In a groundbreaking study published in Nature Communications, researchers have unveiled compelling evidence pointing to a stronger and more prolonged El Niño-Southern Oscillation (ENSO) during the Early Eocene epoch—approximately 56 to 48 million years ago—when the Earth was significantly warmer than today. This revelation is crucial for understanding how climate systems operated under past greenhouse conditions, informing predictions about our planet’s climate trajectory in an era marked by anthropogenic warming.
The ENSO phenomenon, characterized by periodic fluctuations in sea surface temperatures and atmospheric pressure in the equatorial Pacific Ocean, is the most influential mode of interannual climate variability. Modern ENSO events have profound impacts on global weather patterns, ecosystems, and economies. However, its dynamics under drastically different climate regimes remained elusive until now. By leveraging novel paleoclimate proxies and advanced climate modeling, Abhik, Dommenget, McGregor, and their collaborators present a detailed reconstruction and analysis of ENSO’s behavior during the Early Eocene.
This study situates itself within a broader effort to decipher Earth’s climatic past to better anticipate future changes. The Early Eocene represents a “hothouse” period, marked by elevated atmospheric CO2 concentrations and global temperatures surpassing current averages by several degrees Celsius. Existing paleoclimate evidence suggests that oceanic and atmospheric circulation patterns during this era diverged notably from today’s, raising questions about the intensity, frequency, and duration of ENSO events amidst such conditions.
The research team combined empirical data gleaned from sediment cores, marine fossils, and isotopic markers with state-of-the-art climate models. These models were meticulously tuned to replicate Early Eocene boundary conditions, including paleogeography, greenhouse gas concentrations, and solar insolation. The integration of proxy data with simulations allowed the scientists to discern ENSO characteristics distinct from the modern Pacific climate oscillations.
One of the principal findings highlights that Early Eocene ENSO events were not only stronger in amplitude but also exhibited a prolonged duration, sometimes persisting for multiple years. This is in contrast to the typically episodic nature of contemporary ENSO cycles, which commonly span 9 to 12 months. The extended ENSO phases would have significantly modulated global climate patterns, intensifying and extending periods of drought and rainfall across various continental regions.
Mechanistically, the study attributes these changes to altered ocean-atmosphere feedbacks under higher baseline temperatures. The enhanced warming of the tropics intensified the thermal gradient between the western and eastern Pacific Ocean, thereby amplifying the oceanic wave responses and atmospheric convection patterns central to ENSO dynamics. Additionally, Early Eocene ocean stratification and altered thermocline structures played a pivotal role in modulating ENSO behavior.
Particularly intriguing is the implication that the Early Eocene’s prolonged warm episodes may have sustained ENSO events, propagating their climatic influence over extended timescales. This result challenges prevailing assumptions that warmer climates would dampen ENSO variability. Instead, it appears that under elevated greenhouse gas conditions, ENSO could become a more dominant driver of climate variability, with far-reaching consequences for biospheric and geospheric systems.
The study also probes the potential feedback mechanisms linking ENSO with global carbon cycles during the Early Eocene. Stronger and longer ENSO events could have influenced oceanic carbon uptake and release, modulating atmospheric CO2 concentrations and climate feedback loops. This introduces a complex interplay between orbital forcing, Internal climate variability, and biogeochemical processes that governed Earth’s past climate evolution.
Furthermore, the patterns revealed in this research provide a tangible analog for the future, as modern anthropogenic greenhouse gas emissions push global temperatures into uncharted territory. Understanding the response of ENSO—the planet’s most significant climate oscillation—to warmer climates aids in forecasting potential changes to modern weather extremes. Amplified ENSO events in the future could exacerbate droughts, floods, and heatwaves worldwide, posing unprecedented risks to human societies and ecosystems.
Technically, the team’s climate models incorporated coupled atmosphere-ocean general circulation models (AOGCMs) with high spatial resolution and sophisticated physical parameterizations to faithfully simulate Early Eocene climate dynamics. Calibration against proxy reconstructions ensured model fidelity, enabling the isolation of ENSO signals from broader climatic noise. The researchers deployed spectral analysis and statistical methods to quantify ENSO amplitude, frequency, and persistence.
The robustness of the findings stems from the convergence of multiple lines of evidence and the application of rigorous sensitivity analyses. For instance, variations in greenhouse gas forcing, paleogeographic reconstructions, and oceanic nutrient cycles were tested independently to assess their influence on ENSO characteristics. The consistency across simulations and proxies strengthens the conclusion that Early Eocene ENSO was indeed distinctively intensified and prolonged.
This work underscores the importance of paleoclimatology as a vital tool for decoding Earth’s climate system responses under extreme conditions. By probing a deep-time interval with heightened global warmth, the research provides a natural laboratory for exploring how fundamental climate modes, like ENSO, adapt or transform. Such insights become increasingly relevant in light of ongoing climate change and its anticipated impacts on global weather variability.
In essence, this study marks a significant leap forward in paleoclimate research, revealing that the ENSO phenomenon was not merely present but enhanced and extended during a greenhouse world. This reshapes our understanding of past climate variability and compels us to rethink how future ENSO dynamics might evolve. The interplay between oceanic processes, atmospheric circulation, and carbon-climate feedbacks identified here opens new avenues for multidisciplinary exploration.
The findings could also influence the way researchers interpret paleoclimate records from other epochs, as ENSO variability imprints distinct signatures on sedimentation patterns, isotopic compositions, and terrestrial ecosystems. Recognizing stronger and longer ENSO phases in ancient climates enhances the resolution of these reconstructions, allowing for improved correlations between climatic events and their geological archives.
Moreover, this enhanced ENSO model can inform more accurate and reliable predictions of climate extremes, helping policymakers and planners anticipate challenges linked to water resources, agriculture, and disaster preparedness in a warming world. The study’s implications extend beyond academic curiosity, touching on practical considerations for building climate resilience.
In summary, the investigation into Early Eocene ENSO dynamics reveals that global warming in Earth’s deep past amplified the strength and persistence of this critical climate oscillation. This groundbreaking research bridges deep-time climate science with contemporary climate challenges, illuminating pathways for understanding and managing the intensifying impacts of a changing world’s most powerful climatic driver.
Subject of Research: Early Eocene El Niño-Southern Oscillation dynamics under greenhouse climate conditions.
Article Title: Stronger and prolonged El Niño-Southern Oscillation in the Early Eocene warmth.
Article References:
Abhik, S., Dommenget, D., McGregor, S. et al. Stronger and prolonged El Niño-Southern Oscillation in the Early Eocene warmth. Nat Commun 16, 4053 (2025). https://doi.org/10.1038/s41467-025-59263-7
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