Marine heatwaves have a detrimental impact on the marine ecosystem. They bleach corals and lead to mass fish mortality. Photo: Quintana Beach County Park/facebook

On the boil: The unseen risks of marine heatwaves

According to experts, marine heatwaves will have a progressive trend—this means longer durations and increased frequency, with devastating impacts on the ocean’s ecosystems  

After decades of having cast his nets in the Mumbai seas, Kiran Koli is adept at detecting changes in the oceans. The artisanal fisherman from Maharashtra’s Manori Island has been noticing irregularities in the weather and certain changes in the waters for a while now, worried about how this will impact his catch. “We have experienced a decline in fish production due to climate change,” he said, adding that fish tend to go farther away when the water gets too hot.

On the other side of the world, scientists at US’ National Oceanic and Atmospheric Administration’s (NOAA) joined a long list of researchers expressing alarm at ocean temperatures. Researchers at the Physical Research Laboratory publicised worrying forecasts and produced their experimental ocean heat model. By the end of this month, about half the world’s oceanic area is likely to be exhibiting heatwave conditions. This latest warning is almost a milestone in the gradual increase in marine heat content and the increasing threats of marine heatwaves.

As much as 90% of the heat that is trapped by greenhouse gas (GHGs) emissions is stored in the ocean bodies, increasing the ocean heat content (OHC). This is projected to have a devastating impact on the ocean’s ecosystems and biodiversity, adversely impacting the fisheries sector and allied livelihoods such as that of Koli’s.

Tracing the alarming increase in marine heatwaves

A January 2022 study by the Indian Institute of Tropical Meteorology (IITM), Pune, that traced the trend of marine heatwaves between 1982 and 2018, found that “the western Indian Ocean region experienced the largest increase in marine heatwaves (MHWs) at a rate of 1.2-1.5 events per decade,” even though climatologically, it wasn’t the region with frequent heatwaves. While the paper claims a relatively lower rate of increase in MHW events in the Bay of Bengal, a new paper, published last week by researchers from IIT-Kharagpur, that tracked MHWs over the Bay of Bengal from 1982 to 2021, challenges this.

A total of 107 MHW events were identified in this 30-year time span and an increasing trend of 1.11 MHW events/decade was observed. Worryingly though, the paper highlights a stark increase in the frequency of MHWs in the BoB over the past decade. Authors note that if current trends persist, “the BoB will experience heatwaves every month in the next 55–60 years, and it will attain a semi-permanent heatwave state by the end of the century, with extreme temperatures present for more than half of the year.” Interestingly, the paper also shows that while climatic variations like the El Nino Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) have a positive correlation with the occurrence of MHWs, the effect of background warming of oceans as a result of global warming has become the dominant factor in the temperature spikes and sustained MHW events.

 During 1982–2018, the western Indian Ocean region experienced the largest increase in marine heatwaves at a rate of 1.2–1.5 events per decade, followed by the north Bay of Bengal at a rate of 0.4–0.5 events per decade. Source:

MHWs are induced “by increased solar radiation, relaxation of winds, and reduced evaporative cooling” and disrupt the ocean’s ability to cool and sequester carbon. Literature is replete with evidence showing the strong correlation of MHW event frequency and intensity with global warming trends, with a high degree of attribution to anthropogenic climate change. A 2018 study on the global occurrence of MHWs, published in the journal Nature, attributes 87% of the present MHWs to human-induced warming, with the ratio on track to becoming 100% under a 2 degree C global warming scenario.

“Most of today’s marine heatwaves are attributable to anthropogenic emissions leading to temperature rise,” says Roxy Mathew Koll, a climate scientist at IITM. A bi-monthly assessment by Mercator Ocean International, an intergovernmental organisation for July 2023, observed heatwaves in the southern Indian Ocean’s southern hemisphere and the Pacific. These occurrences, according to experts, are slated to continue especially in the Bay of Bengal region and across the Arabian Sea.

 “Our (Central Marine Fisheries Research Institute) studies clearly indicate that MHWs will have a progressive trend—more duration and initially more frequency as well,” says Grison George, principal scientist at CMFRI. But because the duration is longer, he claims the number of events will be fewer. This, however, doesn’t soften the blow for communities reliant on fishing.

There is now evidence that the implications of the rising trend of MHWs cannot be seen in isolation to other atmospheric and oceanic circulations that are vital to the subcontinent. Enter the monsoon. The IITM study from early 2022 for the first time elaborated on this impact. “The marine heatwaves in the western Indian Ocean and the Bay of Bengal are found to result in drying conditions over the central Indian subcontinent. At the same time, there is a significant increase in the rainfall over south peninsular India in response to the heatwaves in the north Bay of Bengal. These changes are in response to the modulation of the monsoon winds by the heatwaves,” the study states.


 Impact on fishing communities

Researchers at UK’s University of Glasgow have found that in response to global warming, the majority of fish populations across oceans are “relocating towards colder waters nearer the north and south poles.” Illustrating the impact of climate change, the study further found this to be a response to ocean warming, where the fish are moving to deeper waters to stay cool.

“Marine heatwaves have a detrimental impact on the marine ecosystem. They bleach corals and lead to mass fish mortality,” states Koll, adding how this can cause a collapse of marine aquaculture farms.

At a national workshop for strategising the inclusivity of fisherfolk and devising climate change policies and plans, the National Fishworkers Forum (NFF) highlighted how the warming of oceans, which has reached 2000 metres, is likely to affect the production of fish. The acidity of water, the report explains, is measured by the pH level. “If the pH value reaches 5 then the fish metabolism decreases drastically, making them sluggish, and affecting their reproduction” and spelling trouble for coastal communities, which are dubbed to be the most vulnerable to “climate-related ocean changes.”

Koli mentions that his community has to remain cognisant of these changes taking place in the ocean. “Our fishing zones are not confined to one particular area, it all depends on the temperature—It (SST) also changes the colour of the water and the kind of fish that are found,” he says. Attesting to this, George says that fishermen have indigenous knowledge, they gauge the changes in the ocean by looking at the water’s characteristics. He says potential fishing zones are also estimated along these parameters, where a technique was developed to “use the remotely sensed sea surface temperature (SST) to identify the locations of fish aggregation”.

“But of late they also say that it’s becoming hard to predict the frequency of these changes,” George says while outlining the need for early warning systems.

Marine heat monitoring in India

In March 2023, when questioned about the government’s plan of action in overcoming the disruption in Indian monsoons due to abnormal oceanic temperature, the MOES replied to the Rajya Sabha that the India Meteorological Department’s monsoon forecast models have incorporated data from the ocean surface temperatures for advance planning and disaster management. 

Roxy says that MHW statistics currently are largely based on sea surface temperatures (SSTs). “We use both satellite measurements of SSTs and ocean observation instruments to detect MHWs,” he says, adding that despite their wide reach, they can only measure surface temperatures. “This is where on-site ocean instruments help, the OMNI (Ocean Moored buoy Network for northern Indian Ocean), RAMA (Research Moored Array for African-Asian-Australian Monsoon Analysis) moorings deployed in collaboration with the US’ National Oceanic and Atmospheric Administration (NOAA), and global Argo floats are some of the observation platforms that measure the ocean environment at the surface and through the depth,” he says, pointing out how one of the OMNI moorings detected record-breaking MHWs of 32-34 degrees Celsius. “Satellites cannot monitor these short-term variations at a high resolution.”

This outlines the need for more platforms to detect the MHW signatures closely and below the ocean’s surface.

Agreeing with this, Dr. Srinivas Rao, a scientist with INCOIS, mentions that for accurate measurement, Argo-profiling devices are deployed by mentioning a predetermined depth such as 2,000 metres from the surface. As per the Indian National Center for Ocean Information Services (INCOIS), Argo is an international project that uses robotic floats that drift below the ocean to collect information of the upper part of the world’s oceans and transmit this data to the satellites.

A 2022 national report mentions that India has deployed 494 floats in total, out of which 83 are active and transmitting data. INCOIS maintains a web-based geographic information system (GIS) site for the Indian Argo Program which monitors the floats’ data and their trajectories. “Argo data are being widely used for many applications to understand the Indian Ocean dynamics, cyclone and monsoon system in relation to heat content, thermosteric component of sea level and validation of ocean general circulation model (OGCM)” the report finds.

Rao says this is used to estimate how much heat content is stored in the ocean. “That parameter gives the inference of cyclone genesis how much energy is stored in the ocean across different seasons.” These conclusions are important to ascertain the impact that MHWs are having on marine biota i.e. the ecosystem and associated organisms.

One such system is the Marine Heatwave Advisory Services by INCOIS, which provides real-time data on MHW intensity and severity in the Indian Ocean, which Rao claims is at a primary stage. “Through climatology, we have average data for over 15 years, and are currently monitoring the locations where there is a difference of 1 degree centigrade,” he says, attributing to how these regions house the probability of a marine heatwave.

Importance of early warning systems

“It is critical to monitor the changes in OHC in order to understand the rate and extent of global warming,” states a chapter on Indian Ocean Warming co-authored by Roxy, where the authors elaborate on how the monitoring of regional variations in OHC can yield key insights into climate variability and change.

Roxy says that warm ocean waters supply the source of energy—the heat and moisture—for the monsoons and cyclones. “MHWs interact and modulate these weather systems. If we do not monitor them closely and integrate this data in the forecast models, then monsoon and cyclone forecasts will be less accurate,” he warns. This will further lead to forecast models failing to simulate the rapid intensification of cyclones in a short time.

In reference to this, George indicates how there are no early warning systems in place currently, so when extreme weather events occur, they severely impact resources and livelihoods. Globally, he adds, the number of warning systems available for MHW is very few. “We have information from the USA where they have come up with a method to provide information on these heatwaves, but we need to adapt these. There are no early warning systems in place as of now, so when such extreme events happen there is a direct impact on the resources and its impacting livelihoods. Globally, the number of warning systems available for MHW is very few.”

“When it comes to ocean heat or MHW, we need to have an understanding of the global circulation system, so we can have early warning systems clearly indicating where the marine heatwaves are generated,” he points out. According to him, we have very few high-resolution models in the northern India ocean that help us generate MHW data.

While granular high-resolution models to predict MHWs are still under development, efforts to produce larger scale models have seen success in recent years. Last year, NOAA scientists produced a global seasonal model to provide targeted warnings with relatively high degrees of confidence. Further development of the science around the causal factors of MHWs and its implications on regional ecosystems, biodiversity, and human populations will likely see louder calls for incorporation into the gamut of global regulatory frameworks and the forums that devise them.