Mediterranean Marine Heatwaves On the Move
Late last month, the Intergovernmental Panel on Climate Change issued a Special Report on the Impacts of Global Warming of 1.5 ºC1 above pre-industrial levels (a rather low-emission pathway), triggering a lot of discussions around its origins and impacts on natural and human systems. In this context, it would be interesting to see how the ocean is likely to respond under - what is considered today as - an "optimistic" scenario for greenhouse gas emissions in the future, relative to more severe projections. Particularly for regions vulnerable to climate change (or else "Hot Spots") like the Mediterranean Sea, such a comparison would be more meaningful to be performed for the anomalous sea surface temperatures rather than the mean temperature evolution. And if you wonder why, let’s dive into the next paragraph.
Episodes of anomalous ocean warming are not as rare anymore in the world and have been associated with widespread ecological impacts and important socio-economic implications. Superimposed on the underlying warming trend of the ocean, these events, referred to as Marine Heatwaves (MHWs), occur regionally from coastal to open oceans and may force changes in marine ecosystems in a matter of weeks or months. For example, they have been linked to extensive mass mortalities of benthic ecosystems in the Mediterranean Sea in 1999, 2003 and 2006, to abrupt tropicalisation of marine communities in Australia in 2011 and to geographical species shifts in the North Atlantic in 2012. Such above-normal ocean temperatures can be persistent in time but also extended in space, like the 2013-2015 Pacific «Blob» that led to mass marine mammals strandings and harmful algal blooms or the 2016 mass coral bleaching (>90 %) in the Great Barrier reef. Cascading effects have also driven commercial fisheries to the point of collapse, with huge financial losses and even economic tensions between nations.
A recent analysis indicated a general increase in MHW occurrence throughout the global ocean surface, over the last century2. Similarly, so far projections in regional3,4 and global5 scale suggest that higher global warming rates will bring more intense and longer-lasting events in the 21st century.
In the case of the Mediterranean Sea, however, little is known about past or future MHW trends, since research has been mostly focused on their local ecological impacts without a systematic assessment of their occurrence. By the end of the 21st century, a significant rise in the annual mean sea surface temperature (+1.5°C to +3°C, depending on the greenhouse gas emission scenario) is projected for the basin and therefore, in the intensity and presence of MHWs.
To systematically assess, for the first time, the evolution of Mediterranean Sea MHWs, we used the dedicated, fully-coupled, Regional Coupled Climate System Model NEMOMED8 and a multi-scenario approach. Simulations were performed under a high-emission scenario (RCP8.5), a low-emission scenario (RCP2.6, representative of mitigation scenarios that aim to limit the global mean temperature likely below 2°C) and one scenario representing, on average, the emissions observed between 1976-2005 (HIST), which is called here as the "present-day" scenario. A comparison between MHW characteristics at the end of the 20st and 21st century is illustrated here.
In response to increasing greenhouse gas forcing, events seem to become stronger and more intense under RCP8.5 than RCP2.6. Particularly for RCP8.5 towards 2071-2100, simulations project at least one MHW every year, up to 4 months longer and about 4 times more intense than present-day events (HIST) (Fig.1). Their maximum surface coverage also seems to increase from an average of 40 % today to almost 100 % in the future and events like the MHW 2003 (well-known, severe event in the basin) appear to become the new normal.
While RCP8.5 is the business-as-usual scenario, RCP2.6 is the closest to Paris agreement limits (<2°C), which could restrain future MHW mean intensity and duration. Yet, even under a low-warming scenario (RCP2.6), events can still be 2 months longer, 1.4 times more intense and 1.5 times larger, on average, than today. Therefore, the risks for marine ecosystems and the marine-dependent societies are likely to be reduced, yet not entirely absent in the years to come. It should be noted, however, that this is just one possible realisation of the future situation in the basin. For more robust estimations multi-scenario runs should be performed with multi-model ensembles, a study that is currently undergoing.
2 .Oliver EC, Donat MG, Burrows MT, Moore PJ, Smale DA, Alexander LV, Benthuysen JA, Feng M,Gupta AS, Hobday AJ, et al (2018a) Longer and more frequent marine heatwaves over the past century. Nature Communications 9(1):1324
3. Oliver EC, Wotherspoon SJ, Chamberlain MA, Holbrook NJ (2014) Projected tasman sea extremes in sea surface temperature through the twenty-first century. Journal of Climate 27(5):1980–1998
4. Galli G, Solidoro C, Lovato T (2017) Marine heat waves hazard 3d maps, and the risk for low motility organisms in a warming mediterranean sea. Frontiers in Marine Science 4:136
5. Frölicher, Thomas L., Erich M. Fischer, and Nicolas Gruber. "Marine heatwaves under global warming." Nature 560.7718 (2018): 360