We live at a unique moment in Earth’s climatic history. Our planet has been both hotter and colder in the past, flipping between periods when it was swathed in ice and when temperatures soared far above current averages. We’re living in an “icehouse” period now, meaning conditions are relatively cool and permanent ice caps crown the poles—though we’re also in a glacial lull where temperatures have warmed a bit, causing the ice to recede.
“There’s no single mechanism that really drives an icehouse. It’s sort of like the perfect storm.”
Creating the oasis that we inhabit today may be no easy task, hints new research that models Earth’s climatic swings over the past 420 million years. Multiple factors must line up for the planet to enter an icehouse.
The study, published in Science Advances in February, is helping to explain why icehouses are so much rarer than greenhouses, said Andrew Merdith, a computational scientist at the University of Adelaide and a coauthor of the paper describing the work.
“There’s no single mechanism that really drives an icehouse,” he said. “It’s sort of like the perfect storm.”
A New Model for Icehouses
Carbon dioxide (CO2) levels are the main factor controlling Earth’s temperature over long periods of time. CO2 is naturally emitted by processes such as volcanic activity and locked away by other processes such as rock weathering. The balance between these processes helps determine whether Earth gets hotter or colder on timescales of millions of years.
Tracking the carbon cycle is complex, and evidence of carbon emissions and sequestrations is buried in rocks tens or even hundreds of millions of years old.
Previous models looking at long-term fluctuations in Earth’s climate were unable to capture the swings between icehouses and greenhouses evident in the geologic record. “[The models] would often get into a greenhouse and then stay there very happily, or they’d get into an icehouse and stay in the icehouse very happily,” Merdith said.
Merdith and his colleagues took a new approach that involved pairing models of historic tectonic plate movements with models of the paleoclimate and the carbon cycle. Their model included CO2 emissions from features such as mid-ocean ridges and continental arcs, as well as features such as basaltic mountain ranges that sequester CO2 through chemical weathering.
Tracking plate movements allowed the researchers to analyze not only the processes themselves but where on Earth they were happening. That work gave them a series of maps reconstructing precipitation, erosion, and temperature on Earth over hundreds of millions of years, Merdith said.
The researchers looked at the past 420 million years, which covers most of the Phanerozoic, our current eon and a time when plants and animals emerged in force. The Phanerozoic has experienced two major icehouse periods, the late Paleozoic and late Cenozoic (our current geologic era). The new model was able to reproduce both of those icehouses, as well as periods of warming in between, Merdith said, giving the researchers confidence in its accuracy.
Then, they ran their model again, turning different features off to see how that would affect the appearance of icehouse periods. Though some factors, such as the amount of CO2 coming from volcanic activity, mattered more at certain times than others, the researchers found that no one factor alone could make Earth cold enough to produce lasting ice caps.
The new model is a step forward for paleoclimate modeling, said Lynn Soreghan, a geologist at the University of Oklahoma who studies the paleoclimate but wasn’t involved in the research.
“It’s just a lot more detailed than some of the past models that, for example, were only looking at carbon buried in different repositories through time,” she said. “I think they’ve made a much more quantitative case” for the drivers pushing Earth into icehouse periods.
Icehouse Mysteries
Though the new model produces reconstructions of past climates that generally agree with evidence from the geologic record, it’s not perfect. The researchers noted a few periods when their model diverged from paleoclimate reconstructions, such as the Permian, when their model showed ice extending farther than it probably did, and the Eocene, when their model indicated colder temperatures and more CO2 than are thought to have existed.
Merdith said future improvements to their model could include better temporal resolution (their model jumped forward in increments spanning tens of millions of years), as well as a more sophisticated representation of the organic carbon cycle, which captures how plants store and bury carbon.
“This is a relatively unusual state of the planet, and so getting into one kind of takes effort.”
The model also doesn’t include non-CO2-related factors such as Earth’s albedo, said Christopher Scotese, a paleogeographer at Northwestern University who wasn’t a part of the research. Albedo, or how reflective Earth is, can be especially important during colder periods because ice caps reflect sunlight and reduce the ability of oceans to absorb it, creating a feedback loop that can perpetuate cold periods.
“CO2 is just part of it,” Scotese said. “Sometimes it’s not the dominant mechanism of climate change.”
Scotese agreed the new model offers a more rigorous approach to simulating past climatic swings and may help shed light on anomalous events in Earth’s history, such as “snowball Earth,” a hypothesized period in which the entire planet was covered in ice.
As for the immediate future of Earth’s climate, the research doesn’t offer many insights. But it does help put into context our interglacial, icehouse present. For humans an icehouse is normal, though it’s not a given by any means, Soreghan said.
“This is a relatively unusual state of the planet, and so getting into one kind of takes effort,” she noted.
—Nathaniel Scharping (@nathanielscharp), Science Writer
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