El Niño in the Anthropocene

We’ve looked at the argument that El Niño is causing global warming, (and we’re hopefully all in agreement that it is obviously not) now let’s take a look to see whether global warming is affecting El Niño. 

Source: Flaming Planet by ImFayth

First, let’s just remind ourselves of a few things. Global warming is the increase in the temperature of the Earth’s surface, oceans and atmosphere. The fourth report from the Intergovernmental Panel on Climate Change (IPCC) stated, with over 90% confidence, that the majority of the observed increase in temperatures since the mid 20^th century is as result of the increased anthropogenic greenhouse gas concentrations. In a study of 11,944 peer-reviewed scientific literatures from 1991 – 2011, it was found that over 97% of climate scientists agree that anthropogenic global warming is as a cause of human activity, and not a natural occurrence (Cook et al., 2013). What the other 3% are thinking..? Well, let’s not get into that again!

This blog started with considering how much the oceans have warmed during the anthropocene, and we now know a little about the interaction between the ocean temperature and the atmosphere, (in particular El Niño episodes), so what, if any, effect is global warming having on El Niño?

One report proposed that a doubling in the occurrences of extreme El Niño events will be seen due to greenhouse warming (Cai et al., 2014). In the report, an extreme El Niño event is characterised by a massive reorganisation of atmospheric convection which leads to austral summer rainfall greater than 5mm per day over a defined area. The result partly comes from the non-linear relationship found between sea surface temperature and rainfall, and meridonial (equator to mid-latitudes) sea surface temperature gradients and rainfall. As can be seen in the plotted points in each of Figure 1 a and Figure 1 b below, if a line were to be drawn joining them they would both be a curve. There is an exponential increase and an exponential decrease in rainfall associated with an increase in SST and meridonial SST gradients, respectively.

Figure 1 a,b: Evolution and nonlinear characteristics of observed extreme El Niño events. (Niño3 area: 5◦ S–5◦ N, 150◦ W–90◦ W).  

Extreme El Niño, moderate El Niño, and La Niña and neutral events, are indicated by red, green and blue dots respectively.

Source: Cai et al., 2014

A report by Hansen et al, 2006, claims there will be an increase in intensity of El Niño events, but the effect on frequency is still unclear. Whilst Kim et al, 2014, deduced that there will be a time-varying response of ENSO to global warming, with an increasing trend in amplitude before 2040, and thereafter a decreasing trend.  

However, another report concluded that it is not yet possible to say whether there would be a change in frequency or strength of events due to the complex year to year variability of ENSO (Collins et al., 2010). Many processes play a part in determining ENSO characteristics, and the effect of global warming would increase them. However, these processes involve both positive and negative feedbacks; hence the uncertainty lies within these feedbacks and, essentially the report suggests they could cancel each other out.

It seems that it we do not currently have enough information to give a unanimous conclusion on whether global warming is going to increase the frequency and intensity of El Niño events (Guilyardi et al., 2012). But what we do know is that potentially global warming will increase the events and hence have catastrophic effects on our environment, from flooding to drought. This highlights the importance of needing to model ENSO activity. Modelling allows us to learn more about the processes by discovering underlying trends, as well as being able to simulate ‘what if?’ scenarios. What if sea surface temperatures rise by X^oC? What if sea levels rise by X inches? Considering these situations is the start of either stopping, or, if unavoidable, preparing for them.