Over the past 539 million years, Earth’s climate and carbon cycle did not change gradually. Instead, they remained in relatively stable states for long stretches, interrupted by sharper transitions. A team led by researchers from Vilnius University has identified five such long-term Earth system regimes and found that life was most vulnerable when the planet moved from one state to another.
The study, published in Nature Communications on August 4, 2026, was carried out by geologist and paleontologist Professor Andrej Spiridonov and doctoral researcher Robertas Stankevičius of the VU Faculty of Chemistry and Geosciences, working with colleagues from institutions in the United Kingdom, the United States and Germany.
During the transitional periods, biological stress increased, rates of species turnover accelerated, and the risk of extinction rose.
“Mass extinctions throughout Earth’s history are usually associated with specific catastrophes: massive volcanic eruptions, rapid global warming, oxygen depletion and acidification of the oceans, or asteroid impacts. However, our study suggests that these events should be viewed within a broader context,” says Prof. Spiridonov.
The long-term climate state, or megaclimate, shaped how the Earth system and the biosphere responded to shorter-term disturbances such as abrupt warming or rising atmospheric carbon dioxide. Life proved more vulnerable under persistently warm conditions. Knowing the immediate trigger of a crisis is not enough; the overall state of the Earth system at the time also mattered.
The researchers stress that transitions between climate regimes were not the direct cause of every mass extinction. Rather, the long-term structure of the climate and carbon cycle created a background of biosphere vulnerability that influenced how strongly life reacted to volcanism, rapid warming, ocean changes or an asteroid impact.
Five long-term regimes mapped across the Phanerozoic
The team examined the entire Phanerozoic Eon, the period in which complex life evolved and diversified. They combined stable carbon and oxygen isotope data, reconstructions of ancient temperatures and atmospheric carbon dioxide levels, and the history of marine biodiversity.
“Using recurrence analysis, early-warning indicators of critical transitions, and a mathematical model of the climate and carbon cycle, we identified five long-term megaclimate regimes. The Earth system remained in these states for tens or even hundreds of millions of years before transitioning to another relatively stable state,” explains Prof. Spiridonov.
These regimes covered very different stages of Earth’s history, from the greenhouse climate of the early Paleozoic and the late Paleozoic ice age to the Mesozoic greenhouse and the later cooling of the Cenozoic. The researchers named the states “Haggis” intervals, a reference to the traditional Scottish dish and to the variegated, block-like appearance of the recurrence plots that revealed the repeating system states.
Prof. Spiridonov was struck by how clearly the Cambrian and Ordovician periods stood out. Many of the principal animal body plans found today appeared during this time, when biosphere vulnerability was also at its highest.
“The recurrence plots showed that the carbon-cycle state of the Cambrian and Ordovician periods was genuinely exceptional. The remaining approximately 450 million years of the Phanerozoic, beginning with the Late Ordovician mass extinction, displayed much greater recurrence. This was probably related to the emergence and establishment of land plants on the continents, as they fundamentally altered the cycling of carbon within the Earth system,” he says.
A new measure of biosphere vulnerability
To track the condition of life across different periods, the researchers developed a biosphere vulnerability index. It incorporates extinction rates, the appearance of new taxa, overall organism turnover, and the level of biological diversity present at the time.
“A high index value means that the biosphere is undergoing rapid restructuring: some groups of organisms disappear, while others emerge or replace them. Environmental stress is usually associated primarily with extinctions, but it can also be reflected in the rapid emergence of new species or genera. When environmental conditions change abruptly, a species may become extinct, but isolated populations may also adapt rapidly to new conditions and eventually develop into new species.
“The vulnerability index therefore captures overall taxonomic turnover and assesses it in the context of total biodiversity: the greater the diversity, the broader the ecological space in which life can exist. A rapidly restructuring biosphere experiences greater macroevolutionary stress and may respond more sensitively to additional environmental disturbances,” explains Prof. Spiridonov.
Peaks in the vulnerability index aligned with almost all periods of elevated extinction examined in the study, including the Big Five mass extinctions. Particularly strong increases appeared during the early and middle Paleozoic and at the Permian–Triassic boundary, the largest mass extinction on record.
The analysis also revealed clear differences between long-term climate states. During the warm early Cambrian, the biosphere remained consistently vulnerable, while vulnerability was lowest during the generally cool Cenozoic. Long-term temperatures and the boundaries of the climate system helped set the broader background against which extinction risk developed.
Separating the trigger from the background state
Until now, mass extinctions have often been examined as isolated events focused on their immediate causes. The new approach distinguishes between the specific event that triggered a crisis and the long-term environmental conditions that may have made the biosphere especially sensitive at that moment.
“Long-term climate regimes created a certain baseline level of biological stress. Shorter-term climatic fluctuations and catastrophic events occurred within an environment already shaped by those conditions. A severe biological crisis could therefore arise without a transition of the entire climate system, but its impact depended on the system’s preceding state,” says the VU researcher.
The authors note that the framework is designed for global and interregional datasets. Geological records vary in resolution and contain gaps, so the precise boundaries of the regimes and their links to individual biological crises will require further testing.
In future work, the method could be applied to specific groups of organisms, ecosystems, regions and different geological periods. That would help show which forms of life were most sensitive to climate-system transitions and whether the same environmental disturbance produced biological consequences of different magnitudes under different long-term Earth system states.
The research involved scientists from Vilnius University, The Open University, the University of New Mexico, the Potsdam Institute for Climate Impact Research, the University of Potsdam and the UK’s National Physical Laboratory.
