Marine heatwave

Topic: Oceans: Hydrological Cycle, Relief of the Ocean Floor, Temperature and Salinity · NCERT: Beyond NCERT

Meaning

A marine heatwave is a period when SST (sea surface temperature, the temperature of the top layer of the sea) stays above the 90th percentile for at least five days in a row. This definition comes from Hobday et al. (2016).

• Marine heatwaves bleach corals, kill fish and make cyclones stronger.
• Global warming is making them more frequent, so they are a major threat to coral reefs, fisheries and coastal people, including in India.

Explanation

What the "90th percentile" means

• Scientists collect SST readings for one place and one season over many years.
• They line up the readings from coolest to warmest. The 90th percentile is the value that 90% of those past readings fall below.
• A marine heatwave starts when the water is warmer than 90% of past readings for that place and season, and it must stay that warm for at least five days.
• The threshold is relative, not a fixed number.
• Each place gets its own threshold, set by its own past readings.
• So the same water temperature can be normal in a tropical sea and a heatwave in a cold sea.
• This is why "SST above 30 °C" is the wrong definition.

How it happens (cause → process → result)

• Cause: global warming.
• Greenhouse gases trap more heat in the atmosphere.

• The ocean takes in much of this extra heat, so the sea surface gets warmer.

• Process: the warm water stays.

• The top layer of the sea stays unusually warm for days or longer.

• Warm surface water mixes less with the cooler, deeper layers, so the heat cannot spread downward.

• Result: the event is long enough to harm life (five days or more by definition).

Effects

• Coral bleaching
• Corals have colourful algae living inside them that give them food.
• Under heat stress, corals push these algae out.

• The corals turn white ("bleach"). If the heat goes on, they may die.

• Fish kills: fish die in large numbers in the hot water.

• Stronger cyclones
• Warm water gives storms more energy.

• So cyclones that pass over a heated sea can grow stronger.

• It adds to other ocean stresses

• Warm water holds less oxygen, which links heatwaves to ocean deoxygenation (the ocean losing its dissolved oxygen).
• Heat, acidification and deoxygenation hit marine life together. Together they are called the "deadly trio".

Trend

• Marine heatwaves became more frequent during the 20th century [3].
• They are expected to increase around the globe during the 21st century (IPCC AR6) [3].

Where it occurs

• Tropical Indian Ocean: it is warming fastest among the tropical oceans. This raises the risk of marine heatwaves near India.
• Coral bleaching hotspots in India:
• Gulf of Mannar (between Tamil Nadu and Sri Lanka)
• Lakshadweep (coral islands in the Arabian Sea)

• Andaman and Nicobar Islands (in the Bay of Bengal)

• Seas with stronger cyclones: the Arabian Sea and the Bay of Bengal.

• Arabian Sea: has one of the world's largest oxygen minimum zones (layers of mid-depth water with very little oxygen), and that zone is spreading. Warming makes oxygen loss worse.
• World: marine heatwaves happen in oceans all around the globe, and they are expected to increase everywhere this century [3]. Tropical and subtropical coral reefs suffer the most.

Don't confuse with

• Heatwave on land (atmospheric heatwave): this is measured by air temperature over land. A marine heatwave is measured by SST, using the 90th-percentile, five-day rule.
• Coral bleaching: this is an effect (corals push out their algae and turn white). The marine heatwave is the cause (the period of unusually warm water).
• Ocean acidification: this is caused by the sea absorbing CO₂, and it is measured by pH (which fell from about 8.2 to about 8.1). A marine heatwave is about temperature, not chemistry.
• El Niño: this is a natural, recurring warming of the central and eastern Pacific Ocean that changes weather patterns. A marine heatwave is any short warm spell that crosses the local 90th-percentile threshold, in any ocean.

Prelims Hooks

• Definition: SST above the 90th percentile for at least 5 days (Hobday et al., 2016). Trap: it is not "above 30 °C" and not "for a month".
• The threshold is relative: it is set by each place's own past readings for that season, not by one global temperature.
• Main cause: global warming. Main victims: corals (bleaching) and fish.
• IPCC AR6: marine heatwaves became more frequent in the 20th century and are expected to increase globally in the 21st century [3].
• Indian bleaching sites: Gulf of Mannar, Lakshadweep, Andaman and Nicobar Islands. The tropical Indian Ocean is the fastest-warming tropical ocean.
• Match the stress to its cause: marine heatwave → global warming; ocean acidification → CO₂ absorption; deoxygenation → warming plus fertiliser runoff.

Mains Points

• The "deadly trio" (GS-III, Environment): marine heatwaves, acidification and deoxygenation act together, and each makes the others worse.
• Heat bleaches corals. Acidic water makes it harder for corals to build skeletons. Warm water holds less oxygen.

• Together they push coral reefs and fisheries towards collapse. More than 40% of the surface ocean had already crossed the acidification boundary by 2020 [2].

• India at risk (GS-III, Disasters and Environment): the fast-warming tropical Indian Ocean threatens India in three ways.

• Bleaching of reefs in the Gulf of Mannar, Lakshadweep and the Andaman and Nicobar Islands.
• Stronger cyclones over the Arabian Sea and the Bay of Bengal, which put coastal lives and property in danger.

• Fish kills that hurt the livelihoods of fishing communities and food security.

• Policy response: marine heatwaves come from global warming, so the main fix is cutting greenhouse gas emissions.

• Reefs can also be helped to cope with heat by removing other stresses: marine protected areas, less fertiliser and sewage runoff, and no overfishing.
• The BBNJ Agreement (in force from 17 January 2026) lets countries create marine protected areas on the high seas, which helps meet the 30x30 target [1].

Related concepts

• Ocean acidification
• Ocean deoxygenation
• Microplastics
• Great Pacific Garbage Patch
• Atlantic Meridional Overturning Circulation
• BBNJ Agreement

Read more

• Oceans and life, oceans under stress, and the High Seas Treaty