Fourth global coral bleaching widest and fastest on record

Sub :Geo

SEC: Oceanography

Context:

The ongoing fourth global coral bleaching event (GCBE4), which commenced in January 2023, is now the most extensive and severe event recorded, surpassing previous bleaching levels observed during the period of 2014-2017 by over 11 percent, according to the United States National Oceanic and Atmospheric Administration (NOAA).
In response to the widespread coral bleaching, scientists from the UN have called for a special emergency session on coral reefs during the upcoming Convention on Biological Diversity summit (COP16) in Cali, Colombia.

Coral bleaching:

When corals face stress by changes in conditions such as temperature, light, or nutrients, they expel the symbiotic algae zooxanthellae living in their tissues, causing them to turn completely white. This phenomenon is called coral bleaching.
The pale white colour of bleached corals is of the translucent tissues of calcium carbonate which are visible due to the loss of pigment producing zooxanthellae.
If heat-pollutions subside in time, over a few weeks, the zooxanthellae can come back to the corals and restart the partnership but severe bleaching and prolonged stress in the external environment can lead to coral death.

About GCBE4:

GCBE4 has been characterized by severe and intense bleaching recorded in nearly half the time compared to GCBE3, which lasted three years and affected over 65.7 percent of the world’s coral reefs.
GCBE3 was previously noted as the longest, most widespread, and damaging event on record.
The event has led to confirmed reports of mass coral bleaching from 74 countries and territories across both hemispheres, impacting regions in the Atlantic, Pacific, and Indian Oceans.
At least 77 per cent of global reef areas and 99.9 per cent of the coral reef areas in the Atlantic Ocean have experienced bleaching-level heat stress during this event.
The full scale of its impact of the event may not be known until a few years later as scientists would need to conduct extensive assessments.

Coral mortality:

If corals stay bleached, they can become energetically compromised and ultimately die.
When corals die or their growth slows, complex reef habitats start disappearing and can eventually erode to sand.
Coral mortality can occur in a matter of days-to-weeks. However, corals can also die 1-2 years after bleaching because they become immunocompromised by the heat stress and thus become far more susceptible to disease-driven mortality.
GCBE4 has seen severe mortality of elkhorn and staghorn corals in certain locations of the Caribbean Sea.
During the 2005 bleaching event in the US Virgin Islands, many corals survived the heat, but then went on to die from subsequent disease outbreaks over the next two years.

Role of climate patterns:

Historically, strong El Nino events have been linked to severe bleaching, but recently, large-scale and severe events have occurred during La Nina periods.
This suggests that ocean temperatures have increased to the point whereby large-scale bleaching events may now occur during any phase of El Nino-Southern Oscillation (ENSO),