Many ecosystems are now reacting to the changes set in motion by ENSO; for example, the climatic phenomenon affects the temperature and availability of food in the ocean, which means consequences forfishing along the coasts of South America and therefore the livelihoods of fishers and the food supply for the local populace. “What we do know is that the nature of ENSO depends on the basic status of the climate,” says Dr Christian Stepanek from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI). “Today we’re experiencing a phase in which the climate is warming. As such, it can’t be ruled out that this warming will change the characteristics of ENSO – and with them, how it impacts human societies – in the future.”
Answering the question of just what these changes might look like poses a challenge for climate research, since the characteristics and behaviour of ENSO aren’t yet fully researched or understood. One approach to understanding its underlying mechanisms is to investigate the phenomenon using the climate of the distant past. In this regard, epochs of Earth’s history in which its basic state was similar to what is projected for the future – like the Pliocene, roughly three million years ago, when the entire world was comparatively warm and humid – are of particular interest. In order to determine which mechanisms shape the characteristics of ENSO in a warm climate, the experts analysed a combination of similar climate simulations and drew on information from geological archives, like sediment cores.
“However, the ENSO cycle in the Middle Pliocene can’t be directly compared to the future climate,” Christian Stepanek explains. “Because of growing greenhouse-gas emissions, the climate of today and tomorrow is constantly changing.” Three million years ago, the climate was much closer to a state of equilibrium. Nevertheless, the study offers insights into how ENSO could behave in the future. For the Pliocene, the consortium has discovered a connection between the position of the Intertropical Convergence Zone (ITCZ) and the characteristics of ENSO. The ITCZ is a low-pressure zone in which the trade winds from the northeast and southeast come together. In the process, numerous convective storms and showers are formed. In the course of the year, the ITCZ shifts to the north or south of its median latitude. In the Pliocene, it shifted to the north, producing a weaker ENSO. This type of shift can also be seen in climate projections – but in the opposite direction, e.g. to the south. “If we assume that the connection between ENSO and the position of the ITCZ, identified in the Pliocene, still applies, then based on the simulations we can deduce that ENSO activity will intensify in the future,” says Stepanek. “Interestingly, this directly contradicts the decline in ENSO observed since 2000.” This raises further questions – such as whether the currently observed weakening of ENSO might just be a temporary effect that reverses itself in the near future.
The researchers conducted their study as part of the international Pliocene Model Intercomparison Project (PlioMIP). One of the project’s central goals is to compare the outcomes of various climate models in order to identify sources of uncertainty in them, and to highlight robust model-based findings regarding our climate. The AWI’s Paleoclimate Dynamics Section has been a project member for more than a decade.