• As per the National Oceanic and Atmospheric Administration (NOAA), prolonged periods of sea surface temperatures rising 3 to 4°C above average for at least five consecutive days.
• IMD defines a marine heat wave (MHW) as a period when the sea’s temperature is above the 90th percentile(seasonally varying threshold) for at least 5 consecutive days.
• Can last from weeks to years, depending on intensity and extent.
• Frequency: Increasing in both occurrence and intensity due to climate change.
• Identified as one of the most significant stressors on global marine ecosystems, causing large-scale disruptions.

• Caused the death of over 4 million common murres (Uria aalge) in Alaska, the largest vertebrate die-off linked to warming oceans.
• Highlighted rapid and severe population declines in marine birds and a lack of ecosystem recovery even years later.
• Increasing frequency and intensity are reshaping ecosystems, creating new survival challenges for marine species.
• Evidence of shifts in marine environments, disrupting population dynamics.

• Mass Mortality: Loss of habitat-forming species like kelps and corals, degrading ecosystems.
• Trophic Cascades: Displacement of lower-trophic species and reduced ecosystem productivity.

On Marine Species: 

• Reduced breeding success and increased mortality.
• Example: 2014–2016 Pacific MHW killed half the common murre population in Alaska.

On Biodiversity: 

• Loss of key predators disrupts food webs.
• Threatened species face survival challenges due to limited adaptability.

PYQ:

[2020] With reference to Ocean Mean Temperature (OMT), which of the following statements is/are correct?

1. OMT is measured up to a depth of 26ºC isotherm which is 129 meters in the south-western Indian Ocean during January-March.
2. OMT collected during January-March can be used in assessing whether the amount of rainfall in monsoon will be less or more than a certain long term mean.

Select the correct answer using the code given below:

(a) 1 only

(b) 2 only

(c) Both 1 and 2

(d) Neither 1 nor 2