Are corals of the WIO adapting to their warming environment?

How hot will 2021 be?

Looking back, 2020 was tied with 2016 as the warmest year on record, making it the sixth in a series of exceptionally warm years starting in 2015, and 2011-2020 was the warmest decade recorded on the planet.

At the start of 2021 the question on all our minds is “how bad might bleaching be this year? The situation is complex, with opposing factors. In early 2021, temperatures across the western Indian Ocean, are 1 to 2 degrees Celsius warmer than they used to be for this time of year (compared to levels in 1985-1990 + 1993), shown by the orange/red colours in the animation. Meanwhile, multiyear climate projections forecast 2021 to be even hotter than the preceding few years (see “Global annual temperature” figure ).  

By contrast, 2021 starts with a moderate La Niña, which means this side of the Indian Ocean is cooler than usual, and the Indian Ocean Dipole (the Indian Ocean’s version of the El Niño/La Niña phenomenon) also in cooler phase.

These tendencies oppose each other – a year predicted to be hot globally, but regional weather patterns predicted to be cooler.

Which will dominate?

Acclimatisation or natural selection!

Despite 2020 being such a hot year, Western Indian Ocean corals seemed to suffer only moderate bleaching and low mortality.

90-day animation SST anomalies 21 Oct-2020-Jan 18-2021 
This low level of bleaching was confirmed by over 100 reports from 25 individuals and organisations in five WIO countries during the 2020 bleaching period (see our interactive dashboard at

An MSc project by Ms. Julian Sitemba (at the University of Milan, Bicocca, and attached to CORDIO) found only 15% of corals on shallow fringing reefs (those most exposed to high temperatures) bleached, however mortality of these was quite high, as about half of those that bleached died, and half recovered. As before, branching corals were most susceptible particularly Stylophora and branching Porites, and less so Acropora, and they presented varied fluorescent pigments (purple, pink, lime green)

This high resistance to high temperatures in coral populations may be due to a combination of several things:

1) corals are becoming acclimated to the higher temperatures they have been experiencing over the last few decades, and

2) the die-off of more vulnerable individuals and species during past bleaching events(particularly in 1998 and 2016) has meant that corals alive now are those which are naturally more capable of coping with higher temperatures.

What does this mean and what next?

While this reduced bleaching susceptibility may be good, the loss in genetic diversity may have significant consequences in other ways in coming years. The effect is that reefs may be losing genetic diversity and species and that this loss is hidden by the robust survivors – until they succumb to another threat, or the temperatures exceed even their tolerance thresholds.

To understand the impacts of these dynamic environmental changes, robust and consistent surveys on coral bleaching are necessary. As such, CORDIO in partnership with other stakeholders in the WIO will continue monitoring temperature and bleaching trends during the summer months.

Now in partnership with the Global Monitoring of Environment in Africa (GMES) through the MarCoSouth project, CORDIO is developing a new co-designed bleaching alert service based on high-resolution Copernicus products to detect thermal stress. 

Meanwhile, bi-monthly bleaching alerts will be shared by email, on CORDIO's website and the GMES website, highlighting the likelihood and intensity of bleaching and potential mortality of corals. Through this activity, we hope to encourage network members and all people, communities, and organizations dependent on coral reefs to monitor their local reefs and share their data during the 2021 bleaching season.

How to participate

At CORDIO, we collect and receive bleaching data through a citizen science approach. To participate in this year’s bleaching alert observation and monitoring, we encourage our partners to continue submitting basic observations on coral bleaching at this online reporting form.

For new or inexperienced observers, you can become part of the coral bleaching observation team by reaching out to us at, and by reading through our resource materials and guidance on responding to coral bleaching and mortality on our web page.