A reconstruction of Pacific Ocean temperatures 21,000 years ago, based on the chemistry of tiny shells, adds hefty support to projections that climate change will make strong El Ni帽o events far more common, leading to more extreme weather around the world.
鈥淲e鈥檙e projecting a pretty dramatic change,鈥� says at the University of Arizona.
The irregular cycle between warmer- and cooler-than-average temperatures in the equatorial Pacific Ocean 鈥� known as the El Ni帽o-Southern Oscillation (ENSO) 鈥� is among the largest influences on the global climate. For years, climate models have projected warming due to greenhouse gas emissions would make this cycle more variable. Some have forecast a substantial increase in the frequency of El Ni帽o events that raise global average temperatures and amplify weather extremes across the planet.
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However, the short record of observed El Ni帽os 鈥� along with incomplete models 鈥� has left the influence of warming on ENSO among the most contentious questions in climate science.
Thirumalai and his colleagues have compiled a record of ENSO鈥檚 variability in the Pacific Ocean 21,000 years ago during the last glacial maximum, when the climate was much cooler than today. This record was largely based on measurements of the shells grown by microscopic plankton called foraminifera. Because temperature affects the chemical composition of the shells, they provide a record of the temperatures at the sea surface, where the plankton grow. When the tiny organisms die, they sink to the depths, where their shells are preserved in the sediment accumulating on the seafloor. 鈥淚t鈥檚 amazing to me these single-cell organisms can tell you about glacial changes,鈥� says Thirumalai.
The researchers compared the modelled projections of ENSO variability at that time with variability in sea surface temperature according to the chemistry of the shells. A match between the two would show the modelled projections of ENSO鈥檚 behaviour in the cooler past are accurate, which would help validate the model鈥檚 projections of ENSO鈥檚 behaviour in the hotter future are also reliable.
鈥淚f cooling made it weaker, then warming has to make it stronger,鈥� says at the University of Colorado Boulder, a co-author of the study.
The results of their analysis suggest this is exactly what we should expect. Not only did the team find a match between the projections of ENSO variability 21,000 years ago and the temperature records as reconstructed using the foraminifera shells, they also found this response to temperature was down to the same mechanism. According to their analysis, temperatures both in the cooler past and the hotter future influence ENSO via feedback between wind patterns in the Pacific and the thickness of a layer of warm water at the surface of the ocean. As global temperatures rise, this warm water layer becomes thinner, which makes it easier for winds and currents to push warm water east to spur an extreme El Ni帽o event.
Under a medium emissions scenario, this model 鈥� its reliability validated by the tiny shells 鈥� projects an extreme El Ni帽o event could occur every decade this century, rather than the roughly once-every-two-decades pattern seen historically. 鈥淲e could be left with this type of climate with a massively active El Ni帽o,鈥� says DiNezio.
at Brown University in Rhode Island says such projections are 鈥渧ery compelling when combined with this palaeoclimate validation鈥�, and the study adds to the 鈥済rowing pile鈥� of evidence that ENSO will become more variable with climate change. However, she says there is much more work to be done before we have a definitive answer on such an important question. 鈥淲hen all lines point to the same conclusion over many studies, that鈥檚 when we put things to bed.鈥�
Nature
