The world experienced a 240 per cent increase in the average number of marine heatwave (MHW) days in the summers of 2023-24 relative to the instrumental record, a new study warned.
MHW, which are extreme rises in ocean temperature for an extended period of time, hit every part of the globe. Close to 10 per cent of the oceans experienced the highest sea surface temperatures (SST) ever — four times higher than the historical annual average, reads the analysis published in Nature Climate Change.
The global average SSTs reached record highs. This rapid warming, according to scientists, is fuelled by human-induced climate change. The other contributing factors include El Niño — warm phase of a natural climate pattern across the tropical Pacific known as the El Niño-Southern Oscillation, or “ENSO” — and record low global cloud cover.
“Summertime MWHs in 2023-24 were unprecedented and had wide-ranging biological, physical and societal impacts. Need to develop better response plans and adaptation and intervention approaches as MHWs intensify under climate change,” Dan A Smale, Marine Biological Association of the United Kingdom, Plymouth, UK, and the author of the paper, wrote on X (formerly Twitter).
The MHWs had knock-on effects. The prolonged MHWs in 2023 influenced weather patterns, for instance. It contributed to extreme air temperatures in the United Kingdom, North America and Japan, and severe flooding in Ecuador, Libya, Japan and Australia.
It also ‘supercharged’ the heat and moisture exchanges between sea and air, with a near-record number of named storms occurring through the 2023 Atlantic hurricane season and several tropical storms made landfall along the Pacific and Indian Ocean coastlines.
For example, MHWs fuelled Cyclone Gabrielle, killing 11 people in New Zealand and with an estimated cost of more than New Zealand $14 billion.
In the Bay of Bengal, MHWs drove extreme weather in 2023, tropical cyclone Mocha in May 2023, and Cyclone Remal in May 2024.
Previous studies have shown that the Indian Ocean has warmed at a rate of 1.2°C per century during 1950-2020. Between 2020 and 2100, climate models predict warming at a rate of 1.7°C-3.8°C per century. The effects are expected to be more pronounced in the northwestern Indian Ocean including the Arabian Sea.
Further, MHWs are expected to increase from 20 days per year to 220-250 days per year, pushing the tropical Indian Ocean into a near-permanent heatwave state, according to the book, Future projections for the tropical Indian Ocean.
Overall, in 2023-2024, MHWs drove 23 records of physical impacts such as cyclones, flooding, atmospheric heatwaves, rainfall and dam collapse.
The MHW events across 2023-2024 were also linked to biological impacts such as coral bleaching event, loss of vital ecosystems off Japan and Peru, and species making inroads into newer geographical locations.
The Gulf of Thailand and the Gulf of Mexico reported mass mortalities of fish. The researchers also recorded impacts on the food web. In the North Atlantic, lower phytoplankton productivity affected the wider food web, raising concerns about potential impacts on fish and seabird populations. Phytoplankton are microscopic marine algae that form the base of several aquatic food webs.
Overall, there were 43 reports of biological impacts driven by MHWs globally. These also include MHWs in October 2023 forcing North Atlantic species in the North Atlantic Ocean to move to the Arctic and an increase in whale and dolphin stranding in the South Pacific Ocean in January 2024. These events also played a role in triggering a disease in the Mediterranean Sea between July and August 2023, threatening the extinction of fan mussel — the largest shellfish in the Mediterranean Sea.
MHWs were also linked to damages to ecosystem services, marine industries and wider society. There were 33 records of societal impact from MHWs globally, shows the study.
For example, the global coral bleaching event negatively affected snorkelling and scuba diving tourism with some areas being closed to visitors. The Mediterranean Sea and the Gulf of Thailand also saw losses to aquaculture. Range shifts in Peruvian anchovies — a type of fish — caused the shutdown of commercial fisheries. The estimated losses were US$1.4 billion, the study highlighted.
In a vast majority of impacted regions, the briefings and response plans were neither issued nor implemented, the researchers noted. This, they added, may be due to issues such as limited resources, disconnection between institutions or organisational bodies and lack of communication.
The researchers stress that the world needs to be better prepared. No known mitigation action was taken in the majority of impacts during the 2023-2024 MHWs.
They also call for improving accuracy of forecasts. In the 2023-2024 period, forecasts were generally accurate with some misses. “Improving forecast accuracy will lead to greater confidence for proactive decision-making, while longer-range forecasts will extend the preparation window,” the researchers wrote.