Written by Sara Cannon
Every two to seven years, the eastern equatorial Pacific climate oscillates between anomalously warm (El Niño) and cold (La Niña) conditions in a process known as the El Niño Southern Oscillation (ENSO). This process influences sea surface temperatures (SSTs), trade winds, and global teleconnection patterns, which together influence weather conditions all over the world (Collins 2010). Some scientists suggest that extreme El Niño events will happen more often with the warming climate (Federov and Philander 2000; Tudhope 2001; Cai 2014; Liu 2017), which would have profound impacts on communities around the world (for example, by altering patterns of global food production). Other scientists are undecided, pointing to the diversity of historical ENSO patterns, which confounds data that could suggest climate change is causing an impact (Collins 2010; Vecchi and Wittenburg 2010; Emile-Geay 2013, 2016). Fortunately, coral reefs hold a treasure trove of paleoclimate data that could be used to solve the mystery of past ENSO diversity, which would allow scientists to make more accurate predictions about how we can expect climate (and therefore weather) to change in the future.
It isn’t, however, an easy puzzle to solve. Scientists around the world have devoted huge amounts of resources to understanding how ENSO patterns will change as the climate continues to warm, but this has proven difficult because ENSO has historically exhibited differences in amplitude, temporal evolution, and spatial patterns (Capotondi 2015). Disagreements about what differences are caused by climate change and what is natural variation caused by radiative or orbital forcing have led to disagreement about future ENSO patterns. One thing that scientists do agree on, other than the absolute certainty that human-caused climate change is happening, is that in order to understand exactly what variations in ENSO are being influenced by a warming climate, scientists must first identify the background diversity of ENSO patterns, which requires going back potentially thousands of years (Collins 2010; Vecchi and Wittenburg 2010; Cobb 2013).This lack of information has limited the predicting power of climate models, leading to conflicting results.
So how can scientists get to the bottom of this? Instrumental records are limited in their usefulness because they tend to be short and sparse, particularly in remote regions of the Pacific where changes in SST are most pronounced (Emile-Geay 2013). Some proxy records, which are preserved physical characteristics of the environment that can stand in for direct measurements like ice cores and sediment records from lakes (NCDC NOAA, N.D.) may also be limited because they lack the temporal resolution needed to resolve ENSO patterns, which may vary seasonally (Cobb 2013). Luckily for us, coral reefs have been recording changes in the climate for hundreds of years at high resolutions. Similar to tree trunks, as they grow, corals record rings in their skeletons that reveal their age (Figure 1), and because corals are so sensitive to environmental fluctations, the chemistry in each ring can tell scientists about the temperature, rainfall, and water clarity from that year. By drilling into old corals and extracting a long sample (called a core), scientists can reconstruct monthly climate data over several hundred years. Corals therefore provide a hugely valuable source of data that could help us finally unravel the complicated history of ENSO, which in turn would help us accurately predict changes in our future climate.
Stable isotopes, which are elements with the same number of protons but different numbers of neutrons, are a power tool to understanding past climate. The environmental conditions at the time a coral grows its skeleton can influence the number of neutrons an element has. For example, a number of scientists have used stable oxygen isotopes (δ18O and δ16O) to reconstruct the history of sea surface salinity (Figure 2) (e.g. Nurhati 2009). Other scientists have used ratios elements, such as Stronium to Calcium (Sr/Ca) to reconstruct temperature (e.g. Thompson and van Woesik 2009). A clearer picture of climate variability has begun to emerge through the use of these climate proxies from coral cores. We know, for example, that there are two different types of El Niño events, one in which warm water is centered over the central Pacific (known as “CP El Niño”) and one where warm water is over the eastern equatorial Pacific (“EP El Niño”), and that CP El Niño, which is projected to increase with global warming, has happened more frequently in the 21st century than EP El Niño (Wang 2016). But data from across the Pacific are limited, and many of the studies identifying ENSO patterns use proxies from just a few coral cores, highlighting the need for more studies.
Another challenge is deciphering the cores themselves. Recent studies have called into question temperature data derived from coral cores using the common Sr/Ca proxy, because biological processes known as “vital effects” can influence and even override Sr/Ca relationships to temperature in corals during the biomineralization process (Alpert 2016, DeCarlo 2016). As a result, DeCarlo (2016) suggested a new proxy record that can be used to record past SST by combining Sr/Ca and the ratio of Uranium to Calcium (U/Ca) to create a new proxy, which they dubbed “the Sr-U thermometer.”
The need to address climate change only gets more urgent as time passes, which emphasizes how important this research is. Scientists cannot accurately predict the ways that climate change will influence humanity without understanding ENSO diversity. Coral have recorded climate variability in their skeletons for hundreds of years and are therefore a source of high-resolution, long-term data that could prove invaluable if we can only figure out the best way to decipher it. If scientists can understand ENSO’s patterns in the past, we can account for those patterns in climate models, and therefore predict how future ENSO will be influenced by climate change. This would allow us to make clear, accurate predictions about climate change in general, such as how rainfall patterns would impact food production, which could prove critical to the future of humanity.
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