The future lifespan of plants just got extended

For now, the future of life on Earth is in human hands. But after the anthropocentric era, the situation starts to get dicey. The sun’s luminosity is increasing over time, about 1% every 110 million years, so the Earth’s surface will gradually get warmer (but at a vastly slower rate than today’s global warming).

This will alter the rate at which silicate weathering takes place—the process whereby silicate rocks transform to carbonate rocks as atmospheric carbon dioxide and rainwater combine to produce carbonic acid.

Carbonate rocks, mostly limestone and dolomite, transform back into silicate rocks through volcanic action and metamorphosis at high temperature. The volcanic activity emits carbon dioxide that replaces that taken out by silicate weathering, and the carbonate-silicate cycle goes on.

So on million-year timescales, the level of carbon dioxide in the atmosphere would be approximately steady, all else being equal (which they seldom are). But as the increasing luminosity of the sun slowly increases the temperature of the Earth, silicate weathering should also decrease, drawing more carbon dioxide out of the atmosphere.

Carbonate burial also removes carbon from the ocean-atmosphere system. That’s bad news for plants, who feed on carbon dioxide, sunlight and water, and they will doubly suffer as surface temperatures increase due to the sun. As plants disappear, large life on Earth would starve and die. Calculating when that happens has been an effort for decades, with timespans obtained from 100 million to 1 billion years, but all the moving parts in such a model make the computation difficult.

A trio of scientists from the University of Chicago and the Weizmann Institute of Science in Israel has now put forth a new model that pushes the terrestrial biosphere’s lifetime out to 1.7 billion years. Their work has been published in The Planetary Science Journal.

“If weathering is weakly temperature-dependent (as recent data suggest) and/or strongly CO₂ dependent,” they write, “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.”

Their results lengthen the period where plants survive to 1.6 to 1.82 billion years, until plants die of either CO₂ starvation or extreme temperatures, perhaps doubling the future lifespan of macro-sized organisms.

Most previous work has assumed that silicate weathering is strongly temperature dependent—exponential with an e-folding time (T_e) of 10 to 20 years, and weakly CO₂ dependent, varying (by the power β) between the fourth root and square root of CO₂ concentration. Smaller T_e means a stronger dependence of the silicate weathering rate on temperature.

They consider two scenarios: plant extinction from inadequate CO₂ {T_e =13.7 Kelvin and β=0.25} as in Caldiera and Kasting in 1992, and extinction from overheating {T_e =31 K and β=0.41} as in Krissansen-Totton and Catling in 2017.

They also looked at C3 and C4 plants separately (they differ in the efficiency with which they use photosynthesis, their carbon fixation processes and how well they tolerate hot, dry conditions—about 95% of the plants on Earth are C3 plants) .

With these parameter pairs they couple global-mean models of plant productivity, the carbon cycle, and climate to determine the eventual extinction mechanisms of land plants—and, of course, all species that rely on them. The second set of parameters, representing the overheating scenario, results in a longer future lifespan of terrestrial plants relative to the first set, 1.8 billion years compared to 1.3 billion years, respectively. This is substantially longer than in either previous work.

The concentration of carbon dioxide decreases from the modern value in both cases, to essentially zero in the first scenario and to about 170 parts per million (ppm) in the second scenario. The Earth’s surface temperature peaks at about 310 K (37°C) in scenario one and 335 K (62°C) in scenario two. (Earth’s current average surface temperature is about 289 K (16°C)). It’s going to be toasty in either picture.

C3 plants were exterminated before C4 plants: 0.5 billion years (scenario 1) at a CO₂ concentration level of 150 ppm, compared to 0.8 Gyr (scenario 2) for C3 plants, and 1.2 Gyr compared to 1.8 Gy respectively for C4 plants. The Earth will see about 500 million years where the only plants that exist are C4 plants, such as sorghum, sugarcane and maize. At least sweets will be around for an extra half-billion years, if there is anyone left to produce them.

The authors are also able to draw some large conclusions about extraterrestrial life. “If life is common beyond Earth,” they write, “our conclusions may be testable with future observations of biosignatures on extrasolar planets.”

A longer future of the biosphere also has implications for the development of intelligent life. They show a longer lifespan suggests fewer “hard steps”—critical, unlikely evolutionary transitions—to produce intelligent life, than previously estimated. Previous researchers had suggested 4–5 for the number of hard steps required, but a longer biosphere reduces this number to 2.4, the authors calculate.

That’s good news for the prospect of intelligent extraterrestrial life. Their results “would suggest that the emergence of intelligent life may be a less difficult (and consequently more common) process than some previous authors have argued.

© 2024 Science X Network