Aug 02 2023
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Consequences of Climate Change, Marine Heatwaves Pose Enduring Threats Both at Sea and on Land

 

The North Atlantic is experiencing an unprecedented marine heatwave, with temperatures between +2°C and +5°C above seasonal norms, as reported by the Copernicus observation program. As a direct consequence of climate change, the ocean is warming and extreme events such as marine heatwaves are on the increase. The intensity of these phenomena, their extent and their devastating impact on marine biodiversity and the populations that depend on it are causes for concern. Meanwhile, the IPCC pointed out in its latest assessment report that the number of days of marine heatwaves has increased by 54% since the beginning of the century.

An extreme phenomenon, exacerbated by climate change

What is a marine heatwave?

Marine heatwaves are unusual periods during which the sea surface heats up for several days to several months, over areas of up to thousands of square kilometres. Although the increase in ocean surface temperatures is the main indicator, they can also extend to the deep sea. Furthermore, while being extreme, localised and temporary climatic events, they are becoming increasingly intense, frequent, long-lasting and spatially widespread as a result of climate change. According to the IPCC, the frequency of marine heatwaves has doubled since the 1980s.

It was in 2003 that the first marine heatwaves were studied in the Mediterranean, revealing summer temperatures ranging from +1 and +3°C above seasonal averages. Other episodes have since made headlines, such as the 2011 episode off the West coast of Australia with temperatures reaching up to +5°C above seasonal norms over a ten-week period. During this event, scientists observed the first occurrences of coral bleaching in the Ningaloo reef, off the Abrolhos Islands and the Pilbara coast. More recently, between 2013 and 2015, the North-West Pacific experienced the strongest heatwaves ever recorded locally. Known as the “Blob”, temperatures spiked by an alarming +2.6°C above the norm.

Fig. 1 Global map of major marine heatwaves since 1995. The intensity scale of each extreme (from moderate to extreme) represents the conditions corresponding to the date of the peak of the event. Adapted from Smith et al. 2021.

 

How do marine heatwaves form and evolve?

Marine heatwaves are the result of a combination of oceanic and atmospheric processes: changes in ocean currents, temporary weakening of winds, reduced cloud cover, increased air and water temperatures, etc.

A correlation also exists between natural climate variability and the occurrence and intensity of marine heatwaves. For example, the “El Niño” phenomenon appears to enhance the emergence of marine heatwaves in the central and eastern regions of the Pacific Ocean.

 

El Niño
The El Niño phenomenon is a natural variability event in the climate system, primarily affecting the Pacific and its neighbouring countries. Unusual warmings of surface waters are thus observed in its central and eastern regions as a result of a weakening of the trade winds. This leads to an increase and displacement of atmospheric disturbances. Heightened rainfall and more frequent storms are recorded in some regions, while others are experiencing an increase in droughts. This situation is also causing areas of precipitation to move eastwards into the Pacific Ocean, preventing the upwelling of cold waters along the west coast of the American continent. These large-scale modifications in the atmosphere-ocean interactions may therefore enhance the emergence of marine heatwaves in the eastern Pacific.

 

By 2021, nearly 60% of the ocean surface had experienced at least one period of heatwaves. Although these events impact the entire globe, they are unevenly distributed, with tropical seas, the Mediterranean, the Arctic Ocean and the western Pacific being particularly exposed to these episodes. These areas are likely to become even more vulnerable to marine heatwaves, as their frequency is expected to rise with climate change. By 2100 – under the most pessimistic greenhouse gas emission scenarios – the probability of experiencing marine heatwaves could be 60 times higher than over the period 1850-1900.

Cascading impacts on marine biodiversity and human societies

 

 

What are the consequences for marine life?

The abrupt rise in temperatures threatens many marine ecosystems, pushing them towards a tipping point: a threshold beyond which species reach the limits of their resilience. The consequences are long-lasting and often irreversible for many species, leading to mass mortality events.

This is particularly visible on coral reefs. Towards the end of the summer in 1999, the Mediterranean was struck by a marine heatwave measured at depths of up to 50 metres, causing significant mortality among gorgonians and other benthic organisms. According to Denis Allemand, scientific director of the Monaco Scientific Centre, “it has been estimated that certain areas of the Ligurian Sea have experienced mortality rates reaching 100%, suggesting that millions of gorgonians have died along the coast. Remarkably, this ecosystem is home to 15 to 20% of species known in the Mediterranean!”. Such levels of mortality were again recorded in August 2022, when a marine heatwave caused temperatures to rise by +4 to +6°C above average in the Mediterranean.

Marine heatwaves also impact species distribution. Due to abrupt changes of conditions, certain ecosystems’ native species disappear or are forced to migrate to cooler waters, while other opportunist and invasive species such as lionfish, sea urchins and jellyfish thrive. Higher temperatures, combined with water deoxygenation, can also lead to toxic algae bloom and profoundly alter plankton populations. As a result, the composition and functioning of marine ecosystems undergo profound transformations in every part of the ocean, extending well beyond the area directly affected by marine heat waves.

Eventually, these extreme events could exacerbate climate change by impairing the mitigation capacity of the ocean and its ecosystems. Marine heatwaves alter the ocean carbon cycle by reducing the density of CO2 flows between the air and the sea, particularly during extreme weather events such as El Niño. The so-called “blue carbon” ecosystems, which capture large quantities of CO2, are also affected. As the IPCC points out, “if degraded or lost, blue carbon ecosystems are likely to release most of their carbon into the atmosphere”.

 

Human societies are not spared from the impacts of marine heatwaves

Human societies are not immune to the consequences of marine heatwaves. The livelihoods, sources of income, employment and food, as well as cultures of coastal populations are directly at threat.

One particularly vulnerable sector is fishing. The collapse of certain fish populations, the sudden development of opportunist species and the loss of diversity impact the very availability and composition of the catch. Likewise, aquaculture undergoes profound modifications: as farmed species endure significant heat stress, their behaviour, reproduction capacity and immune systems are likely to be affected. As Didier Gascuel, Director of the Fisheries Unit at the Agro Institute, points out: “Heatwaves contribute to the erosion of functional biodiversity in marine ecosystems and to the proliferation of ‘natural chaos’: unpredictable phenomena involving rapid changes in species abundance that make fisheries management particularly challenging”.

Between 2013 and 2015, the Blob phenomenon in the north-western Pacific profoundly altered the abundance, distribution and nutritional value of invertebrates and fish. The fishing industry, particularly abalone fishing, has bore the full force of these ecosystem changes. Along the west coast of the United States, toxic algae blooms were observed, while kelp forests largely declined and seabird populations collapsed.

The economic costs are colossal. In 2016, heatwaves in southern Chile caused more than 800 million dollars in direct losses for aquaculture. On a global scale, the economic losses caused by mass mortality amount to several billion dollars every year.

How to address the challenges raised by marine heatwaves?

In the face of marine heatwaves, it is imperative to reduce our greenhouse gas emissions. Every fraction of a degree matters, thereby keeping the 1.5°C target alive is of utmost importance to limit the impacts on ecosystems and human societies.

Meanwhile, it is critical to better prepare people for the occurrence of new marine heatwaves by adapting their livelihoods and activities. Strengthening monitoring and early warning systems, improving the collection and sharing of real-time data on sea surface temperatures, ocean currents and other relevant parameters can help anticipate and respond rapidly to changing conditions. The fishing and aquaculture sectors could thus be prepared to adjust fishing seasons and to plan for potential temporary closures.

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Authored by: Arno Coblentz, Anaïs Deprez, Sarah Palazot, Niagara Poulain

With the contributions of: Denis Allemand (Centre Scientifique de Monaco), Didier Gascuel (Institut Agro Rennes-Angers), Sabrina Speich (ENS-PSL)

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