Research Highlight: Recent El Niños are the Most Intense in 700 Years

Researchers including Scripps Institution of Oceanography, UC San Diego, climate scientist Shang-Ping Xie have constructed a 700-year record of the climate phenomenon known as El Niño that may be the most accurate ever made.

The international team representing three continents reconstructed a climate history using more than 2,200 tree ring records and has concluded that anthropogenic climate change made the late 20^th century an unusually active period for El Niño.

The team also said it has demonstrated that El Niño is more sensitive to fluxes in solar energy than climate models had suggested. The researchers found strong correlations between major volcanic eruptions and immediate spikes in El Niño intensity in following years. That discernible sensitivity strengthens the researchers’ conclusion that recent global warming is influencing long-term El Niño-Southern Oscillation (ENSO) patterns. Climate models vary in predicting how ENSO activity will change in the face of such trends, but Xie said some of the things places like California can expect include a heightened risk of coastal flooding, stronger storm surges and increased beach erosion as storm tracks move southward.

“Many climate models do not reflect the strong ENSO response to global warming that we found,” said Xie, the first Roger Revelle Chair in Environmental Science at Scripps. “This suggests that many models underestimate the sensitivity to radiative perturbations in greenhouse gases. Our results now provide a guide to improve the accuracy of climate models and their projections of future ENSO activity. If this trend of increasing ENSO activity continues, we expect to see more weather extremes such as floods and droughts.”

Spawning droughts, floods, and other weather disturbances worldwide, ENSO impacts the daily life of millions of people. During El Niño, Atlantic hurricane activity wanes and rainfall in Hawaii decreases while Pacific winter storms shift southward, elevating the risk of floods in California.

The ability to forecast how ENSO will respond to global warming thus matters greatly to society. Providing accurate predictions though is challenging because ENSO varies naturally over decades and centuries. Instrumental records are too short to determine whether any changes seen recently are simply natural or attributable to man-made greenhouse gases. Reconstructions of ENSO behavior are usually missing adequate records for the tropics where ENSO develops.

The study, “El Niño modulations over the past seven centuries” led by Jinbao Li of the University of Hong Kong and the International Pacific Research Center, University of Hawaii at Manoa, appears in the June 30 advance online publication of the journal Nature Climate Change. The National Science Foundation, NOAA, and the National Basic Research Program of China were among the funders of the research.

In the year after a large tropical volcanic eruption, our record shows that the east-central tropical Pacific is unusually cool, followed by unusual warming one year later. Like greenhouse gases, volcanic aerosols perturb the Earth’s radiation balance, said Li. In the case of eruptions, volcanic aerosols reflect sunlight, reducing solar radiation that reaches the Earth surface.

“This supports the idea that the unusually high ENSO activity in the late 20^th century is a footprint of global warming,” he said.

Tree rings have been shown to be very good proxies for temperature and rainfall measurements. The inclusion of tropical tree-ring records enabled the team to generate an archive of ENSO activity of unprecedented accuracy, as attested by the close correspondence with records from equatorial Pacific corals amassed by Scripps geologist Chris Charles and others and with an independent Northern Hemisphere temperature reconstruction that captures well-known climate patterns of El Niño influence that extend over multiple ocean basins.

These proxy records all indicate that ENSO was unusually active in the late 20^th century compared to the past seven centuries, implying that this climate phenomenon is responding to ongoing global warming.

“Though the tree-ring archive has several advantages for the reconstruction of inter-annual climate variability – for example, the continuity of records, the near-absolute accuracy in dating, and the ubiquity in regions of societal importance – none of the previous attempts to reconstruct ENSO using tree rings have been too convincing,” said Charles. “This is probably because, as the authors point out, records from the proper locations were either unavailable or not included in the analyses.  Now, however, with the inclusion of tropical tree ring records, the authors show encouraging agreement between their approach and completely independent marine-based (coral) approaches to ENSO reconstruction. This agreement suggests that we might finally be able to capitalize on the unique advantages of the tree-ring archive for understanding the evolution and global influence of the ENSO system.”

Scripps will also further ENSO research and operational ENSO forecasting through an unrelated program recently funded by NOAA that is planned for launch later this year. Researchers with the Cooperative Institute on Marine Ecosystems and Climate (CIMEC) will augment the existing El Niño observing system with a suite of measurements spanning the equatorial Pacific Ocean, tracking El Niño signals across the full width of the Pacific. The equatorial ocean will be the subject of enhanced observations by the Argo network of profiling floats as well as profiling gliders. An ocean model will be used to assimilate these and other satellite and in situ datasets to demonstrate their impact on observing and understanding tropical Pacific variability.

Besides Xie and Li, report co-authors include Edward Cook and Roseanne D’Arrigo of Lamont-Doherty Earth Observatory; Mariano Morales of the Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA) in Argentina; Duncan Christie of the Universidad Austral de Chile and the Center for Climate and Resilience Research in Chile; Nathaniel Johnson of University of Hawaii at Manoa, Fahu Chen, Xiaohua Gou, and Kevan Fang of Lanzhou University in China; and Anthony Fowler of the University of Auckland in New Zealand.

 – Robert Monroe