How long will Earth remain habitable? From an astronomical perspective, the answer is not forever. In roughly 5 billion years, the Sun will use up the hydrogen in its core and enter the next stage of its life as a red giant. As it expands, the Sun could engulf and destroy Earth.
For now, the Sun remains in its main-sequence phase, fusing hydrogen into helium in its core. Although the Sun is relatively stable during this stage, it gradually becomes brighter over time. Increasing solar brightness will expose Earth to more heat and radiation, eventually raising global temperatures beyond the limits that most life can tolerate.
One of Earth’s natural systems that helps regulate the planet’s surface temperature is the carbon cycle. Over millions of years, carbon moves from Earth’s interior into the atmosphere as carbon dioxide, or CO2. This greenhouse gas traps heat and helps regulate Earth’s climate, while plants absorb CO2 through photosynthesis and circulate carbon throughout the biosphere.
Rainfall and the weathering of rocks also remove carbon dioxide from the atmosphere, storing it in the oceans and rocks. As the Sun becomes brighter, this natural process could gradually reduce Earth’s atmospheric CO2 levels and slow planetary warming. However, over billions of years, the carbon cycle could remove so much CO2 that plants would no longer have enough carbon dioxide to carry out photosynthesis.
To investigate how long Earth’s biosphere could survive, researchers Jacob Haqq-Misra and Eric Wolf modeled the planet’s distant future. Their computer simulations calculated the balance between the energy Earth receives from the Sun and the energy it emits into space. The models also accounted for increasing solar radiation and changing atmospheric CO2 concentrations.
The researchers then used the ExoCAM climate model to estimate future temperatures and atmospheric conditions at different latitudes. These results were compared with a habitability metric to determine how changing conditions could affect Earth’s biosphere.
To estimate the maximum possible lifespan of life on Earth, the team conducted 29 simulations covering the next 2 billion years. In one group of simulations, atmospheric CO2 remained near the modern concentration of 400 parts per million, while global temperatures continued to rise as the Sun grew brighter. In another group, the carbon cycle reduced atmospheric CO2 enough to keep Earth’s average surface temperature near the modern level of 59℉ (15℃).
In the scenario where CO2 remained fixed, Earth’s average surface temperature reached approximately 122°F (50°C) after about 1.7 billion years. These extreme temperatures would be too hot for most land plants. After roughly 1.9 billion years, the average temperature climbed to approximately 149°F (65°C), making conditions too severe for all land plants to survive.
The simulation in which Earth’s average temperature remained stable produced a different threat to life. Atmospheric CO2 levels fell below the 150 parts per million threshold that most plants need for photosynthesis within approximately 500 million years. After 1.4 billion years, CO2 concentrations dropped below 10 parts per million, leaving only a small number of land photosynthesizers and aquatic plants. By about 1.8 billion years, atmospheric CO2 had fallen to approximately 1 part per million, potentially leaving only photosynthetic microbes.
These findings suggest that Earth’s future biosphere could survive longer than earlier studies estimated. Previous, simpler models predicted that Earth’s biosphere might last between 100 million and 1.5 billion years. However, Haqq-Misra and Wolf emphasized that additional research is needed to confirm their results using other climate and atmospheric models, particularly under extreme conditions involving intense solar radiation and very low CO2 levels.
The study also did not account for major evolutionary changes that could help life adapt. Over time, plants might evolve to tolerate higher temperatures or lower atmospheric CO2 concentrations. Future humans, or another intelligent species, could potentially use large-scale geoengineering to delay the loss of habitability. If life adapts successfully, Earth’s biosphere might endure until the Sun expands into a red giant and fills the daytime sky.
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Source: sciworthy.com