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Effects & Impacts : ENSO (El Niño / La Niña)

ENSO (El Niño / La Niña)

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Summary

ENSO is a cycle of warm El Niño and cool La Niña episodes that happen every few years in the tropical Pacific Ocean. It is the most dramatic year-to-year variation of the Earth’s climate system, affecting agriculture, public health, freshwater availability, power generation, and economic activity – McFadden et al (eds), 2020

Although they begin in the Pacific, the impacts are global. During La Niña, the Pacific Ocean retains more heat, so average global temperatures are cooler. During El Niños, the ocean releases heat into the atmosphere, so global temperatures are higher.

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Summary

Graph showing Global temperature anomalies during El Niño and La Niña conditions 1950-2026

Fig. 1: Global temperature anomalies ENSO (El Niño / La Niña) 1950-2026. Image: Screen grab from Our World in Data

When the global climate was stable at temperatures lower that today, these peaks and troughs in temperatures average out. This is clearly visible in the period 1950-1978 (Fig. 1).

By the mid 1970s, the increasing amount of greenhouse gases in the atmosphere prevented some of the heat from the sun escaping back into space (see Energy Imbalance below). About 90% of this excess heat—around 23 Zettajoules annually the equivalent heat of 12 Hiroshima sized atomic bombs exploding in the ocean every second of every day of every year (Fig. 2)—is absorbed by the oceans.

Water has a very high specific heat capacity, so the ocean can store massive amounts of this thermal energy with relatively small changes in temperature—although temperatures are rising fast, and due to thermal expansion, this heat is one of the causes of accelerating sea level rise.

El Niños periodically release some of this stored heat from the oceans into the atmosphere. While La Niñas take some of this heat back into the ocean. In 2023-2024 the effects of El Niño helped set the highest global ocean temperatures on record (Fig 3). But the ‘cooling effects of a subsequent La Niña barely made a difference.

“Cool” years are now hotter than the “warm” years of the past: tracking global temperatures through El Niño and La Niña. – OurWorld in Data, 03 March 2025

In 2026, instead of heading into a neutral ENSO phase, a ‘super’ El Niño is on it’s way .The Pacific basin covers one third of the planet, so changes in this area have profound effects on east Asia and the western areas of the Americas. That leads to a domino effect around the planet.

Exactly what drives El Niño and La Niña to flip back and forth is still unclear. Recent research suggests that the increasing loss of sea-ice around Antarctica is leading to more warming in the eastern equatorial Pacific, which is where ENSO patterns form. The long term trend is for “more persistent ENSO damages“.

Ocean heat content compared to atmospheric heat.Image: von Shuckmann et al.

Fig. 2 The amount of heat that the ocean, land, ice, and atmosphere has been absorbing 1960-2020 as measured in Zetajoules. Image: von Schuckmann et al. 2023

Fig. 3: Annual average temperatures to 2025. Image: Copernicus

Sea surface temperature anomaly 01 June 2026.

Fig. 4: Sea Surface temperatures to 01 June 2026 heading into a ‘super’ El Niño. Click the image for daily updates and select ‘Sea Temperatures’. Image: Copernicus

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Radiative Forcing (RF) - why the energy imbalance is increasing

Some incoming radiation is reflected back into space due to the albedo effect of the cryosphere (ice caps, permafrost, glaciers) and atmospheric aerosols much of which, paradoxically, comes from burning fossil fuels.

About 89% of the retained excess heat is being absorbed by the oceans. The land takes up ~6%. The cryosphere absorbs ~4%, which is why it’s rapidly melting, and the remaining 1% of excess heat is in the atmosphere.

The imbalance leads to energy accumulation in the atmosphere, oceans and land, and melting of the cryosphere, resulting in increasing temperatures, rising sea levels, and more extreme weather. – Mauritsen et al 2025

The role of the land, ocean, and cryosphere in taking up most of the heat is included in climate models. However:

Worryingly, the observed energy imbalance is rising much faster than expected, reaching 1.8 W/m2 in 2023—or twice that predicted by climate models—after having more than doubled within just two decades. – op. cit.

While the land and oceans absorbed 95% of the excess heat, they’re also doing double duty by absorbing (mostly via plants) 38% of the excess CO₂ we’re putting into the atmosphere. If not for them, temperatures would be much higher (see Greenhouses gases and how they work: Video 1).

In 2024, the global annual growth rate of atmospheric CO₂ surged to a record of 3.73 ppm year—the highest since 1959—exceeding the 1.5°C climate threshold for the first time. – Dang et al April 2026

Human emissions were still climbing, but not nearly enough to account for all that extra CO_2. Dang et al all found that from 2014-2023 the land had been absorbing and average 3.22Gt/year of carbon. But in 2024—the year that temperatures spiked above 1.5°C (Fig. 3)—the land suddenly stopped absorbing about a third of the CO₂ that it had done over the previous ten years of measurements.

Fig. 5: The imbalance between the incoming radiation (black line) and outgoing infrared radiation (red line). The imbalance leads to energy accumulation in the atmosphere, oceans, and land, leading to rising temperatures (Image: Leon Simmons, February 2025).

….the underlying mechanism for the reduction was caused by hotter and drier conditions leading to a larger increase in respiration than photosynthesis….These results challenge previous assumptions about the long-term stability of the terrestrial carbon sink. – op cit.

This explained the increasing energy imbalance and corresponding sudden increase in temperatures. The land had been taking up ~6% of the heat, which was driving up ground surface temperatures, increasing soil respiration, and triggering decomposition of soil organic matter, which released CO₂. Together with melting permafrost and an increase in forest fires and ongoing destruction of forests, the land had not only reduced the amount of CO₂ that it had once absorbed, it was now releasing long-held CO₂ back into the atmosphere. A tipping point had been crossed.

This had already been raised in 2021 by Prof. Johan Rockström, but as with all COPs it was ignored:

The only reason why the IPCC provides the world with the remaining carbon budget* of roughly 4 – 500 gigatonnes, is that the models assume that the carbon sink capacity of intact nature will continue. So not only are we assuming that biological systems will not cross tipping points, we’re also assuming that the stocks of carbon in forests in soils, in wetlands and permafrost, remain reasonably intact over the next 50 years. That is quite an optimistic assumption because it means we need to invest in conserving the remaining intact ecosystems. – Rockström, COP 26 (see Tipping points: Video 2).

* The ‘carbon budget’ was the assumed amount of carbon that we could keep releasing into the atmosphere to keep temperatures under 1.5°C , on the assumption that the land and ocean would keep absorbing it.

References and further reading
2026; Tsuchida et al; Multi-year La Niña–El Niño transition influenced Earth’s extreme energy uptake in 2022–2023 Nature Geoscience 19 pp432–438

2025: Peng et al; Strong 2023–2024 El Niño generated by ocean dynamics Nature Geoscience 23 May

2025: Lu et al; Increased frequency of multi-year El Niño–Southern Oscillation events across the Holocene Nature Geoscience 18 pp337-343 (Open access)

NIWA: What are El Niño and La Niña

NIWA: The impact of El Niño and La Niña on New Zealand’s climate
2022: Lopez et al; Projections of faster onset and slower decay of El Niño in the 21st century, Nature Communications v13 no. 1915
2020: McFadden et al (eds) El Niño Southern Oscillation in a Changing Climate, AGU book series
2020: Cai et al; Butterfly effect and a self-modulating El Niño response to global warming Nature 585 pp68–73

Effects

ENSO (El Niño / La Niña)

Summary

Effects

Summary

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