Earth's Climate Thermostat May Have Flipped Modes During the Age of Pangea
University of Chicago researchers say the planet's carbon-cycle control switched from heat-driven to water-driven weathering for roughly 100 million years, then switched back.
Life arose on Earth at least 3.5 billion years ago, and the planet has stayed habitable ever since, neither too hot nor too cold. Part of the reason is a built-in climate thermostat based on the carbon cycle, which controls how much carbon dioxide sits in the atmosphere. A new study in Nature Communications argues that this thermostat did not always run the same way. It may have operated in one mode for most of the past 480 million years, flipped modes during the age of Pangea, and then flipped back.
"There appears to be some kind of balance in what sustains the main greenhouse gas in Earth's atmosphere, keeping the climate at a pretty nice temperature, habitable for life over really long timescales," said R.J. Graham, a postdoctoral scholar in the University of Chicago's Department of the Geophysical Sciences and first author of the paper. The authors, from UChicago and the University of Southampton, include professor Dorian Abbot and assistant professor B. B. Cael.
The mechanism is silicate weathering. Carbon dioxide combines with rainwater, making it slightly acidic, and the acid breaks down rock. The dissolved minerals and carbon wash into rivers and oceans, where they form carbonate rocks on the seafloor. Over millions of years those rocks are carried into Earth's interior by subduction, and volcanoes release the carbon as CO2 again. Weathering speeds up to cool the planet and slows down to warm it.
Scientists have long debated what sets the pace. In one version, temperature is the main driver, because chemical reactions run faster when it is warmer. In the other, rainfall matters most. When water passing over rock becomes saturated with dissolved minerals, temperature stops mattering, and only more water allows weathering to continue. The distinction is important because the water-based thermostat is weaker. "Hotter temperatures increase rainfall, but the amount by which that speeds weathering is less than the amount by which you would directly increase the dissolution of rock just by turning the temperature up," Graham explained. A hydrologic thermostat "might allow for larger climate changes while maintaining that balance."
To tell the two apart, the team looked for signatures tied to the layout of landmasses. They analyzed 120 climate model simulations that were identical except for randomly generated continents, and calculated weathering rates under both assumptions. Temperature-driven weathering depended on how much land sat in the tropics, where land gets hotter but not necessarily wetter because extra rain evaporates under more sunlight. Water-driven weathering tracked how consolidated or broken up the continents were, since scattered landmasses with more coastline allow more runoff.
The researchers then compared a 480-million-year reconstruction of Earth's temperatures with reconstructed maps of ancient continents. For most of that time the temperature-driven signature fit. For roughly 100 million years, the pattern switched to the hydrology signature, a window that overlaps the period when most of Earth's land was joined in the supercontinent Pangea. The scientists suggest Pangea's aridity may explain the change. With less water moving through the soil, weathering was limited by water rather than heat.
The authors caution that their results rely on simulations with idealized conditions. The models include no mountain ranges, differences in rock type, plants or other variables, and there are competing reconstructions of ancient climate. They say that if the thermostat did flip, it offers a new view of Earth's history and of how life's long-term climate control may behave as the sun brightens.
