Kenneth Glad
Distant environmental changes could reshape the conditions supporting vegetation and the organisms that depend on it. New modelling explores several possible limits rather than identifying one fixed extinction date.
Earth could retain some form of plant life for up to about 1.8 billion years, according to research using a three-dimensional climate model.
The estimate depends on how the planet’s carbon cycle responds as the ageing Sun becomes steadily brighter.
Heat may ultimately overwhelm vegetation, but falling atmospheric carbon dioxide could become the decisive barrier first.
Heat could become the first barrier
Published in the Journal of Geophysical Research: Atmospheres, the study examined 29 ExoCAM simulations covering stronger sunlight and carbon dioxide concentrations from present-day levels to almost zero.
Unlike simpler calculations, ExoCAM represents atmospheric circulation, clouds, rainfall, regional temperatures and interactions with a slab-ocean surface. Comparisons indicated that some earlier models may have exaggerated the warming produced by increasing solar radiation.
Under the weak-weathering scenario, carbon dioxide remains near current levels while the planet heats. Average conditions would become unsuitable for most land plants in roughly 1.68 billion years. Using a more optimistic thermal threshold, some vegetation might persist until about 1.87 billion years from now.
The study does not point to one firm end date. Its estimates depend on how strongly weathering removes carbon dioxide and how much heat future vegetation can withstand.
When carbon dioxide becomes too scarce
A different outcome emerges if warmer conditions accelerate rock weathering. That process removes carbon dioxide from the atmosphere and could restrain global warming, while gradually depriving plants of material required for photosynthesis.
The conventional threshold for C4 plants would be reached after about 1.35 billion years. Lower thresholds proposed in recent research could extend that period.
CAM plants, including cacti, agave and some orchids, use a specialised pathway to take in and store carbon dioxide while reducing water loss. Some can also reuse part of the carbon released during respiration. Aquatic plants and diatoms may obtain carbon from bicarbonate dissolved in water.
Allowing survival at concentrations near one part per million produces an upper estimate of approximately 1.84 billion years. Evolution and technology were excluded from the calculations, although either might prolong the survival of some organisms.
“We suggest that the default story for our planet’s future is that life will survive at least as long as Earth,” the researchers wrote.
The broader finding is that vegetation may endure until Earth approaches climatic thresholds linked to major ocean loss, although models disagree substantially about when those conditions would begin.
Source: “Maximum Lifetime of the Vegetative Biosphere,” Journal of Geophysical Research: Atmospheres
