Climate change is strengthening El Niño, corals reveal
A new study using fossil and living corals from the Galápagos Islands finds that El Niño events over the past 40 years have been substantially stronger — roughly 36-40% more intense — than at any point in the preceding thousand years
Researchers analysed 13 coral specimens (both living colonies and ancient boulder corals), reconstructing sea-surface temperature variability going back roughly a millennium using chemical signatures preserved in the coral skeletons
Climate models simulating natural variability alone (volcanic activity, solar cycles) could not reproduce the observed intensification, indicating that post-industrial, human-caused warming is the likely driver
The findings were published in the journal Science, led by researchers at the University of Michigan
Stronger El Niño events are linked to intensified droughts, floods, wildfires, and food insecurity across affected regions, including India's monsoon-dependent agriculture
El Niño-Southern Oscillation (ENSO)
ENSO is a recurring climate pattern involving changes in sea-surface temperature and air pressure across the tropical Pacific Ocean, alternating between a warm phase (El Niño), a cool phase (La Niña), and a neutral phase. El Niño refers to anomalous warming of surface waters in the central and eastern equatorial Pacific, occurring irregularly every 2-7 years, and is a key example of an ocean-atmosphere "teleconnection" — a link between climate anomalies in geographically distant regions.
Key Details
- El Niño events recur roughly every 2 to 7 years; the current study's coral sampling protocol targeted specimens at least 20 years old to reliably capture multiple ENSO cycles
- El Niño years are generally associated with weaker Indian summer monsoon rainfall, while La Niña years tend to strengthen the monsoon
- The India Meteorological Department (IMD) uses ENSO status as one of the primary inputs for its seasonal Southwest Monsoon forecast models
- ENSO interacts with the Indian Ocean Dipole (IOD) — a positive IOD phase can partially offset the monsoon-suppressing effect of El Niño
The study's finding that El Niño is intensifying under climate change has direct bearing on India, since a stronger El Niño typically correlates with a weaker and more erratic monsoon, affecting agricultural output, water security, and food inflation.
Paleoclimate Proxy Records — Coral Sr/Ca and Oxygen Isotope Analysis
In the absence of direct temperature measurements extending back centuries, scientists use natural "proxy" archives — such as coral skeletons, tree rings, and ice cores — to reconstruct past climate. Corals build their calcium carbonate skeletons in growth bands, incorporating trace elements from surrounding seawater in ratios that vary with temperature, providing a dateable, high-resolution climate record.
Key Details
- Strontium-to-calcium (Sr/Ca) ratio: corals incorporate fewer large strontium atoms into their skeleton during warmer conditions (such as El Niño events), so a lower Sr/Ca ratio indicates warmer seawater
- Oxygen isotope ratio (δ¹⁸O, ratio of oxygen-18 to oxygen-16): warmer water favours incorporation of the lighter oxygen-16 isotope, so the ratio is used to cross-validate temperature reconstructions
- The Galápagos Islands sit in the eastern equatorial Pacific, the region where El Niño originates and is most strongly expressed, making local corals a particularly sensitive proxy archive
- This proxy method is analogous to techniques used in Antarctic ice-core studies (oxygen isotopes in ice) for reconstructing much longer paleoclimate records
These proxy techniques allowed researchers to extend the El Niño intensity record back roughly 1,000 years — far beyond the ~150-year instrumental record — enabling a robust comparison between pre-industrial and post-industrial El Niño strength.
Anthropogenic Climate Change and Extreme Weather Attribution
Climate change "attribution science" seeks to determine whether and to what extent human-caused greenhouse gas emissions have altered the frequency or intensity of specific climate phenomena, as distinct from natural variability. This study forms part of a growing body of attribution research linking warming to changes in established climate oscillations like ENSO, not just to extreme weather events.
Key Details
- The study ruled out natural drivers (volcanic eruptions, solar variability) by comparing observations against 1,000-year climate model simulations of natural-only forcing
- The IPCC's Sixth Assessment Report (AR6) similarly links unprecedented warming since the mid-20th century to human activity with "unequivocal" confidence
- Stronger El Niño events amplify downstream risks assessed under India's National Action Plan on Climate Change (NAPCC, 2008), particularly the National Water Mission and National Mission for Sustainable Agriculture
The study strengthens the evidence base that climate change is not only raising baseline temperatures but also amplifying natural climate oscillations like ENSO, compounding risks to monsoon-dependent economies such as India's.
- El Niño intensity increase measured: approximately 36-40% stronger over the last 40 years compared to the period before around 1850
- Number of coral specimens analysed: 13 (living colonies and fossil boulder corals)
- Proxy indicators used: Strontium-to-calcium (Sr/Ca) ratio and oxygen isotope (δ¹⁸O) ratio
- Historical record reconstructed: approximately 1,000 years
- El Niño recurrence interval: every 2 to 7 years
- Study location: Galápagos Islands, eastern equatorial Pacific
- Published in: Science journal, led by University of Michigan researchers (Julia Cole, Department of Earth and Environmental Sciences)