Approximately 56 million years ago, our planet experienced a significant and abrupt intensification of heat. Over a period of roughly 5,000 years, atmospheric carbon levels surged dramatically, leading to a global temperature escalation of approximately 6°C.
In accordance with our recent investigation, disseminated through the esteemed journal Nature Communications, a notable consequence of this climatic shift was the diminished capacity of numerous global plant species to flourish.
Consequently, the absorption of atmospheric carbon by these plants was curtailed, a factor that may have perpetuated a peculiar characteristic of this ancient planetary heatwave: its protracted duration, extending beyond 100,000 years.
Presently, Earth is undergoing warming at a rate approximately ten times faster than that observed 56 million years ago, a pace that could present even greater challenges for contemporary flora to acclimate.
A Journey Back 56 Million Years
Vegetation plays a pivotal role in climate regulation via a mechanism termed carbon sequestration. This process entails the assimilation of carbon dioxide from the atmosphere through photosynthesis, followed by its storage within plant tissues, including leaves, wood, and root systems.
However, precipitous global warming events can exert a temporary impediment on this regulatory function.
Examining the response of Earth’s vegetation to the swift global warming episode that occurred approximately 56 million years ago—formally designated the Paleocene-Eocene Thermal Maximum (PETM)—presents considerable methodological hurdles.
To address this, we devised a computational model designed to simulate the evolution, distribution, and carbon cycle dynamics of plant life. The outcomes generated by this model were cross-referenced with fossilized pollen and plant trait data acquired from three distinct geographical locations, enabling the reconstruction of alterations in vegetation characteristics, such as stature, leaf mass, and deciduousness, throughout the warming period.
The selected study sites encompass the Bighorn Basin in the United States, the North Sea region, and the Arctic Circle.
Our research specifically targeted fossil pollen due to its distinctive and advantageous attributes.
Primarily, pollen is generated in prodigious quantities. Secondly, it is disseminated widely by aerial and aquatic currents. Thirdly, its robust structural composition renders it resistant to decomposition, facilitating its exceptional preservation within ancient geological strata.
A Transformation in Flora
At the mid-latitude study locations, including the Bighorn Basin—a broad and deep valley nestled within the northern Rocky Mountains—evidence indicates a diminished capacity of the local vegetation to influence climate regulation.
Analysis of pollen records points to a transition towards smaller plant forms, such as palms and ferns. Concurrently, leaf mass per unit area, a metric reflecting leaf density and thickness, saw an increase, coinciding with a decline in deciduous tree species. Furthermore, fossil soil examinations revealed a reduction in soil organic carbon content.
The collected data suggests that smaller, arid-adapted flora, including various palm species, prospered in this environment due to their resilience to escalating temperatures. However, their prevalence was associated with a reduced efficiency in sequestering carbon within biomass and soil reservoirs.
In stark contrast, the high-latitude Arctic site exhibited an augmentation in vegetation height and biomass following the warming event. Pollen data revealed a succession from conifer forests to broad-leaved swamp flora, alongside the persistence of certain subtropical species, such as palms.
Our model and empirical data indicate that high-latitude regions possessed the capacity to adapt and even enhance their carbon dioxide assimilation and storage rates under the ameliorated climatic conditions.
A Foresight into Future Scenarios
The disruption of vegetation patterns during the PETM era may have resulted in a prolonged period, spanning 70,000 to 100,000 years, of reduced terrestrial carbon sequestration. This was attributed to the impaired ability of both flora and soil ecosystems to effectively capture and retain carbon.
Our findings suggest that vegetation species exhibiting superior climate regulatory capabilities required an extensive temporal scale to re-establish themselves, a factor that significantly contributed to the protracted duration of the warming event.
A global temperature increase exceeding 4°C during the PETM overwhelmed the adaptive capacity of mid-latitude vegetation. The current anthropogenic warming trend, proceeding at ten times the rate, further constrains the timeframe available for plant adaptation.
The historical episode on Earth 56 million years ago underscores the critical importance of comprehending the inherent capabilities of biological systems to keep pace with rapid climatic transformations and sustain effective carbon sequestration processes.
