Earth’s climate is shifting at a pace unmatched in recent geological history, surpassing the intense warming of the Paleocene-Eocene Thermal Maximum (PETM) that occurred 56 million years ago. New research reveals that today’s human-caused surge in greenhouse gases and temperatures proceeds about 10 times faster than during the PETM, challenging plants—the planet’s primary climate stabilizers—to adapt quickly enough. This accelerated rate risks undermining Earth’s natural carbon absorption mechanisms, potentially extending the duration and severity of current climate disruptions.
The PETM marked a profound shift at the boundary between the Paleocene and Eocene epochs, driven by massive carbon releases equivalent to thousands of gigatons into the atmosphere and oceans. Global temperatures spiked by 5-8°C over just a few thousand years, superimposed on already warming early Paleogene conditions, with effects lingering for 150,000-200,000 years. Evidence from oxygen isotopes in foraminifera shells and sea surface temperature proxies like TEX86 confirms this rapid onset, with tropical oceans hitting over 36°C and high-latitude Arctic waters reaching 23°C—levels comparable to modern subtropics.
A Planetary Past and Present at Breakneck Speed
During the PETM, carbon flooded the system over about 5,000-6,000 years, likely from methane hydrate destabilization, volcanic activity like North Atlantic Igneous Province eruptions, or orbital forcings during eccentricity maxima. This pushed average global temperatures up by 5-6°C or more, reshaping ecosystems worldwide and causing the largest deep-sea extinction in 93 million years alongside bursts of diversification on land and surface oceans. In North America’s Bighorn Basin, detailed pollen and fossil records document the collapse of large, carbon-storing forest trees, replaced by resilient, smaller drought-tolerant species such as palms, ferns, and increased herbaceous plants.
High-latitude regions experienced dramatic greening. Arctic areas shifted from conifer forests to dense subtropical swamp vegetation with taller, more productive plants, while the southwestern Pacific and Australo-Antarctic Gulf saw sea surface temperatures climb from 26°C to 33°C at 65°S paleolatitude. North Sea waters jumped 10°C to 33°C, and West Siberian seas warmed to 27°C, indicating reduced polar amplification compared to today but still extreme conditions. Sediment cores from Spain’s Tremp-Graus Basin show fluvial systems expanding and alluvial deposition rates rising after a 3,800-year lag, with mercury and osmium anomalies pointing to intense volcanism as a trigger.
Today’s crisis accelerates this pattern dramatically. Over the last 150 years, atmospheric CO2 has risen over 40%—from 280 ppm to more than 420 ppm—driving 1.3°C of warming already, with rates 10 times those of the PETM per unit time. Unlike the PETM’s gradual multi-millennial rollout, modern changes compress equivalent shifts into decades, leaving scant migration or evolutionary time for long-lived plant communities. This disparity, backed by models comparing carbon release rates, underscores why current ecosystems face heightened stress.
How Fast Warming Overwhelms Plants and Slows Recovery?
Plants regulate climate by absorbing CO2 via photosynthesis, sequestering it long-term in biomass, roots, and soils—processes that faltered severely during the PETM. Fossil evidence indicates a 70-100,000-year loss of biospheric carbon stocks, as vegetation biomass dropped and soil carbon storage weakened, prolonging elevated temperatures. In mid-latitude sites like Wyoming, leaf fossils show increased insect damage from nutrient-poor, CO2-enriched plants, signaling stressed food webs and reduced overall sequestration efficiency.
Terrestrial ecosystems underwent profound restructuring semi-arid paleosols with carbonate nodules formed from heightened weathering and runoff, while glauconite formation halted unlike in slower hyperthermals. Benthic foraminifera suffered mass die-offs, but surface ocean microbes and land mammals diversified, with many species range-shifting poleward into new continents. Recovery lagged, as smaller plants dominated, limiting carbon drawdown and contributing to the event’s 200,000-year span.
Contemporary observations mirror these vulnerabilities. Mountain species struggle to migrate upslope fast enough, with experiments showing heat stress overwhelming even resilient types like dinoflagellates that perished in PETM tropics. Climate models predict widespread forest dieback and vegetation loss without deep emission cuts, as restoration to pre-disruption carbon sink capacity demands tens of thousands of years—impossible on human timescales. Elevated CO2 may briefly boost some growth, but rapid warming disrupts this, echoing PETM patterns where productivity gains were short-lived and regionally uneven.
An Uncertain Future: Lessons from Deep Time
Deep-time records from the PETM warn that threshold crossings overwhelm biological feedback loops essential for climate stability. While high-latitude swamps thrived temporarily with enhanced density, most regions—especially mid-latitudes—saw persistent declines in vegetation function, amplifying heat through failed sequestration. Spatial climate patterns reveal uneven warming, with stronger tropical and subtropical responses, and estimates suggest equilibrium climate sensitivity during PETM exceeded modern values due to potent cloud feedbacks in an ice-free world.
Today’s 10-fold faster pace heightens these dangers, as plants cannot track changes via migration or adaptation, risking cascading ecosystem failures. Post-PETM temperatures eventually cooled to baseline over millions of years into the Eocene Optimum, but gradual declines followed toward the Eocene-Oligocene boundary, driven by shifting vegetation zones equatorward. Current orbital minima predict our warming phase could persist 50,000 years absent interventions.
Experts call for immediate action. Protecting carbon-rich habitats, advancing conservation, and achieving rapid emission reductions can bolster resilience against irreversible biosphere damage. “What happened on Earth 56 million years ago highlights the need to understand biological systems’ capacity to keep pace with rapid climate changes and maintain efficient carbon sequestration,” state Vera Korasidis of the University of Melbourne and Julian Rogger of the University of Bristol, urging integration of paleo-insights into modern strategies. These ancient lessons demand proactive measures to safeguard Earth’s regulatory networks.
