Climate

84% of the world’s reefs recently experienced bleaching-level heat stress, but not all were ‘completely bleached’

Posted on: 2025-05-02

Key takeaway

Roughly 84% of the world’s reefs recently experienced bleaching-level heat stress – which can lead to coral bleaching. However, that does not mean 84% of Earth’s total reef area was actually bleached. Widespread bleaching – driven by high ocean temperatures – occurred across Earth’s oceans, with 82 different countries reporting bleaching. Although satellite data showed bleaching-level heat stress in many areas of the ocean, that does not necessarily mean all reefs in those areas were bleached. Years of research and coral surveys will be needed to assess how much bleaching occurred and the full impact of this event on reefs at a global scale.

Review

It’s projected that, by 2050, over 98% of coral reefs will experience bleaching-level heat stress every year^[1]. But this is not only a future problem – corals are already feeling the impacts.

On 15 April 2025, the National Oceanic and Atmospheric Administration (NOAA) confirmed that the world is undergoing its fourth global bleaching event on record. In partnership with the International Coral Reef Initiative (ICRI), NOAA explained in a later update that bleaching-level heat stress has affected roughly 84% of the world’s reefs, with 82 countries and territories reporting mass levels of coral bleaching.

But what happens when reefs experience ‘bleaching-level heat stress’? Some news outlets, such as The Guardian and CBS, wrote headlines suggesting that ‘84% of the world’s reefs have been bleached’ as a result of this heat stress.

To check these headlines against reality, Science Feedback contacted NOAA Coral Reef Watch and several scientists who research coral. Below we will share what we learned from these scientists about the recent event and what it means for the world’s coral reefs.

84% of the world’s reefs recently experienced harmful ‘bleaching-level heat stress’ – but this doesn’t mean 84% are now ‘bleached’

Before diving into specifics on the bleaching event, let’s clarify – what exactly is coral ‘bleaching’?

Coral reefs are especially sensitive to temperature changes. When water temperatures rise, corals can experience a breakdown and loss of algae which normally live inside their tissue and provide them with energy. As they dispel these symbiotic algae, they lose their color^[2], giving the appearance of being ‘bleached’ white – hence the term ‘bleaching’ (Figure 1). When this occurs, corals become more vulnerable to death and disease^[2].

**Figure 1 –** The same coral community in Halfway Island, Great Barrier Reef, before and after a bleaching event in 2002, where approximately 95% of the coral community was “severely bleached”. Source: IPCC (2014)^[3]

Following NOAA’s update about the recent global bleaching event, The Guardian and CBS published articles with interesting insights about the recent bleaching event. That being said, the headlines they used could unfortunately mislead readers.

For example, CBS’s headline reads “Latest coral reef bleaching is worst on record, turning 84% pale white”, and The Guardian’s headline reads “More than 80% of the world’s reefs hit by bleaching”. Readers could easily interpret this to mean that if you look at 100% of the Earth’s reef area, 84% of it is now bleached. But scientists we interviewed explained that this is not necessarily the case.

Science Feedback contacted several scientists – including Dr. Derek P. Manzello, Coordinator of NOAA’s Coral Reef Watch program (which shared the ‘84%’ figure) – who explained that, while it is true that 84% of coral reefs experienced bleaching-level heat stress, this does not necessarily mean 84% of Earth’s reef area was entirely ‘bleached’.

In an email to Science Feedback, Manzello pointed out that it is not 84% of reef area that has been bleached, but rather that 84% of the world’s reefs experienced high ocean temperatures that are known to trigger bleaching. As Manzello explained:

Derek P. Manzello

Coordinator, National Oceanic and Atmospheric Administration Coral Reef Watch

“From 1 January 2023 to 30 April 2025, bleaching-level heat stress has impacted 83.7% of the world’s coral reef area and mass coral bleaching has been documented in at least 83 countries and territories. This makes the 4th Global Coral Bleaching Event (GBE4) the most spatially expansive on record. […]

What we are tracking/reporting here is how much area of the ocean with coral reefs has experienced heat exposures that are known to elicit coral bleaching, as well as the number of countries/territories where mass bleaching has been confirmed to be happening via in-water diver observations. It’s important to understand that the latter is a qualitative measure of bleaching presence/absence.

It is going to take a few years before we will truly know the full impact of this event because this requires in-water monitoring, or field assessments of the impacts, which are generally completed months after a bleaching event has stopped. Once the follow up surveys are done to quantify mortality and recovery, then those data need to be analyzed and interpreted.”

Science Feedback confirmed this with Dr. Ken Caldeira, Senior Scientist (Emeritus) in the Carnegie Institution for Science and Senior Scientist at Gates Ventures, who explained:

Ken Caldeira

Senior Scientist, Carnegie Institution for Science

“The data and model reanalysis indicate that about 84% of the world’s reefs have experienced temperature sufficient to cause coral bleaching. As there are well over 10,000 coral reefs in the world, not all of them have been inspected and had their bleaching quantified. However, the expectation is that some corals in each of these 84% of reefs experienced bleaching. 84% of reefs experiencing some bleaching is not the same as 84% of reef area bleached.”

This suggests that the headlines are a bit misleading, but to confirm this, we asked Dr. John Bruno, Distinguished Professor in the Department of Biology at The University of North Carolina at Chapel Hill, who studies coral reef ecology and conservation. Bruno explained:

John Bruno

Professor, The University of North Carolina at Chapel Hill

“I think they [CBS] botched this headline but, also, the ICRI press release was misleading:

‘From 1 January 2023 to 30 March 2025, bleaching-level heat stress impacted 84% of the world’s reefs, with 82 countries, territories and economies suffering damage.’

[…] in summary the statement that ‘84% of the world’s reefs were turned pale white’ is an exaggeration and lame extrapolation. We don’t actually know what proportion of reefs [were] bleached.”

This does not mean that the event isn’t problematic for coral – it was, as we will explain in next section – but this headline error is not just a minor detail because, as Bruno explained, 84% of global reefs ‘turning white’ would be unprecedented:

John Bruno

Professor, The University of North Carolina at Chapel Hill

“I have not seen any evidence of that, and I think it’s extremely unlikely. For the reasons I explained below (not enough coral remaining for that to happen). But also, even during the worst bleaching episodes, there’s a lot of variability among reefs (and corals) in bleaching severity. They never all turn white. Maybe third to a half at most. Never 84% across a region like the GBR, or globally.”

While the headlines were a bit misleading, it is true that this recent event is problematic for the world’s reefs, like many of the other recent events, as we will explain below.

Rising ocean temperatures are causing frequent mass bleaching events, making reef recovery very difficult

Bleaching events have greatly increased since the early 1980s. For example, “from 2016 through 2020, the Great Barrier Reef experienced mass coral bleaching three times in 5 years”^[4]. There have also been major losses of coral; there has been at least a 50% decrease of warm-water coral reefs over the last 30 to 50 years in many of the world’s tropical regions^[2].

Putting this recent trend in perspective, Dr. Terry Hughes, Professor at the ARC Centre of Excellence for Coral Reef Studies at James Cook University, explained to Science Feedback:

Terry Hughes

Professor, ARC Centre of Excellence for Coral Reef Studies, James Cook University

“Severe bleaching has occurred in different regions every summer now for nearly two decades, including on the GBR [Great Barrier Reef] in 1998, 2002, 2016, 2017, 2020, 2022, 2024 and 2025. Of these 1998, 2016 and 2017, and 2024 and 2025 were during the 1st, 3rd and 4th global events.”

So was the most recent (fourth) global bleaching event the worst on record? It depends how you define ‘worst’. One way of measuring it is the geographic or spatial extent of heat stress (Figure 2), and another is measuring how much of the living coral is now dead or lost. In terms of spatial extent, the recent event is the worst on record, according to NOAA (see earlier quote from Manzello).

Heat stress map NOAA coral reef bleaching, 84% of coral reefs experienced bleaching-level heat stress — **Figure 2 –** This map illustrates the maximum accumulated heat stress (based on satellite data) that coral reefs have experienced globally from January 1, 2023 – the start of the fourth global bleaching event – to April 30, 2025. Areas shaded in dark red or brown represent zones where sustained heat has reached levels capable of triggering widespread bleaching and mortality among heat-sensitive coral species. Regions shown in light brown (Alert Level 3 [AL3]), pink (AL4), and deep purple (AL5) are locations where heat stress poses a risk of extensive loss across certain coral species or total mortality. Source: NOAA Coral Reef Watch

But when it comes to actual coral that died or was lost, it might be a different story. When Science Feedback asked Bruno if this was truly the worst bleaching event on record, he replied:

John Bruno

Professor, The University of North Carolina at Chapel Hill

“I am skeptical of this claim. Some of my colleagues also argued this after 2017/2018. I do not know what criteria it’s based on. It could be based on geographic extent or the absolute (in terms of living cover) coral loss/mortality. I think it should probably be based on the latter.

Even though ocean temperature is increasing and the heatwaves that cause bleaching are undoubtedly getting more frequent and severe (in terms of the maximum temps reached), bleaching should be getting less severe, not more. That’s because there’s much less coral left to lose than there was for the first global event in 1988.

Moreover, most of the warming sensitive genotypes and species have been removed from communities over the last 40 years. There is a very strong relationship between how much living coral is present and much is lost due to a heat wave. Healthy reefs lose MUCH more coral than degraded reefs.

This is because there’s fewer coral colonies left, and the remaining species are more tolerant of heatwaves.”

In other words, this may be the worst event on record in terms of extent of bleaching-level heat stress – or conditions that make coral bleaching more likely – but it may not be the worst in terms of how much bleaching actually occurred. That will take more time to investigate.

Hughes explained to Science Feedback why there is an issue with assuming bleaching-level heat stress directly translates to bleaching:

Terry Hughes

Professor, ARC Centre of Excellence for Coral Reef Studies, James Cook University

“The 84% figure is based on satellite data on sea surface temperatures in summer, and by the duration of high levels of heat. The standard metric of heat exposure is Degree Heating Weeks [DHW]. However, NOAA’s global maps of DHW are not a reliable proxy of how corals are responding to heat. The response of corals (whether they bleach or die) to a given level of heat exposure varies over time, and from place to place.”

Regarding the scale of recent bleaching from this event, Bruno explained:

John Bruno

Professor, The University of North Carolina at Chapel Hill

“There are papers in review now that document that in detail, including in terms of how much coral was lost. When that data comes out, quantitative comparison to 1998, 2017, etc. can be made”

Manzello pointed out to Science Feedback that understanding the global scale impacts will take time:

Derek P. Manzello

Coordinator, National Oceanic and Atmospheric Administration Coral Reef Watch

“We are actively compiling data on the impacts right now. We do anticipate a few publications in the near future from select locations, but the global scale perspective will still be a year or two away.”

Can the world’s reefs recover from recent events? Total recovery is unlikely but reducing future global warming can help

Regardless of which bleaching event was the ‘worst’, this event – and the general rise in bleaching events – is problematic for the future of coral reefs. But can these reefs recover? Bruno explained the following to Science Feedback:

John Bruno

Professor, The University of North Carolina at Chapel Hill

“Many reefs, populations, and species will recover, but some will not. If this was the final event, I’m confident that things would get back to “normal” in a decade or two. The problem is that this is happening nearly every year now. It used to only happen every ten to twenty years.

There’s obviously no way a forest can regrow if all burns every summer. That’s what’s happening on coral reefs. And in nearly every other kind of marine habitat.

These human-caused heat waves are becoming so frequent populations and communities can’t possibly recover or keep up with all the change.

So, it’s a grim reality for reefs. But it is also happening to kelp forests, seagrass meadows, mangroves, mussel beds and oyster reefs… everywhere we look we see the same destruction and loss.”

Caldeira similarly explained the following to Science Feedback:

Ken Caldeira

Senior Scientist, Carnegie Institution for Science

“Most reefs can recover from rare, infrequent, bleaching events. However, they often cannot recover from frequent bleaching, say, several bleaching events in a decade. And this is where most reefs in the world are heading, if they are not there already.

Hughes explained:

Terry Hughes

Professor, ARC Centre of Excellence for Coral Reef Studies, James Cook University

“Coral reefs are never going to recover to their former condition due to global heating. The reefs of today are already degraded compared to the era before mass bleaching. What we really need to see is a stabilization of global temperatures as soon as possible, which would allow reefs to recover to a new mix of species.”

To learn more about coral reefs, check out our past reviews, including an insight article about why climate change is an existential threat to coral reefs.

Scientists’ feedback

Questions from Science Feedback:

Is this most recent bleaching event (January 2023 to April 2025) truly the worst one on record? Do we know why it happened?
A global bleaching event update from NOAA notes that 84% of the world’s reefs have experienced ‘bleaching-level heat stress’ – does this mean that 84% of the world’s reefs have been ‘hit’ by or experienced bleaching, as The Guardian title [or CBS title] suggests? If not, how and when will scientists determine how much actual bleaching has occurred as a result of this heat stress?
What does the magnitude of this event mean for the future of global reef health? Do scientists expect good recovery or permanent impacts?

Derek P. Manzello

Coordinator, National Oceanic and Atmospheric Administration Coral Reef Watch

1)“My answer to this question covers your 2^nd question as well.

From 1 January 2023 to 30 April 2025, bleaching-level heat stress has impacted 83.7% of the world’s coral reef area and mass coral bleaching has been documented in at least 83 countries and territories. This makes the 4th Global Coral Bleaching Event (GBE4) the most spatially expansive on record. During the first global coral bleaching event in 1998, 21% of reefs experienced bleaching-level heat stress, rising to 37% in the second event in 2010 and 68% during the third event (2014-2017)

For instance, the Caribbean experienced severe bleaching in 2023 and 2024. To quantify mortality, many of our collaborators are still going out to the field now, or just wrapping up their surveys, to determine how much coral died from the 2024 bleaching.

An additional challenge is that corals become highly susceptible to disease after undergoing the stress of bleaching. For instance, in the Virgin Islands in 2005, many corals survived the heat stress, but died over the next 1-2 years due to disease outbreaks (https://link.springer.com/article/10.1007/s00338-009-0531-7). Thus, the full impacts from a severe bleaching event may not be fully realized for up to 2 years after the event!

We are actively compiling data on the impacts right now. We do anticipate a few publications in the near future from select locations, but the global scale perspective will still be a year or two away. Graham Readfern’s article has links to 4 of the studies that have come out showing impacts: https://www.theguardian.com/environment/2025/apr/23/coral-reef-bleaching-worst-global-event-on-record

I do anticipate that the number of papers documenting the mortality from this event will increase greatly over the latter half of this year, as I am aware of some studies in review now. We will be learning a lot more on just how severe this event has been very, very soon.

2) See response to question 1.

3) The warming of the oceans poses a serious threat to coral reef ecosystems around the planet, as bleaching events continue to increase in severity, frequency, and magnitude.

Corals can recover if the heat stress is not too severe, or too prolonged. However, corals that bleach and recover generally have depressed growth for 2-4 years, impaired reproductive output for up to 5 years, and become highly susceptible to disease for 1-2 years. Thus, there are lasting physiological impacts for the survivors. All of these impacts act to impair the resilience, or recovery potential of a coral reef to added stressors. Some reefs in the Indo-Pacific region have shown the ability to recover from severe bleaching events in as little as 7-15 years. Recovery is certainly possible, but becomes increasingly challenging as bleaching events become more frequent and more severe. Current models suggest that every reef on planet Earth will experience severe, annual bleaching sometime between 2040-2050. This is very alarming because this implies that corals will be in a constant state of stress, as they won’t have enough time to recover from one bleaching event to the next.

Coral reefs are the rainforests of the sea and represent the most biodiverse ecosystem in the oceans. It is estimated that roughly 25% of all marine species associate with coral reefs at some point in their lives, and recent estimates have put these number >30%. Thus, the large-scale death of corals and degradation of coral reefs threatens the survival of 1 in 4 of every living organism in the ocean! It is estimated that the ecosystem services of coral reefs are contributing more than 3 billion USD to the US economy every year and this number is in the trillions of dollars for the entire globe. These ecosystem services include things like coastal protection from storms and sea-level rise, as well as tourism (diving, snorkeling, recreational fishing etc). Coral reefs are big source of protein for many island and coastal communities, supporting subsistence fishing in places like the Pacific Islands. Finally, many new pharmaceuticals, including experimental cancer drugs, are being developed from novel organic compounds discovered on coral reefs. As reefs lose biodiversity, the human race may be losing medicines that have the potential to save lives.

Thus, there are severe economic ramifications to the large-scale decline of reefs, as well as downstream impacts that could affect land-locked people that have never seen coral reefs.”

Terry Hughes

Professor, ARC Centre of Excellence for Coral Reef Studies, James Cook University

1) There are several issues here – uncertainty over start and end dates, and using satellite data as a proxy for bleaching (see #2).

Bleaching occurs in summer, so bleaching events in the northern and southern hemispheres typically occur 6 months apart (e.g. from around August in Florida, or February on the Great Barrier Reef). Most of the world’s reefs are in the southern hemisphere or near the equator. Therefore mild or moderate bleaching in a relatively small number of locations in one hemisphere could artificially extend the duration of a global event. For example, it’s unclear how many southern-hemisphere reefs bleached in early 2023 before Florida, the Caribbean and eastern Pacific all bleached severely later in the northern summer.

Severe bleaching has occurred in different regions every summer now for nearly two decades, including on the GBR in 1998, 2002, 2016, 2017, 2020, 2022, 2024 and 2025. Of these 1998, 2016 and 2017, and 2024 and 2025 were during the 1^st, 3^rd and 4^th global events. The GBR did not bleach in 2^nd global event in 2010.

2) Obviously the 84% figure is sensitive to how many summers are included in the calculation (see #1). Even before this latest global event, almost 100% of the world’s coral reefs had already experienced bleaching, on average 3-4 times up to 2016^[6] (Fact-check: see Fig. 2 in this paper). 84% over the past 3 years is not unexpected.

The 84% figure is based on satellite data on sea surface temperatures in summer, and by the duration of high levels of heat. The standard metric of heat exposure is Degree Heating Weeks. However, NOAA’s global maps of DHW are not a reliable proxy of how corals are responding to heat. The maps assume that bleaching is initiated at 4DHW, and that corals begin to die at 8DHW. But it’s not that simple.

The response of corals (whether they bleach or die) to a given level of heat exposure varies over time, and from place to place. For example, on the GBR, it took twice as much heat exposure to trigger bleaching in 2017 as it did in 2016. Why? Because dead corals don’t bleach a second time. Earlier bleaching events winnow out thermally-sensitive corals leaving tougher survivors. The opposite can happen if there’s a long enough gap between one bleaching event and the next when vulnerability to heat exposure can increase again if populations of heat-sensitive branching corals partially recover – we saw that on the GBR in 2022.

In places like Florida, only tougher species remain, and it takes far more heat exposure now to bleach or kill them.

DHW predicts bleaching – it doesn’t measure it. It would take a huge effort to collate underwater measurements from around the tropics to show where NOAA’s prediction are accurate. The satellites give us the big picture, but the picture is a bit fuzzy. Yes, the last 2 or 3 years constitute a global event, and it has probably been worse than the one centred on 2016.

3) Coral reefs are never going to recover to their former condition due to global heating. The reefs of today are already degraded compared to the era before mass bleaching. What we really need to see is a stabilization of global temperatures as soon as possible, which would allow reefs to recover to a new mix of species.”

Ken Caldeira

Senior Scientist, Carnegie Institution for Science

I am not sure what is bad and what is worse:

2014-2017:

2023-2025:

They [NOAA] added the additional levels is 2023:

‘Coral bleaching response plans, incident action plans, and restoration plans around the world rely on NOAA CRW’s Bleaching Alert Levels (recently expanded, in December 2023, to include Bleaching Alert Levels 3-5, in response to the extreme marine heatwaves of 2023)’

Most reefs can recover from rare, infrequent, bleaching events. However, they often cannot recover from frequent bleaching, say, several bleaching events in a decade. And this is where most reefs in the world are heading, if they are not there already.”

John Bruno

Professor, The University of North Carolina at Chapel Hill

1)“I am skeptical of this claim. Some of my colleagues also argued this after 2017/2018. (And I think it’s to an extent a message coming from younger scientists that only recently witnessed bleaching)

I do not know what criteria it’s based on.

It could be based on geographic extent or the absolute (in terms of living cover) coral loss/mortality. I think it should probably be based on the latter.

Even though ocean temperature is increasing and the heatwaves that cause bleaching are undoubtedly getting more frequent and severe (in terms of the maximum temps reached), bleaching should be getting less severe, not more. That’s because there’s much less coral left to lose than there was for the first global event in 1988.

Moreover, most of the warming sensitive genotypes and species have been removed from communities over the last 40 years.

There is a very strong relationship between how much living coral is present and [how] much is lost due to a heat wave. Healthy reefs lose MUCH more coral than degraded reefs.

This is because there’s fewer coral colonies left, and the remaining species are more tolerant of heatwaves.

I was working in Palau during the 1998 global mass bleaching, and I’ve worked on coral reefs and mass bleaching ever since, and honestly, I’ve never seen anything like that again. Those reefs were pristine and nearly everything died. Nowadays in the Caribbean, what’s left is like 10-20% of the living coral that was there in the late 80s – so there’s just not as much to lose. Sadly.

My lab recently published a paper (linked here) that found that hurricanes no longer have any measurable effect on Caribbean coral communities, again, because all that remains are a few weedy, disturbance-tolerant species^[5].

2) Yes, it means nearly all countries / regions with coral reefs experienced bleaching. There are papers in review now that document that in detail, including in terms of how much coral was lost.

When that data comes out, quantitative comparison to 1998, 2017, etc. can be made (to address your first question).

I am confident nobody is measuring or even estimating the geographic extent of reef bleaching, or if they are, that’s only happening in a very few places. We don’t even do that in the US and the Australian’s do not do it. (It would be possible but massively expensive and there’s little $ to do this kind of monitoring.)

‘84% of the world’s reefs were ‘turned pale white’ – I have not seen any evidence of that, and I think it’s extremely unlikely. For the reasons I explained below (not enough coral remaining for that to happen). But also, even during the worst bleaching episodes, there’s a lot of variability among reefs (and corals) in bleaching severity. They never all turn white. Maybe third to a half at most. Never 84% across a region like the GBR, or globally. (That would be truly insane).

I think they [CBS] botched this headline but, also, the ICRI press release was misleading:

‘From 1 January 2023 to 30 March 2025, bleaching-level heat stress impacted 84% of the world’s reefs, with 82 countries, territories and economies suffering damage.’

So in summary the statement that ‘84% of the world’s reefs were turned pale white’ is an exaggeration and lame extrapolation. We don’t actually know what proportion of reefs [were] bleached.”

3) Many reefs, populations, and species will recover, but some will not. If this was the final event, I’m confident that things would get back to “normal” in a decade or two. The problem is that this is happening nearly every year now. It used to only happen every ten to twenty years.

There’s obviously no way a forest can regrow if all burns every summer. That’s what’s happening on coral reefs. And in nearly every other kind of marine habitat.

These human-caused heat waves are becoming so frequent populations and communities can’t possibly recover or keep up with all the change.

So, it’s a grim reality for reefs. But it is also happening to kelp forests, seagrass meadows, mangroves, mussel beds and oyster reefs… everywhere we look we see the same destruction and loss.”