Climate Change Impact on India and World – International Reports, Key Observations, etc.

What are Marine Heat Waves (MHWs)?


From UPSC perspective, the following things are important :

Prelims level: Marine Heat Waves

Mains level: Read the attached story

heat wave

Central Idea

  • MHWs have engulfed regions like the north-east Pacific, southern Indian Ocean, the Atlantic, and the Mediterranean, as reported by Mercator Ocean International.
  • In April, the ocean’s surface temperature peaked at 21.1 degrees Celsius, breaking the previous record set in 2016.

What are Marine Heat Waves (MHWs)?

  • While we often associate heatwaves with the atmosphere, they can also occur in the ocean, known as Marine Heatwaves (MHWs).
  • These prolonged periods of excessively warm ocean temperatures can have significant consequences for marine ecosystems and industries.
  • MHWs can happen in both summer and winter, with “winter warm-spells” affecting specific regions and species.

How are MHWs measured?

  • Threshold Criteria: A marine heatwave is characterized by seawater temperatures exceeding a seasonally-varying threshold (often the 90th percentile) for at least 5 consecutive days.
  • Continuity of Events: Successive heatwaves with gaps of 2 days or less are considered part of the same MHW event.

heat wave

Causes of Marine Heatwaves

  • Air-sea heat flux: Ocean currents and air-sea heat flux are common drivers of MHWs, leading to the build-up of warm water in specific areas.
  • Influences of Wind and Climate Modes: Wind patterns can enhance or suppress MHWs. They influence the likelihood of events occurring in certain regions.
  • Large-Scale Climate Drivers: Events like the El Nino Southern Oscillation (ENSO) can also contribute to the formation of MHWs.
  • Intensification with Global Warming: Rising global temperatures have resulted in longer-lasting, more frequent, and intense MHWs in recent decades.
  • Human Influence: 87% of MHWs can be attributed to human-induced warming, with the oceans absorbing significant amounts of heat due to greenhouse gas emissions.
  • Oceans as Heat Sink: Oceans have absorbed 90% of the additional heat caused by greenhouse gas emissions, increasing global mean sea surface temperature by nearly 0.9 degrees Celsius since 1850.

Impacts of Marine Heatwaves

  • Ecosystem Structure: MHWs can disrupt ecosystem structure, supporting certain species while suppressing others.
  • Kelp Forest Destruction: MHWs along the Western Australian coast in 2010-2011 devastated kelp forests and fundamentally altered the ecosystem of the coast.
  • Economic Losses: MHWs can cause economic losses, particularly in fisheries and aquaculture industries.
  • Vulnerability of Temperature-Sensitive Species: Species such as corals are highly vulnerable to MHWs. The 2016 marine heatwaves in northern Australia caused severe bleaching of the Great Barrier Reef.

Impact on Marine Ecosystems

  • Catastrophic Effects: MHWs have led to the death of numerous marine species, altered migration patterns, and caused coral bleaching, endangering coral reefs.
  • Coral Bleaching: High ocean temperatures in the tropical Atlantic and Caribbean in 2005 led to a massive coral bleaching event, impacting over 80% of surveyed corals, with severe consequences for marine life.
  • Invasive Species and Ecological Imbalance: MHWs fuel the growth of invasive alien species, disrupting marine food webs and posing threats to wildlife. Ex. Whale entanglements in fishing gear.

Consequences for Humans

  • Amplifying Storms: Higher ocean temperatures associated with MHWs make storms like hurricanes and cyclones stronger, leading to severe weather events and flooding.
  • Threat to Coral Reefs: Half a billion people depend on coral reefs for food, income, and protection, but MHWs pose a grave threat to these ecosystems, impacting human livelihoods.
  • Socio-Economic Impact: Coastal communities are particularly vulnerable to the socio-economic impacts of MHWs, affecting fisheries and tourism.

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