Marine heatwave

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Major marine heat waves 2000–2016

A marine heatwave (MHW) , also known as sea ​​heatwave , is a relatively long period of unusually high sea temperatures in a region. According to a more specific definition proposed in 2016, one speaks of a marine heat wave if the temperatures are higher than 90% of the temperature values over a period of at least five days that were determined in a 30-year comparison period for the same calendar days and the region.

Marine heat waves pose a threat to important ecosystems , particularly coral reefs , kelp forests and seagrass beds . They can trigger species migration and mass extinctions and lead to significant economic losses in fisheries and aquaculture . The frequency, duration and intensity of marine heat waves are increasing significantly with the current anthropogenic global warming .

term

Similar to the atmosphere, there are extreme events in the sea. These include the effects of floods and strong storms, anoxic events or extreme anomalies in water temperatures. Extremely high temperature deviations over a long period of time in one place are called marine heat waves (MHW) . They are a phenomenon that has only been researched in more detail since the late 2000s.

In 2016, Australian oceanographer and marine ecologist Alistair J. Hobday and others proposed a definition based on atmospheric heat waves :

A marine heat wave is an event of at least five consecutive days with sea ​​surface temperatures that are higher than 90% of the values ​​of a 30-year comparison period for the respective calendar day and the same location.

The definition uses the 90 percentile of daytime temperatures as the temperature threshold T 90 . It is a relative threshold that includes regional and seasonal variability (→ annuality ). It allows comparisons across different seasons and regions. Even in winter and at high latitudes, marine heat waves can occur and have serious consequences - for example, the survival rate of young plants in some species of seagrass depends on low water temperatures in the cool season. The minimum duration of five days is chosen so that marine heat waves are currently rare occurrences in a location. The deviation of the measured temperatures T MHW from the long-term daily mean temperatures T c is called the intensity .

Due to its thermal inertia, the temperature of the water usually changes more slowly than that of the air. This is why one speaks of a marine heat wave only when the temperature threshold has been exceeded for at least five days, while a shorter period of three days is definitive for atmospheric heat waves. The level of fluctuations in sea temperatures is also usually less than that of the air. In order to be able to speak of a marine heat wave, comparatively smaller temperature deviations are sufficient.

Categorization of marine heat waves based on their maximum intensity

Hobday and others suggested in 2018 that marine heat waves should be classified into four categories based on their intensity.

  • Category I: moderate MHW
  • Category II: strong MHW
  • Category III: severe MHW
  • Category IV: extreme MHW

Let ΔT be the difference between the long-term mean temperature T c and the temperature threshold T 90 . The category C of the heat wave indicates by how many times ΔT the maximum intensity I max exceeds the temperature threshold:

Triggers and Spread

Marine heat waves can be caused by various combinations of atmospheric and oceanic processes. In connection with climate events such as El Niño , they occur more frequently and over a larger area. Another cause of marine heat waves can be stable weather conditions that divert wind systems and thus prevent the water from mixing. Overland heat waves can also contribute to the warming of adjacent ocean surfaces.

Marine heat waves can occur worldwide. One to three events per year are typical, depending on the region. There are particularly intense MHWs near the equator in the eastern and central Pacific and in areas of eastern and western marginal currents. In the tropics, MHWs usually last 5–10 days, in the area affected by the El Niño-Southern Oscillation (ENSO) a longer duration of up to 60 days is observed.

The oceans absorb over 90% of the heat, which also remains in the earth system due to the increasing greenhouse gas concentrations in the atmosphere (→ heat content of the oceans ). The upper 75 m are warming worldwide by 0.11 ° C per decade compared to the mean for the years 1971–2001. This makes marine heat waves more frequent and more intense. The sea area affected by heat waves is increasing. Regional and global fluctuations such as El Nino , the Pacific Decade Oscillation (PDO) and the Atlantic Multi-Decade Oscillation (AMO) also contribute to the variability in the occurrence of marine heat waves .

An evaluation of satellite data and on-site measurements carried out in 2018 showed that the frequency of marine heat waves has increased by more than a third since 1925, and their duration has increased by more than half. The number of heat wave days in the period 1987–2016 was 54% higher than in the period 1925–1954. Most noticeable is the increase in the sea area affected by Category II heat waves by almost a quarter by the mid-2000s. Except in some areas south of 50 degrees south latitude and the eastern Pacific, the frequency of marine heat waves increased everywhere. The greatest increase in frequency was in the North Atlantic. Over 80% of the heat waves that occur can probably be traced back to man-made global warming. If the warming continues unabated, the frequency of marine heat waves would increase by more than 40 times by the end of the century, their spatial extent would be more than 20 times greater and their duration would extend over 100 days.

consequences

Starving young sea lion, stranded on the US Pacific coast in 2015 during the MHW called "The Blob"

Marine heat waves as short-term extreme events superimpose the long-term trend of rising sea temperatures. The extremely high temperatures can be above the thermal tolerance ranges of important habitat-forming species and entire marine ecosystems. They then have destructive effects on biological processes and species groups. Some regions are particularly vulnerable, they are characterized by high biodiversity and the occurrence of species for which the typical regional temperatures are already close to their tolerable temperature limit. Other human influences also increase the vulnerability. Important habitats - coral reefs, kelp forests and seagrass meadows - have been damaged on a large scale and in parts possibly irreversibly by marine heat waves, partly in connection with other anthropogenic stressors, such as pollution or harmful fishing practices, since the 2000s. Species have changed their range and phenology . Regional mass extinctions occurred along food chains . This has had socio-economic implications too, particularly in the sea-dependent economic sectors of fisheries, aquaculture and tourism.

Faded corals in the Great Barrier Reef, 2016

Corals live in symbiosis with unicellular algae, from which they draw oxygen and energy and which give them their color. When the water temperature is too high, corals repel the algae and coral bleaching occurs . In the summers of 2016 and 2017 off Australia in the Great Barrier Reef , weeks of heat waves resulted in a loss of 50% of the corals. In the northern part of the reef, which was mainly affected by the 2016 heat wave, 90% of the antler and table corals died. Corals take 10 to 15 years to recover from a bleaching event. Until the beginning of the 1980s, there was an average of 25 to 30 years between two events, most recently they occurred about every six years. There is a fear that MHW will appear too often in the future for the regenerative ability of the corals and that these will finally die off.

Seagrass meadows form important habitats and are at the beginning of marine food chains (→ producer (ecology) ). They are an important carbon sink , in the top meter of the sediments formed by seagrass beds, 4.2–8.5 Gt of carbon are stored worldwide. During marine heat waves, stocks of habitat-forming species such as the Neptune grass in the Mediterranean and Amphibolis antarctica in Western Australia were lost. When the soil below seagrass meadows is exposed, erosion and remineralization of the carbon-rich sediments occur.

Rhizoid of Durvillaea antarctica ( bull kelp ) near Oaro, South New Zealand. Above: healthy organ, Nov. 2017. Below: decomposing specimen, March 2018, after a heat wave

Kelp forests occur outside of the tropics, they are dependent on cooler waters. The rapidly growing, up to 45 m high macroalgae form complex, species-rich marine habitats. Similar to seaweed, they are a basal species at the beginning of food chains. They are important fishing areas and tourist destinations. Up to 80% of the kelp is torn loose and driven into regions with low primary production , where they are also an important source of food. Parts of the drifted seaweed sink to the bottom of the deep sea. Each year they remove 1.73 Gt of carbon from the carbon cycle. In regions where kelp forests are at the upper edge of their thermal tolerance range and where the seas are warming particularly quickly, the decline or even collapse of kelp forests has often been observed. Both the gradual increase in sea temperatures, for example in the North Atlantic, off Spain or in the western Pacific, and marine heat waves, for example when the kelp forest collapses off Western Australia, play a role. Since the 2000s, kelp forests have been increasingly replaced by mats of small-sized algae ( turf ). It is feared that tipping points have been exceeded in some regions and that an irreversible collapse of the ecosystems there has occurred. In some cooler regions, the kelp forests there survived episodes of higher sea temperatures or there were signs of recolonization of turf areas.

Significant heat waves

Since the end of the 2000s, with the beginning of closer research into marine heat waves, some have been observed in all oceans. Only a few have been analyzed in more detail so far, one of the first was an MHW in the Mediterranean in 2003. In addition to the heat waves listed here as examples, these include other heat waves that occurred up to 2018, including MHW in the years 1998, 2002, 2016 in the Western Pacific, which also hit the Great Barrier Reef or a prolonged heat wave in the Tasman Sea in 2015.

MHW in the Mediterranean 2003

In the summer of 2003, a strong atmospheric heat wave over Europe led to increased heat input into the northern Mediterranean. This, in combination with only a slight wind, triggered a thermal stratification of the seawater in the region: the exchange of surface water with deeper water layers largely came to a standstill, and it warmed up by 2–3 ° C. The heat wave lasted more than 10 weeks. The result was a mass extinction of bottom-dwelling invertebrates in shallow waters. Seagrass meadows were lost.

Ningaloo-Niño MHW 2011

In the southern hemisphere summer of 2011, an extreme marine heat wave, also known as the Ningaloo- Niño, occurred off the coast of Western Australia. The cause was probably an extraordinary La Niña event off the coast of South America in 2010/2011. About an oceanic and atmospheric teleconnection strengthened the Leeuwin -Strömung off the coast of Western Australia and brought tropical warm water towards the poles in the temperate latitudes along the coast.

This triggered a mass extinction of species that are crucial for the benthic habitats in the transition zone between tropics and temperate latitudes. As a result, benthic ecosystems changed fundamentally. Kelp forests along an over 100 km long stretch of coast between Kalbarri and Jurien Bay have been lost and have been replaced by mats of low-growing algae ( turf ).

The Ningaloo heat wave damaged more than a third of the significant seagrass beds in Shark Bay , a UNESCO World Heritage Site on the west coast of Australia. An extensive area of ​​seagrass had developed there over the past 8,000 years, accounting for 0.7–2.4% of the world's seagrass area. Shark Bay is home to a large population of the Indo-Pacific Bottlenose Dolphin . The heat wave reduced the number of newborn calves and the number of animals living there fell by a total of 12%. The stock had not yet recovered in 2017. The fishing industry has suffered significant losses.

Northwest Atlantic MHW 2012

In 2012, as a result of a strong marine heat wave in the northwestern Atlantic, and particularly in the Gulf of Maine, a drastic change in the distribution and seasonal development of some species important for fisheries was observed. Heat-loving species migrated north, other species prematurely migrated fish . Squids appeared off Maine . Lobsters migrated into the shallow waters three weeks early. This resulted in earlier and larger catches in the fishery, a drop in prices, and threatened US and Canadian lobster fishermen.

Fishing practices then had to be adjusted, with economically and politically significant implications.

The Blob in the Northeast Pacific 2014–2016

The "Blob" in May 2015

From spring 2014 to summer 2016 there was a severe and particularly extensive heat wave in the northeastern Pacific - after a horror film of the same name from 1959 - The Blob , which at times stretched from Alaska to Mexico and from the US coast to Hawaii was enough. The water temperatures off the coast of California were at times six degrees higher than normal. A long-lasting blocking high over Alaska in the fall of 2013 ensured that the otherwise common arctic storms could not reach the Northwest Pacific and could not mix the water. In addition, the heat was not removed from the sea surface. The stably stratified water warmed up strongly in the upper layer and nutrient-rich deep water no longer penetrated the surface. In the winter of 2014/2015, southerly winds brought unusually warm air. An El Niño event was added in 2015 and 2016.

The consequence was a mass extinction of copepods and krill as well as other animals such as seabirds or whales, which depend on krill in the food chain. The cod stocks fell by 70%. An estimated 1 million guillemots died of starvation because the quantity and quality of the fish stocks they serve as a food source declined. Poison-producing diatoms, on the other hand, increased strongly. Due to the algal bloom , fishing on the US west coast had to be suspended for several months. Kelp forests off the California coast collapsed, after which they resembled a wasteland characterized by sea ​​urchins . With the loss of this food source, the stocks of the red abalone also collapsed , its catch came to a standstill off Oregon and California. The high sea surface temperatures likely affected the weather along the US west coast and contributed to the 2011-2017 drought in California .

literature

Web links

Individual evidence

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