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The life span of the biosphere revisited

Ken Caldeira and James F. Kasting · Nature 360, 721-723 · 1992

Key finding. Because C4 photosynthesis can persist at atmospheric CO2 below 10 parts per million, a C4-plant-based biosphere could survive at least another 0.9 to 1.5 billion years — depending on whether carbon dioxide or temperature becomes the limiting factor — rather than the roughly 100 million years implied by a C3 limit.

Two stacked panels against time in billions of years from the present, 0.0 to 1.6. The upper panel plots soil and atmospheric carbon dioxide partial pressure on a logarithmic axis from 1 to 10,000 parts per million, both falling steeply and reaching 1 part per million near 1.0 billion years, against a rising solar luminosity line. The lower panel plots biological productivity, falling from 1.0 to zero at about 0.9 billion years, against temperature rising from about 293 to 373 kelvin.
Two clocks running at once. Carbon dioxide falls as the brightening Sun accelerates rock weathering, and productivity follows it down to zero near 0.9 billion years; temperature then climbs toward the boiling point. Which curve ends the biosphere — starvation or heat — is what sets the 0.9-to-1.5-billion-year range. Figure 2 from Caldeira and Kasting (1992), Nature 360, 721-723. Reproduced under author reuse rights. Extracted from the published PDF and resized for web display.

What question did this research address?

As the Sun brightens, silicate rocks weather more readily, drawing carbon dioxide out of the atmosphere. That feedback stabilises Earth's temperature, but it also means carbon dioxide falls over geological time — eventually below what plants need.

Lovelock and Whitfield had put that limit about 100 million years away, when carbon dioxide drops below the 150 parts per million required for C3 photosynthesis. This paper re-examined the calculation with a better model.

What did we find?

Three improvements drive the difference: a more accurate treatment of the greenhouse effect of carbon dioxide, a biologically mediated weathering parameterization, and the recognition that C4 photosynthesis operates far below the C3 threshold.

Whether the limit is 0.9 or 1.5 billion years depends on which constraint binds first — carbon dioxide starvation or rising temperature.

Within about a further billion years beyond that, Earth may lose its water to space through photodissociation and hydrogen escape, following the path of Venus. That loss would end the biosphere unambiguously.

The model is constructed to give a *minimum* estimate: it maximises both the greenhouse effect and stratospheric water loss, and takes conservative temperature and carbon requirements for the biosphere.

Why does it matter?

It changes the answer to how long Earth remains habitable by roughly an order of magnitude, and does so by attending to plant biochemistry rather than to planetary physics.

The bearing on astrobiology is direct, as the paper notes: a habitable period measured in billions rather than hundreds of millions of years changes the odds of finding biologically active planets elsewhere.

It is also an early demonstration that the carbonate-silicate thermostat has a termination condition. The feedback that keeps Earth temperate is the same one that will eventually starve its biosphere of carbon.

Citation

Ken Caldeira and James F. Kasting (1992). The life span of the biosphere revisited. Nature 360, 721-723.

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