MARINE HEAT WAVES: A silent threat
For the second time this summer, an intense heat spell is on the rise in many European countries as temperatures go beyond 40 C. Official heatwave warnings and instructions are issued repeatedly for european citizens who often flee to the sea breeze to cool off. But exactly how cold is the water that we are turning to for a bit of comfort in days like that? We are all familiar with the idea of global warming, but have you ever wondered what happens once such extreme heat penetrates the sea surface into the marine world? Does the ocean ever develop heatwave ''fever ''?
After all, it is only natural for the ocean to receive heat and sustain it for long periods of time, as part of its interaction with the atmosphere. There is, therefore, no physical barrier that inihibits or protects near-surface marine environment from intense atmospheric signals. Signals which can trigger anomalously warm sea temperatures, more extreme in fact, than the gradual underlying ocean-warming trend. Can you imagine living in a world where heatwaves break out without any notice and with not many means of escaping from them? Surely, marine inhabitants cannot receive extreme weather warnings but testimonies of heatwaves in the ocean have recently emerged.
We all remember the record-breaking heat wave in summer 2003 that claimed the lives of 30,000 people across Europe. One of the longest and hottest regional events appears to have also caused mass mortality events in benthic ecosystems, across thousands of kilometres of coastline in the NW Mediterranean, with sea surface temperatures reaching 1-3 C above normal . A similar, anomalously warm water event hits the west coast of Australia back in 2011, lasting for more than 10 weeks. This hot surge was responsible for the eradication of kelp forests that never recovered, mass deaths of seaweed meadows and knock-off effects on wildlife depending on these ecosystems. It was then that the term ''marine heat wave'' was first coined (by Australian researchers) to describe similar extreme events that would follow. Incidents of washed up seabirds, dehydrated lions, dead whales followed by a large toxic algal bloom along the California coast and species migrations, summarize the infamous extremely warm ''blob'' of 2013 - with its very own dedicated Wikipedia page - that swept the Pacific for 1.5 years. Then, only a few years later, the Great Barrier Reef would be blasted from what was experienced as possibly the worst coral bleaching ever recorded in the region, in one fell swoop.
Extreme events in ocean properties happened before but received little attention until their adverse impacts started to increase in scale and frequency (in parallel with the improving observational systems). In principle, a marine heat wave is a prolonged period of anomalously warm water which extends regionally and can have severe impacts on marine ecosystems. They can occur almost everywhere in the ocean and by default, an objective universal index is nearly impossible to define for comparisons between every event. However, researchers have recently proposed a standardised framework for their assessment. The idea is to measure characteristics like frequency, duration, (cumulative) intensity across the available observational record in an effort to understand how marine heat waves have evolved. Meanwhile, work is also conducted using climate models to study their future evolution in the 21st century and to disentangle their remote and/or local driving mechanisms.
Take for example the Mediterranean Sea. A ‘’hot spot’’ region for climate change, biodiversity, tourism and a heavily inhabited area. Inspired by the rapid changes already in place, regional climate models (models with incorporated air-sea interactions) are used to examine how surface marine heat waves in the Mediterranean Sea will evolve in the next 100 years, under different greenhouse gas regimes. We are interested in understanding their present and future spatiotemporal patterns and how quickly these are transformed. But the big question, we are facing is whether (and how much) will their severity and frequency change under climate change and what will be the consequences in the relevant ecosystems and coastal communities. Preliminary results suggest longer and more severe extreme events in the future that follow up greenhouse gas emissions. For this reason, researchers are also investigating what it seems to be a complicated combination of local oceanic and large-scale atmospheric forcing components of these events, in an attempt to identify the modulating factors behind them. Deep understanding will hopefully lead to more accurate predictions.
And while initially this might not look very disturbing (who doesn’t prefer tropical ocean temperatures?), implications for society are worrying. Aside from considerable losses in biodiversity, “tropicalisation” of (Mediterranean) waters can lead to abrupt shifts in economic balance between fisheries, due to redistribution of resources. The collapse of lobster fisheries due to a marine heat wave (high temperatures moved the lobsters closer to shore, increasing catch size, lowering the prices ) in the northwest Atlantic (summer 2012) is only a first taste of what will follow if these events keep intensifying. In return, marine-based tourism and coastal communities will be less resilient to environmental change due to the absence of vidal protection and food security offered by the currently vulnerable marine ecosystems.
One thing we know for sure is that as anthropogenic climate change escalates, so will marine heatwaves, like their atmospheric counterparts, spreading silently chaos in the oceans, initially hidden from view but eventually reaching our communities. We also know that a small increase in average temperatures forces big changes in extreme events, notably in intensity and frequency. As such, it is important not to forget that under the seemingly calm and cool 70% of our planet, marine heat waves can and will spread devastation unless we start reducing our greenhouse gas emissions. Let’s all take a small step towards that initiative before we have to deal with irreversible conditions.
 Garrabou, Joaquim, et al. "Mass mortality in Northwestern Mediterranean rocky benthic communities: effects of the 2003 heat wave." Global change biology15.5 (2009): 1090-1103.
 Mills, Katherine E., et al. "Fisheries management in a changing climate: lessons from the 2012 ocean heat wave in the Northwest Atlantic." Oceanography 26.2 (2013): 191-195.