The El Niño–Southern Oscillation (ENSO) has been a preoccupation of scientists in recent years. It has also entered the public consciousness because this climatic phenomenon in the tropical Pacific Ocean has worldwide effects.
It is uncertain how ENSO will behave in a future warmer world, but one source of possible clues is warm climates of the past. About 3 million years ago, during the mid-Pliocene warm period, global temperatures were 2°–3° warmer than present, and polar ice extents were about one third smaller, making this period interesting as a comparison to the near future.
Understanding climate conditions of the past is no simple task. There are two approaches to this—using proxy data or climate models—but each has strengths and weaknesses, and correlating various measures between the two can be difficult.
Good quality proxy data are accurate and can reconstruct long timescales but are collected from limited locations and may not have broad geographic relevance; translating measurements from proxy data into useful climate variables (such as temperature and precipitation) may also be difficult. On the other hand, climate models can be run on regional and global scales and examine climate variables directly, but they cover shorter timescales and may have errors.
Tindall et al. compare and combine proxy data and climate models in an attempt to reduce the uncertainties in our understanding of ENSO in a warmer climate of the past. Their hope is that their results and analysis might also help us to better understand how warm climates will influence ENSO in the near future.
They focus on two maritime sources of proxy data: coral and individual planktonic foraminifera (monocellular organisms that float in seawater at various depths). The ratio of stable isotopes oxygen-18 and oxygen-16 in the foraminifera is an indicator of ocean temperature. From these ratios, scientists can infer El Niño (warmer than average sea surface temperatures in the tropical Pacific) and La Niña (cooler than average sea surface temperatures) conditions.
The scientists used climate model results representing the mid-Pliocene warm period to verify that signals in the proxy data that appeared to be due to ENSO were being correctly interpreted. For example, extreme values of the ratio of stable isotopes in foraminifera from the eastern Pacific have previously been interpreted as ENSO activity, but modeling suggests that this interpretation may not always be suitable for past climates due to variability caused by other factors.
By simulating the presence of the two types of proxy data over a whole grid of locations in the Pacific, the team identify, theoretically, the best potential locations for actual collection of samples to provide further evidence of ENSO variability in the Pliocene era. They also suggest that at many locations, ENSO may be easier to detect in proxy data of Pliocene age than similar data from more recent times, as the model suggests that ENSO was stronger in the Pliocene.
As we face an uncertain climatic future, the past can provide some illumination of possible changing conditions, and a combination of data and modeling brings the best of science together. (Paleoceanography, https://doi.org/10.1002/2016PA003059, 2017)
—Jenny Lunn, Contributing Writer