The Role of El Niño in Shaping the Global Mean Temperature During 2026 and 2027

Desislava Petrova is coordinator of the Climate and Planetary Change Hub of ISGlobal.

El Niño Southern Oscillation (ENSO) is widely recognized in the scientific literature as the dominant driver of year-to-year variability in global mean surface temperature (GMST). Its influence extends across oceans and continents through atmospheric teleconnections. Typically, GMST increases by 0.1-0.2°C during the warm phase of ENSO, with exceptionally strong events raising GMST even more. Currently, this additional rise in GMST as a result of El Niño and the release of stored heat from the tropical Pacific Ocean occurs on top of steadily rising temperatures due to anthropogenically driven global warming. Therefore, the sizable El Niño that is about to peak in the final months of 2026 is expected to result in a significant boost of GMST, potentially well above the Paris Agreement’s 1.5°C threshold relative to pre-industrial levels (GMST has stayed close to this level for the last 2-3 years: 1.45+/-0.12°C in 2023, 1.55°C in 2024, and 1.44+/-0.13°C in 2025).

How El Niño amplifies the annual global mean temperature

Despite being weaker (or of lower amplitude) than the strongest warm events on record, the previous El Niño in 2023/24 produced an unprecedented annual global mean surface temperature anomaly and contributed to the staggering additional 0.36°C year-to-year rise, as GMST reached 1.58°C above pre-industrial levels during the period from July 2023 to June 2024. This unprecedented increase was found to be largely driven by extreme sea surface temperature anomalies across tropical oceans, contributing over 90% of the observed interannual warming.

One of the direct culprits for this substantial increase in sea surface temperatures was the subsurface ocean heat accumulation during the preceding triple-dip La Niña event that occurred back to back for three consecutive years between 2020 and 2023. During La Niña excess subsurface ocean heat accumulates in the western equatorial Pacific, typically released to the ocean surface and transmitted to the atmosphere during the next warm phase of the oscillation.

In this way, the 2023/24 El Niño and preceding triple-dip La Niña turned into an important contributor to the record-breaking GMST in 2023 and 2024. Interestingly, observed GMST increased by 0.31°C from 2022 to 2023, which was also a record since measurements started, with almost the whole additional warming coming from a jump in tropical surface temperatures, with some additional contributions from extratropical warming over the Northern Hemisphere in central Asia, northern North America and the northwestern Pacific Ocean. A recent study demonstrated that when prolonged cold La Niña conditions were present in the tropical Pacific, a jump in GMST of 0.25°C or more was much more probable to occur with the next strong El Niño event, and this was exactly the scenario that developed between 2020 and 2024.

What are the implications for the 2026-2027 global mean temperatures?

Considering this context, what are the prospects for GMST in 2026 and 2027, provided that we are most probably witnessing the culmination of the strongest El Niño since records began? One particular aspect of importance is that in the beginning of this year we emerged from two consecutive La Niñas, which could be seen as a relatively prolonged period of the cold phase of ENSO that could set a stage reminiscent of 2023. However, it should also be noted that the two La Niñas were comparatively weak events, the Relative ONI index measuring ENSO strength reached an anomaly of about -1°C during both boreal winters of 2024 and 2025.

Because the decaying year of warm ENSO events is typically hotter than the developing one, 2027 is projected to reach unprecedented warming levels, approaching 1.7°C.

Another point to note is that sea surface temperatures (and surface air temperatures) were significantly lower in the first four months of 2026 than in 2024. Therefore, for 2026 to rank as the warmest year on record, a substantial number of the remaining months would need to be considerably warmer than measurements in 2024. Thus, it is not straightforward to project record warming in 2026. However, due to recent acceleration of global warming, global sea surface temperature in spring was about 0.13°C (up to 0.17°C depending on the data product) warmer than in 2023.

This temperature gap was expected to be maintained, and additionally we know now that the El Niño is much stronger than the previous one in 2023/24, which increases the likelihood that 2026 will set a new GMST record after all. This is because temperature in 2024 (the standing record year) was only 0.11°C higher than in 2023 (the previous record holder), and this means that 2026 is set to beat the record by about 0.02°C to 0.06°C or more.

Outlook for 2026 and 2027: Are We Heading for a New Record?

The decaying year of warm ENSO events is typically hotter than the developing one, which is why 2027 has been projected to reach an even greater warming close to 1.6°C or 1.7°C above pre-industrial. At the moment, no peer-reviewed article that forecasts the 2027 GMST has been published, but studies are expected to emerge from the scientific community in the next few months. If realized, such a 2027 outcome would mark the first time GMST approaches 1.7°C above pre-industrial levels, further narrowing the remaining margin to the 1.5°C long-term threshold.

The coming months of observations will therefore be critical in determining whether the post-El Niño decay delivers the record warmth currently anticipated, and this strongly depends on when the event will terminate. Some of the forecasts point to a prolonged El Niño, possibly lingering into the boreal winter of 2027 and strengthening the likelihood of a substantial annual overshoot of the 1.5°C mark, as we have studied in a new paper summarizing these forecasts by Petrova, Koopman and Rodó now under revision in the Bulletin of the American Meteorological Society.

For now, what we know is that GMST in 2026 has exceeded monthly temperatures in 2024 since July this year, and given that the El Niño is set to peak in the October-December season, this tendency is expected to continue until year-end and beyond.