When La Niña Years Break Heat Records: The Global Warming Baseline Shift

Published: June 3, 2026 · 8 min read

There's a basic rule in climate science: El Niño years are hot, La Niña years are cooler. That's the textbook answer. But the textbooks were written before the 2020s happened. The last few years have fundamentally challenged that framework — La Niña years that should have been relatively cool have instead broken global temperature records. This isn't a glitch. It's the baseline shifting under our feet.

The simple version: global warming has pushed the baseline so high that even the "cool" phase of ENSO is now warmer than the "hot" phase was a few decades ago. Thinking of La Niña as a "cool event" in today's climate is like thinking of a 30°C day as "cool" — it depends entirely on what you're used to.

The Data That Broke the Rule

Let's look at what actually happened. The 2020-2023 La Niña was a rare "triple-dip" — three consecutive La Niña years, which typically means sustained below-average global temperatures. Historically, La Niña years are 0.1-0.2°C cooler than the preceding and following years.

Yet 2021 — a La Niña year — had a global mean temperature of about 1.11°C above pre-industrial levels. 2022, also a La Niña year, was at 1.09°C above pre-industrial. For context, the 1997-98 El Niño — one of the strongest on record — pushed global temperatures to about 0.63°C above pre-industrial levels. A La Niña year in 2021 was nearly twice as warm as the strongest El Niño was in 1998. Let that sink in.

The year 2023 transitioned from La Niña to El Niño and became the hottest year on record at 1.45°C above pre-industrial levels. But the early months of 2023 were still under La Niña conditions, and those months were already among the warmest on record for their respective calendar positions.

Why This Happens: The Baseline Has Moved

The concept is straightforward. Imagine a bathtub that has been filling with hot water for decades. That's greenhouse gas warming. Every so often, you turn on the cold tap (La Niña) or the hot tap (El Niño). The cold tap still cools the tub relative to the hot tap — but the overall water level is higher and hotter than it was before, regardless of which tap you're running.

This is called "baseline drift." The long-term warming trend from CO₂, methane, and other greenhouse gases has raised the global average temperature by roughly 1.3°C since pre-industrial times. ENSO modulates that baseline, but it doesn't determine it. The 2020-23 La Niña tripped on top of a baseline that was already warmer than any El Niño peak in the 20th century. Calling it a "cool event" is technically correct but practically misleading.

The 2023-24 Case Study: La Niña's Legs on a Record-Breaking Year

The 2023-24 El Niño started with a baseline that had been elevated by three years of La Niña. Usually, La Niña suppresses global temperatures enough that the subsequent El Niño has more room to run. But in this case, the suppression wasn't strong enough — global temperatures were already near record levels when the El Niño began. The result was an El Niño that broke annual temperature records by a wide margin.

What this tells us is that the cooling effect of La Niña is being progressively weakened as the climate warms. A study published in Nature Communications in 2024 showed that the global cooling impact of La Niña has diminished by roughly 30% since the 1980s, because the background warming overwhelms the ENSO signal in absolute terms. The relative difference between El Niño and La Niña hasn't changed much — but the absolute temperatures in both phases are climbing.

This has real implications for how we interpret ENSO forecasts. When NOAA says "La Niña brings cooler conditions to the central Pacific," they mean cooler relative to the current baseline, not cooler in absolute historical terms. A moderate La Niña in 2030 might produce global temperatures that would have been considered a severe El Niño in 1990.

What This Means for Future ENSO Predictions

Honestly, this shift makes ENSO forecasting harder in ways that forecasters are still figuring out. The traditional approach of comparing current conditions to historical analogs becomes less useful when the baseline is no longer stationary. An El Niño that looks "moderate" by the Niño 3.4 index might produce impacts more typical of a strong event, simply because the base it's building on is warmer.

This is already happening. The 2023-24 El Niño peaked at +2.0°C in the Niño 3.4 region — technically a strong event but not a super El Niño like 1997-98 or 2015-16. Yet its global impacts — 2023 as the hottest year on record, unprecedented ocean heat, extreme weather across multiple continents — were in some ways more severe than those super events. The same ENSO amplitude produced more extreme outcomes because the starting point was higher.

Some research groups are now calling for a "warming-adjusted ENSO index" that accounts for the secular warming trend. The idea is to subtract the background warming from the Niño 3.4 index so that you're measuring the ENSO signal against a moving baseline rather than a fixed one. It makes scientific sense, but it hasn't been adopted operationally yet.

The Bottom Line

Calling La Niña a "cooling event" in the 2020s climate is technically correct but practically misleading. The global warming baseline has shifted so much that even the cool phase of ENSO now produces temperatures that would have been record-breaking a generation ago. This doesn't mean ENSO is broken — it means the frame of reference has changed. Every La Niña from here forward will be warmer than the last, and every El Niño will have higher baseline to launch from.

If you're reading ENSO forecasts, here's the practical takeaway: pay more attention to the absolute temperature impacts than to whether it's "El Niño" or "La Niña." The phase tells you the direction of the anomaly. The baseline tells you how hot it's actually going to be.

Further Reading

• 2026 El Niño Watch — current data and forecasts
• 2023-2024 El Niño review
• Indian Ocean Dipole — ENSO's partner

References: NOAA NCEI global climate reports 2021-2024; WMO annual state of the climate reports; IPCC AR6 WG1 Chapter 3 (ENSO in a warming climate); Wang, B. et al. "Understanding the recent increase in multi-year La Niña" (Nature Climate Change, 2024); Cai, W. et al. "ENSO and greenhouse warming" (Nature Climate Change, 2021); Copernicus Climate Change Service ERA5 reanalysis data.