What are marine heat waves?

Marine heat waves are defined as sharp rise of anomalously warm temperature that last at least five days, although many persist for weeks or months.

Fueled by climate change-caused ocean warming they can impact marine ecosystems for years after the water has cooled again.

How marine heat waves are caused?

Marine heatwaves are caused by a range of ocean processes, acting either separately or in combination, but there are two main drivers:

1 . Surface heat flux: heating from the atmosphere, which tends to occur when an atmospheric high-pressure system sits above a region of water for an extended period.

2 . Advection: movement of warmer waters into the region by ocean currents.

Marine heatwaves driven by surface heat flux tend to be shallower and shorter in duration, whereas as those driven by advection can be deeper and longer in duration. Oceanic mixing processes play a key role in their occurrence, intensity, and persistence.

Do the marine heat waves are increasing in frequency and intensity?

As the effects of climate change become increasingly embedded around the world, marine heat waves are increasing in frequency and intensity—posing a very big concern for ocean environments.

A 2020 study in the journal Science concluded that marine heat waves have increased more than 20-fold as a result of climate warming.

In the first decade after satellites began recording ocean temperatures (i.e., after 1981), there were 27 large marine heat waves, with an average duration of 32 days and an average peak temperature anomaly of 8.5°F; in the 2010s, there were 172, which lasted 48 days on average with an average peak temperature almost 10°F above normal.

Why many marine heat waves persist for weeks or months?

As the ocean surface warms, it radiates heat into the atmosphere that prevents cloud cover from forming, exposing the seawater to increased sunlight and further warming.

What kind of damages are done by marine heat waves to the marine ecosystems?

Marine Heat Waves (MHWs) have repercussions throughout marine ecosystems. MHWs have been associated with the mass mortality of marine invertebrates, and may force species to change behaviour in a way that puts wildlife at increased risk of harm.

MHWs have been linked to whale entanglements in fishing gear, for example. Changing conditions can also help invasive alien species to spread, which can be devastating for marine food webs.

Marine heat waves can cause mass coral bleaching, toxic algal blooms, and other heat-related disruptions of ocean ecosystems. These impacts can lead to mass die-offs of fish, marine mammals, and seabirds. Billions of dollars are lost in such events around the world each year.

Impact of Marine Heat Waves on Indian climate

How are corals impacted by marine heat waves?

Marine heat waves cause stress to corals and other marine ecosystems. Exposure to extreme temperature for long periods of time causes a breakdown in the relationship between coral and the algae that live inside of them.

The coral is left pale or white, i.e., bleached. The lack of food from the algae can lead to the death of the coral. If the heat stress does not subside, the coral will die.

How humans are impacted by marine heat waves?

Higher water temperatures associated with MHWs can cause extreme weather events such as tropical storms and hurricanes, and disrupt the water cycle; making floods, droughts and wildfires on land more likely.

MHWs have other profound socio-economic impacts for coastal communities. Aquaculture, for instance, requires water temperatures to remain suitable for farmed species, while fisheries rely on species that often relocate in response to changing environmental conditions. MHWs have been shown to kill or reduce the productivity of economically important species including lobster and snow crab in the northwest Atlantic and scallops off Western Australia.

MHWs can also harm regional tourism.

How global warming and climate change are related with increased incidence of marine heat waves?

Heat waves have become more frequent and more intense due to climate change. The ocean has absorbed 90 percent of the extra heat added to the atmosphere by burning fossil fuels, and as a result, the top 2,300 feet of the world’s oceans—where most of that absorbed heat is concentrated—has warmed by about 1.5°F on average since 1901.

An ocean that is already warmer could reasonably be expected to be more susceptible to marine heat waves, and that indeed appears to be the case.

Some recent examples that reflect a growth in reports of marine heat wave events.

Examples that reflect a growth in reports of marine heat wave events:

1 . A warming event during the summer of 2010-11 in which the waters off western Australia heated up an astonishing 11°F, killing huge swathes of kelp forest and large numbers of animals, from abalone and scallops to penguins.

2 . In 2016, a marine heat wave off Chile triggered algal blooms that devastated fish farms.

3 . From 2015 to 2019, a series of heat waves in the Mediterranean Sea led to multiple mass mortalities of seagrass and coral, heralding what has been called a new normal in the region.

4 . In 2021 and 2022, New Zealand experienced its highest ocean temperatures on record, resulting in the bleaching of “millions” of sponges

5 . Gulf of Mexico marine heat wave in 2023.

External link: https://www.iucn.org/resources/issues-brief/marine-heatwaves