime of the terrestrial biosphere,” and it’s been accepted for publication in the Planetary Science Journal. It’s in pre-print now, and the lead author is R.J. Graham, a postdoctoral researcher in the Department of Geophysical Sciences at the University of Chicago.

“Approximately one billion years (Gyr) in the future, as the Sun brightens, Earth’s carbonate-silicate cycle is expected to drive CO₂ below the minimum level required by vascular land plants, eliminating most macroscopic land life,” the authors write.

As the Sun brightens and warms the Earth’s surface, scientists expect the carbonate-silicate cycle to draw more CO₂ out of the atmosphere because of carbonate-silicate weathering and carbonate burial. Rainwater is enriched with atmospheric carbon, which reacts with silicate rocks and breaks them down. The products of the chemical reactions that break them down find their way to the ocean floor, where they form carbonate minerals. As these minerals are buried, they effectively remove carbon from the atmosphere.

Normally, the cycle acts as Earth’s natural thermostat. However, higher temperatures make the reactions more efficient, meaning the carbonate-silicate cycle will remove more CO₂ from the atmosphere. That’s what led scientists to conclude that the CO₂ will become so low that planet life will perish. However, the authors examined these ideas and found that it may not quite work out that way.

“Here, we couple global-mean models of temperature- and CO₂-dependent plant productivity for C₃ and C₄ plants, silicate weathering, and climate to re-examine the time remaining for terrestrial plants,” they write. C₃ and C₄ plants are two main plant groups that are classified based on how they perform photosynthesis and absorb carbon. They’re relevant because they respond differently to higher temperatures.

The researchers say recent data shows that the carbonate-silicate cycle isn’t as temperature-dependent as previously thought. Instead, it’s only weakly temperature-dependent and more strongly CO₂-dependent. In that case, “we find that the interplay between climate, productivity, and weathering causes the future luminosity-driven CO₂ decrease to slow and temporarily reverse, averting plant CO₂ starvation,” they explain.

Instead of a one billion-year outlook for Earth’s plant life, the researchers say atmospheric CO₂ levels will mean plants have another 1.6-1.86 billion years. When plants can no longer survive, it won’t be because of plummeting CO₂ levels. Instead of CO₂ starvation, it’ll be because of what scientists call the moist greenhouse transition.

When that transition happens, a planet’s atmosphere becomes saturated with water vapour as the planet warms. Since water vapour is a potent greenhouse gas, it creates a feedback loop of increased warming. Eventually, it’s simply too hot for plants to survive. The consequences don’t end there. As the Earth’s upper atmosphere becomes more saturated with water vapour, UV energy splits water apart, and the hydrogen drifts off into space. In this situation, there’s a gradual and irreversible loss of water into space.

According to the authors, Earth won’t experience this transition for between about 1.6 and 1.86 billion years.

“We show that recent data indicating weakly temperature-dependent silicate weathering lead to the prediction that biosphere death results from overheating, not CO2 starvation,” the authors write. “These findings suggest that the future lifespan of Earth’s complex biosphere may be nearly twice as long as previously thought.”

These results also affect our understanding of exoplanet habitability. It has to do with what are called ‘hard steps’ in the appearance and evolution of life. The hard steps model says that certain evolutionary transitions were difficult and unlikely to happen twice. Some examples are the appearance of multicellular organisms and the Cambrian explosion.

But if Earth’s biosphere has a much longer lifespan than thought, that affects the hard steps model.

“A longer future lifespan for the complex biosphere may also provide weak statistical evidence that there were fewer “hard steps” in the evolution of intelligent life than previously estimated and that the origin of life was not one of those hard steps,” the authors conclude.

If that’s the case, then exoplanet habitability could be less rare than thought.