A study published in the journal “Nature Communications” shows that life was most vulnerable precisely when transitioning from one long-term climate and carbon cycle state to another.
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During these transitional periods, biological stress increased, species turnover rates rose, and extinction risk grew.
What determined the scale of mass extinctions?
“Mass species extinctions in Earth’s history are usually associated with specific catastrophes: massive volcanic eruptions, sudden climate warming, oxygen depletion and acidification of oceans, or asteroid impacts. However, our study suggests looking at these events more broadly,” says Prof. A. Spiridonov.
According to him, the long-term climate state, or megaclimate, determines how the Earth system and biosphere respond to much faster disturbances, such as sudden warming or increased carbon dioxide concentration.
During different geological periods, the Earth’s system sensitivity to such changes varied.
The study showed that life was more vulnerable under long-term hot climate conditions, so knowing just the direct cause of the crisis is not enough – it is also important to understand the state of the entire Earth system at that time.
The researchers emphasize that the transition between climate regimes was not a direct cause of each mass extinction.
The long-term structure of the climate and carbon cycle rather formed the overall background of biosphere vulnerability, which determined how strongly life responded to volcanism, rapid warming, ocean changes, or asteroid impacts.
Five long-term regimes identified in Earth’s history
Scientists analyzed the last 539 million years – the entire Phanerozoic eon, during which complex life evolved and diversified.
They combined data on stable isotopes of carbon and oxygen, reconstructions of ancient temperature and atmospheric carbon dioxide, and the history of marine organism biodiversity.
“Using recurrence analysis, early warning indicators of critical changes, and a mathematical model of climate and the carbon cycle, we identified five long-term megaclimate regimes. The Earth system remained in these for tens or even hundreds of millions of years, then transitioned to another relatively stable state,” explains Prof. A. Spiridonov.
These regimes covered very different stages of Earth’s history – from the early Paleozoic greenhouse climate and late Paleozoic ice age to the Mesozoic greenhouse and later Cenozoic cooling.
The researchers named these states “Haggis” intervals. The name was inspired by the traditional Scottish dish and the patchy, block-like structure of the data recurrence (state repetition) graphs.
The scientist was most surprised by how clearly the Cambrian and Ordovician periods stood out from other stages of Earth’s history.
It was during this time that many of the main current animal phyla appeared, and biosphere vulnerability was greatest: “In the recurrence graphs, we saw that the carbon cycle state of the Cambrian and Ordovician periods was truly exceptional. The remaining approximately 450 million years of the Phanerozoic, starting with the late Ordovician mass extinction, were much more repetitive. This is likely related to the emergence and establishment of land plants on continents, as they fundamentally changed the carbon cycle in the Earth system.”
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A biosphere vulnerability index created
To assess the state of life during different periods, scientists calculated a biosphere vulnerability index.
It includes not only the extinction rate but also the emergence of new taxa, overall organism turnover, and the biological diversity existing at the time.
A high index means the biosphere is rapidly reorganizing: some groups of organisms disappear, others appear or replace them. Environmental stress is usually associated first with extinctions, but it can also be indicated by rapid emergence of new species or genera.
“When the environment changes suddenly, species may go extinct, but isolated populations can also quickly adapt to new conditions and eventually become new species. Therefore, the vulnerability index covers all taxonomic turnover and evaluates it in the context of overall biological diversity: the greater the diversity, the broader the ecological space in which life can exist. A rapidly reorganizing biosphere experiences greater macroevolutionary stress and may respond more sensitively to additional environmental disturbances,” explains Prof. A. Spiridonov.
According to the speaker, vulnerability peaks coincided with almost all periods of increased extinction evaluated in the study, including the so-called Big Five – the five largest mass extinction crises in Phanerozoic history. Significant vulnerability jumps were recorded in the early and middle Paleozoic and at the Permian-Triassic boundary, when the largest known mass species extinction occurred.
Moreover, the study showed large differences between long-term climate states. During the warm early Cambrian period, the biosphere was consistently quite vulnerable, while overall in the cooler Cenozoic its vulnerability was lowest. This suggests that long-term temperature and climate system boundaries contributed to forming the overall extinction risk background.
Allows separating the cause of the crisis from the state of the biosphere
Until now, mass extinction crises were often studied as separate events, mainly searching for their direct cause. The new system allows distinguishing two things: the specific event that caused the crisis and the long-term environmental conditions that may have made the biosphere especially sensitive at that time.
“Long-term climate regimes created a certain baseline level of biological stress. Shorter climate fluctuations and catastrophic events acted within this environment shaped by those conditions. Therefore, a strong biological crisis could arise even without a complete reorganization of the entire climate system, but its impact depended on the previous system state,” says the VU scientist.
The study authors note that their created system is intended for global and interregional datasets, not individual geological sections. Old geological data vary in resolution, contain gaps and chronological uncertainties, so the boundaries of identified regimes and their connection to various biological crises will need to be verified with additional research.
In the future, this method could be applied to specific organism groups, ecosystems, regions, and different geological periods. This would help clarify which life forms were most sensitive to climate system reorganizations and whether the same environmental impact during different long-term Earth states truly causes biological effects of varying scale.
The study was conducted by scientists from VU, the Open University in the United Kingdom, the University of New Mexico, the Potsdam Institute for Climate Impact Research, the University of Potsdam, and the UK National Physical Laboratory.
Prof. A. Spiridonov contributed to formulating the overall methodological approach to linking climate states with macroevolutionary indicators, also proposed the concept of biosphere vulnerability, formalized it, and performed empirical calculations.
PhD student R. Stankevičius performed important recurrence analysis for the study and contributed to developing new methods for assessing climate and evolutionary state recurrence.
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