“Lo and behold, we end up finding that we can explain this apparent discrepancy,” said Hilton. In other words, weathering of organic carbon amplified the warming initiated by volcanic CO₂, so the planet warmed more than it would have if only the volcanic CO₂ had been emitted.

But Rugenstein is skeptical of the amount of CO₂ released by the weathering. “Their estimates of the total amount of carbon delivered by this feedback are enormous,” said Rugenstein. “I find it difficult to believe that these carbon fluxes are going to be as big as they think they are.”

Where does that leave Earth’s “thermostat”?

If rock weathering emits CO₂, where does this leave our widely accepted understanding of Earth’s climate “thermostat,” where weathering of rock (specifically weathering of silicate minerals) reduces atmospheric CO₂, preventing runaway global warming?

“Silicate weathering is still playing a major role. We’re not challenging that,” said Hilton. “It means that silicate weathering has to work harder.”

“While [organic carbon weathering] could be a big positive feedback, in the end, that tells you that the silicate weathering feedback has to be even stronger,” said Rugenstein.

The strength of the competition between silicate weathering and organic carbon weathering depends on the amount of organic-carbon-rich sediment that’s exposed on land. “At some point, you’re going to run out of organic carbon to oxidize, and that’s then going to place a hard limit on the strength of this feedback,” said Rugenstein.

By contrast, the volume of silicate minerals available to weather and draw down CO₂ is much greater. “That’s why that feedback ultimately is a stronger one—we have a much bigger buffer to play with,” said Rugenstein.

A small, long-term uncertain impact for humans

The study suggests that this feedback is likely to apply to other climate warming events, including our own today. Hilton foresees organic carbon weathering adding CO₂ to the atmosphere over the next few centuries, with a small amplifying effect to human-caused warming. “It’s not disaster stations,” said Hilton, “but it is an amount of carbon that could be released at a rate more than it is right now, and that does eat into our carbon budget.”