The way Earth's 'thermostat' works may have changed during Pangea era

Life arose on Earth at least 3.5 billion years ago, and our planet has remained habitable ever since—not too hot, not too cold. These life-sustaining conditions are possible in part thanks to a built-in, carbon cycle–based climate "thermostat" that controls the amount of carbon dioxide in the atmosphere.

"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.

The process that manages this balance, however, may not be an unchanging mechanism.

In a new study published in Nature Communications, Graham and colleagues from UChicago and the University of Southampton present evidence that this so-called climate thermostat may have operated in one mode for most of the past 480 million years, briefly—by geological standards—flipped modes during the age of Pangea, and then flipped back.

If the thermostat mechanism did in fact flip, the scientists said, that change could provide a new perspective on our planet's history and climate mechanisms, as well as the future of life on our planet as the sun brightens.

Thermostat controls

Carbon dioxide is the main long-term controller of Earth's greenhouse effect. Removing it from the atmosphere essentially turns on the air conditioning, and putting it back turns on the heat.

The feedback cycle relies on a process known as silicate weathering. CO₂ in the atmosphere combines with rainwater, turning it slightly acidic, which chemically breaks down rock. The dissolved minerals and carbon from the CO₂ wash into rivers and oceans, where they form carbonate rocks on the seafloor.

Over millions of years, those rocks get carried into Earth's interior through subduction, and carbon gets released by volcanoes as carbon dioxide back into the atmosphere, completing the cycle.

Silicate weathering speeds up to cool the planet and slows down to warm it. But scientists have long debated what exactly is driving the rate—temperature or rainfall? Does warmer rock simply react more quickly, or is weathering more affected by the amount of water washing over rock and through soil?

In the version of events where temperature is the main driver, weathering is primarily affected by heat. Chemical reactions occur faster in warmer conditions.

In the version where rainfall is the main factor, though, an absence of water can become a limiting factor. When water passing over rock becomes saturated with dissolved minerals, temperature doesn't matter; only more water will allow weathering to continue.

Scientists care about this distinction because the water-based thermostat is weaker. "It's less efficient," explained Graham, the study's first author. "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.

"So the hydrologic mechanism might be a less efficient thermostat, and it might allow for larger climate changes while maintaining that balance between the influx and outflux of the carbon," Graham explained.

The UChicago team, which included professor Dorian Abbot and assistant professor B. B. Cael, hypothesized that they could distinguish between the two "modes" by searching for signatures related to the position and distribution of landmasses.

To identify the signatures, the team analyzed 120 climate model simulations, all identical except for the layout of randomly generated continents, and calculated weathering rates under both the temperature and the hydrology assumptions. They then looked for correlations between weathering rates and landmass distribution.

They found that temperature-driven weathering depends on how much land sits in the tropics, where the land gets hotter but not necessarily wetter because extra rain evaporates with more sunlight.

Water-driven weathering, however, correlated with how consolidated or broken up the continents were. Scattered landmasses with more coastline regions allow for more runoff.

Patterns of the past

Because faster weathering cools the planet, the signatures of each mechanism should be reflected in the geologic record of global temperatures, so the team searched the past for those clues.

Comparing a 480-million-year reconstruction of Earth's temperatures against reconstructed maps of ancient continents, the researchers found signs that for the majority of that time, Earth's thermostat was running on the temperature-dominated mode, where weathering was primarily driven by heat.

Yet for roughly 100 million years, the pattern switched to the hydrology signature. This overlaps the period when most of Earth's land was combined into the supercontinent Pangea.

The researchers suggest Pangea's aridity may explain the shift: with less water moving through the soil, weathering was controlled primarily by water—or lack thereof—rather than temperature.

The scientists also said, however, that these results rely on simulations with idealized factors—no mountain ranges, rock type differences, plants, or other variables were considered. There are also different reconstructions of ancient geography, and volcanic carbon dioxide output was assumed to stay steady for the purpose of the modeling, though it may have increased as the supercontinent broke apart. Studying ancient soils from the Pangea era for the thermostat mode signatures could corroborate the hypothesis.

In addition to offering new insight into our planet's climate regulation, the possible dynamic nature of our thermostat could also inform our understanding of the "Goldilocks zone," the range of orbits around a star where a planet could have liquid water and might support life.

"The concept of a habitable zone is based on assumptions about the functionality of this CO₂ feedback," said Graham. "If your thermostat isn't as efficient as it could be, then it's plausible that it won't operate well enough to keep planets across the whole habitable zone habitable."