Understanding how human-induced climate change interacts with the natural El Niño–Southern Oscillation is among the most vital and complex challenges in contemporary climate science.
What the IPCC AR6 Concludes
The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) synthesized thousands of peer-reviewed studies and paleoclimate records, reaching key consensus findings:
- ENSO Persistence: The El Niño–Southern Oscillation will remain the dominant driver of global year-to-year climate variability in the 21st century.
- Rainfall Extremes Amplify: Under greenhouse gas warming, the moisture-holding capacity of air increases by roughly 7% per degree Celsius of warming (the Clausius-Clapeyron relation). Consequently, the precipitation anomalies associated with both El Niño and La Niña will become significantly more intense.
- Increased Frequency of "Extreme" Events: State-of-the-art climate models project an increased frequency of extreme El Niño and La Niña events under high-emission scenarios.
The Shift Toward Relative ONI (RONI)
Historically, the Oceanic Niño Index (ONI) measured Niño 3.4 sea surface temperatures against a 30-year base period updated every five years. However, because the entire tropical ocean has warmed over recent decades, a traditional fixed threshold could mistakenly identify permanent background warming as an ongoing El Niño event.
To address this, NOAA and climate researchers increasingly utilize the Relative ONI (RONI), which subtracts the average tropical ocean warming from the Niño 3.4 anomaly. This isolates the true east-west temperature gradient that actually forces atmospheric circulation shifts.
Compound Extremes
Background climate warming interacts with natural El Niño phases to produce compound disasters: heatwaves become hotter, droughts become more dehydrating, and rainfall events over saturated river basins become more torrential.