The Sun is a middle-aged star, about five billion years old. While its life is far from over, it will undergo significant changes as it ages. Over the next billion years, the Sun will continue to brighten. This brightening will have a profound impact on Earth. As the Sun fuses helium into hydrogen, the ratio of hydrogen to helium in its core changes, leading to increased density and more efficient hydrogen fusion. This results in an increase in the Sun's luminosity, which has already increased by about 30% since its formation.
How The Sun's Brightness Impacts Earth
The Sun's brightening will affect Earth's environmental cycles, including the carbonate-silicate cycle, which regulates carbon dioxide levels in the atmosphere. Scientists have long believed that the increasing Sun's luminosity would disrupt this cycle, leading to a decline in CO2 levels and the eventual demise of complex land life. This scenario was based on the assumption that the carbonate-silicate cycle is strongly temperature-dependent. However, recent data suggests a different picture.
A Longer Lifespan For Earth's Plants
New research published in the Planetary Science Journal, titled “Substantial extension of the lifetime of the terrestrial biosphere,” sheds light on the interplay between climate, plant productivity, and weathering. The study, led by R.J. Graham, a postdoctoral researcher at the University of Chicago, found that the carbonate-silicate cycle is only weakly temperature-dependent and more strongly CO2-dependent. This means that the cycle won't remove CO2 as quickly as previously thought, leading to a longer lifespan for Earth's plants.
The researchers used 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. C3 and C4 plants are two main plant groups that are classified based on how they perform photosynthesis and absorb carbon. The new study predicts that plant life could survive for another 1.6-1.86 billion years, considerably longer than the previously estimated one billion years.
The Moist Greenhouse Transition
However, the demise of plant life won't be due to CO2 starvation. Instead, it will be caused by the 'moist greenhouse transition.' This transition occurs when a planet's atmosphere becomes saturated with water vapor due to rising temperatures. Water vapor is a potent greenhouse gas, creating a feedback loop that amplifies warming. Eventually, the planet becomes too hot for plants to survive.
The moist greenhouse transition also leads to a gradual and irreversible loss of water into space. As the Earth's upper atmosphere becomes more saturated with water vapor, UV energy splits water molecules, causing hydrogen to escape into space.
Implications for Exoplanet Habitability
This extended lifespan for Earth's biosphere has implications for understanding exoplanet habitability. The 'hard steps' model of life's evolution suggests that certain evolutionary transitions, like the appearance of multicellular organisms, are difficult and unlikely to occur twice. The extended lifespan of Earth's biosphere challenges this model, implying that the origin of life and the emergence of intelligent life might not be as rare as previously thought.
Earth's Future: A More Optimistic View
The new research paints a more optimistic picture for Earth's future. While the Sun's brightening will inevitably lead to the demise of life on Earth, the process is likely to take much longer than previously thought. This extended time allows for the possibility of more complex life forms to evolve and for Earth's biosphere to continue flourishing for billions of years.
The study's findings also suggest that the conditions necessary for the emergence and persistence of complex life on exoplanets may be more common than previously thought, expanding the potential for discovering life beyond our own planet.
