The El Niño–Southern Oscillation (ENSO) is the strongest year-to-year climate fluctuation on the planet. It is spawned in the tropical Pacific Ocean, but its societal and environmental impacts are felt worldwide. The character of ENSO, which is a naturally occurring phenomenon alternating between warm (El Niño) and cold (La Niña) phases, depends on the background climatic conditions in which it develops.
The climate conditions associated with ENSO are changing as the planet warms through unabated atmospheric greenhouse gas emissions, raising questions about whether anthropogenic greenhouse forcing has affected ENSO already, or will in the future. These questions have been debated for nearly 30 years, but they take on greater urgency as the manifestations of climate change become ever more apparent.
Sixty research scientists from around the world gathered to address these questions at a symposium held at CSHOR in Australia.
Participants considered suggestions from the instrumental record and paleoproxy data—primarily of tropical Pacific sea surface temperature—that climate change has affected the observed ENSO cycle.
The evidence was deemed to be inconclusive.
The past 40 years have witnessed three extreme El Niños (1982–1983, 1997–1998, and 2015–2016) unlike any comparable period in the nearly 150-year-long instrumental record. However, 150 years is too short a period to unambiguously determine a climate change effect, considering natural variability and data reliability before 1950.
Paleoproxies can provide much longer records, with some studies finding that 20th-century ENSO variance is higher than in the distant past. However, paleoproxy records are relatively limited in geographical distribution, and their interpretation is complicated by the convolution of biological, geochemical, and physical factors not related to climate, leaving large uncertainties.
For future projections, scientists rely on climate models whose performance has been improving with regard to representation of ENSO, although the climate sensitivity varies widely across models and systematic errors persist. However, the latest modeling studies coupled with theoretical understanding suggest that under the usual emission scenarios, occurrences of strong ENSO events may increase by the end of the 21st century.
Regardless of whether the ENSO cycle has been affected already or will be affected in the future, the effects of ENSO today appear to be compounded by climate change simply because of the superposition of ENSO conditions on a warmer background state. This became most evident during the extreme 2015–2016 El Niño, which coincided with record-breaking cyclone activity in the tropical Pacific, an unprecedented global coral bleaching event, and extreme disruption of ecosystems and fisheries in the central Pacific (linked to record high sea surface temperatures there).
The ENSO Science Symposium participants discussed the need to reduce uncertainty in ENSO predictions and long-term projections, which will require ongoing efforts to sustain satellite and in situ climate observing systems, expand the database of paleoreconstructions, and improve models. There is likewise a need for better understanding of climate interactions across ocean basins, across timescales spanning decadal variability, and interactions with biogeochemistry. Significant progress has been made on many of these topics, but more work remains to be done.
The symposium was followed by a 2-day coordination workshop on an AGU monograph for the AGU Centennial titled ENSO in a Changing Climate, which will cover the latest science on ENSO dynamics, effects, prediction, and future projections.
CSHOR sponsored the symposium and the book workshop. CSHOR is a joint research center between the Qingdao National Laboratory for Marine Science and Technology and the Commonwealth Scientific and Industrial Research Organisation. This is Pacific Marine Environmental Laboratory (PMEL) contribution 4951.
—Michael J. McPhaden ([email protected]), PMEL, National Oceanic and Atmospheric Administration, Seattle, Wash.; Agus Santoso, Australian Research Council (ARC) Centre of Excellence for Climate Extremes and Climate Change Research Centre, University of New South Wales, Sydney, Australia; and CSHOR, CSIRO Oceans and Atmosphere, Hobart, Tas., Australia; and Wenju Cai, CSHOR, CSIRO Oceans and Atmosphere, Hobart, Tas., Australia; and Key Laboratory of Physical Oceanography/Institute for Advanced Ocean Studies, Ocean University of China and Qingdao National Laboratory for Marine Science and Technology, Qingdao, China