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Extending Earth’s Biosphere Lifespan: New Research Paints a Brighter Future for Life on Our Planet
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Caption: Time until land plant extinction (Gyr) vs. CO2 levels. Relative degassing rate compared to modern for different weathering parameters. Credit: Planet Science Journal (2024). DOI: 10.3847/PSJ/ad7856
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Mankind currently holds the fate of life on Earth in its hands, but the future seems uncertain after the Anthropocene era. As the luminosity of the Sun increases over time, approximately 1% every 110 million years, the Earth’s surface will gradually warm at a much slower rate than the current global warming. This gradual warming will alter the rate of silicate weathering, the process by which silicate rocks transform into carbonate rocks and atmospheric carbon dioxide (CO2) is absorbed by rainfall to form carbonic acid.
Carbonate rocks, predominantly limestone and dolomite, are converted back into silicate rocks through vulcanism and high-temperature metamorphism, releasing CO2 that replaces the CO2 lost due to silicate weathering. Thus, the carbonate-silicate cycle continues, and atmospheric CO2 levels remain roughly stable over millions of years, assuming all other factors remain constant.
However, as the Sun’s luminosity gradually increases, causing Earth’s temperature to rise, silicate weathering will also slow down, drawing more CO2 out of the atmosphere. Burial of carbonates also removes carbon from the ocean-atmosphere system, which is bad news for plants, which consume CO2, sunlight, and water. Rising surface temperatures will further stress plants, leading to mass extinctions and widespread famine among life forms that depend on them. Estimates for when this will occur have ranged from 100 million to one billion years, but variations in models make these calculations challenging.
Now, a trio of scientists from the University of Chicago and the Weizmann Institute of Science in Israel has presented a new model that extends the lifespan of Earth’s biosphere to 1.7 billion years. Their research has been published in Planet Science Journal.
"[I]f weathering depends strongly on temperature and/or CO2 strongly depending," they write, "we find that interactions between climate, productivity, and weathering cause future CO2 levels driven by luminosity to decrease and slow, delaying CO2 starvation." Their findings extend the period during which plants can survive from 1.6 to 1.82 billion years, until they perish due to CO2 starvation or extreme heat, possibly doubling the lifespan of macroscopic organisms in the future.
Previous research had assumed that silicate weathering depends strongly on temperature, with an Arrhenius-dependent rate that varies exponentially with the inverse of temperature, and a weak dependence on CO2, which varies between the fourth and square root of CO2 concentrations. However, the authors consider two scenarios: plant extinction due to CO2 limitation, with T_a = 13.7 Kelvin and β = 0.25 (as in Caldiera and Kasting, 1992), and extinction due to overheating, with T_a = 31 K and β = 0.41 (as in Krissansen-Totton and Catling, 2017).
They also examine C3 and C4 plants separately, which differ in their photosynthetic efficiency, carbon fixation processes, and tolerance of hot and dry conditions; roughly 95% of Earth’s plants are C3. With this pairing of parameters, they combine global average plant productivity, carbon cycle, and climate models to determine the mechanisms of land plant extinction and, consequently, all species that depend on them.
The second parameter, representing the overheating scenario, produces a longer lifespan for plants in the future, 1.8 billion years versus 1.3 billion years, far longer than previous estimates. In both cases, CO2 concentrations drop from modern levels to around 170 ppm in the second scenario and zero in the first. Surface temperatures peak at approximately 310 K (37 °C) in the first scenario and 335 K (62 °C) in the second, much hotter than Earth’s current average surface temperature of around 289 K (16 °C).
C3 plants will be the first to go, with extinction occurring around 500 million years in the first scenario, with CO2 levels around 150 ppm, compared to 0.8 Gyr in the second scenario, and 1.2 Gyr for C4 plants. For roughly 500 million years, the only plants remaining on Earth will be C4 plants like sorghum, sugarcane, and corn. At least, mankind’s favorite sweetener could still be around for another 500 million years.
The authors also draw important conclusions about extraterrestrial life. "If life is common beyond Earth," they write, "our conclusions might be testable through observations of biosignatures on extrasolar planets in the future." Furthermore, their findings suggest that the emergence of intelligent life is not as difficult or rare as some previous researchers have suggested, potentially increasing the likelihood of finding intelligent life elsewhere in the universe.
