How much more frequent did marine heat waves become in the last decades?

How can we spot a marine heat waves in a warming climate? 

Marine heat waves (MHW) consist of prolonged, anomalously warm water in a particular location of the ocean. They are examples of extreme, unanticipated, rare natural events. Hobday et al. (2016) have proposed quantitative global metrics to detect and classify MHW as a discrete prolonged anomalous warm water event relative to a baseline climatology in a given location, that is an event exceeding a chosen high percentile threshold (typically 90% or greater) for a period of at least five days. Based on this definition, Bensoussan et al. (2017) found a long-term increasing trend in MHW total duration in the Mediterranean Sea. This is confirmed in recent work by Pastor and Khodayar (2022), where a trend of 3.4 days/year is estimated for the period 1982–2021 (note that the year 1981 marks the beginning of systematic observations of the sea surface temperature from satellites).  
 
In a climate change scenario, with increasing temperatures and more frequent atmospheric extreme events, we may expect that the frequency and intensity of the MHW would also increase. In the Mediterranean Sea, unusually long and intense MHW have occurred in 2003 and in 2022, extending their presence through the summer season with a sequence of intense events that interested fractions of the Mediterranean area between 30% to 60% on a monthly mean basis.  
 
The intensity of those two MHW was comparable, but their duration was quite different. In 2003, the threshold of 30% was exceeded over four months, from May to August. In 2022, that threshold was exceeded for a significantly longer period, from May to, at least, December. By the time this news article was published (early January 2023), such MHW event was still ongoing.

Figure 1: Time series of monthly fraction of Mediterranean Sea where a marine heatwave was detected based on the Hobday criterion (observed temperature exceeded 90% percentile). 

© CAREHeat/ENEA

Chiswell (2022) has stressed that the choice of the long-term baseline used to compute the anomalies may be a critical issue. He noted that if present MHW appear stronger, longer, and more frequent, this may be partly due to the positive bias resulting from the strong ocean warming of the last decades. Consequently, it may be argued that this climate change signal should be removed to correct the events’ detection. Using a linear trend to remove the climate signal from the 40-year reference baseline, Chiswell found a shift towards weaker MHW in the tropics (especially in the eastern Pacific), whereas the tendency towards stronger MHW persisted in the northern Pacific and in the Atlantic.

A similar analysis has been performed by Ciappa (2022) for the Mediterranean Sea. He found that, removing the long-term trend, the number of events per year (obtained using the classical Hobday method) and their intensity resulted stable, indicating that the increasing occurrence of actual MHW in the last decades is mainly due to the climatic warming rate.

How is CAREHeat adding to the debate?

The CAREHeat project has repeated Ciappa’s exercise of recomputing the frequency occurrence of Mediterranean MHW after removing the long-term warming trend from the sea surface temperature time series of the last 41 years (1981 – 2022), finding similar results.

Figure 2: Time series of monthly fraction of Mediterranean Sea where a marine heatwave was detected based on the Hobday criterion (observed temperature exceeded 90% percentile of the detrended time series). 

© CAREHeat/ENEA

Whether the approach of Ciappa and Chiswell (Fig. 2) is the appropriate one, or whether it makes more sense to leave climate trend components in the time series (Fig. 1) is still up for debate. Both criterions may be useful, depending on the specific application under study. Marine species seem to be more sensitive to absolute temperature values rather than to anomalies, even though the abruptness in temperature variation and the persistence of anomalous higher temperature levels should also be carefully considered.

In the next few months, the CAREHeat project will continue research on this matter. Our results are expected to be of help in assessing and understanding the risks for the health of the marine environment due to the current global warming. Stay tune to our website and Twitter for new possible answers to the title of this news article.