June 9, 2025 — Unusual heat waves have occurred in all of the major ocean basins around the planet in recent years. And some of these events have become so intense that scientists have coined a new term: super marine heat waves.
“The marine ecosystems where the super marine heat waves occur have never experienced such a high sea surface temperature in the past,” said Boyin Huang, an oceanographer at the National Oceanic and Atmospheric Administration in an email.
The seas off the coasts of the United Kingdom and Ireland experienced an unusually intense marine heat wave, one of the longest on record, starting in April and the temperature rise happened much earlier in the year than usual. Australia and its iconic coral reefs were recently struck by heat waves on two coasts.
Scientists define marine heat waves in different ways. But it’s clear that as the planet’s climate changes, the oceans are being fundamentally altered as they absorb excess heat trapped in the atmosphere from greenhouse gases, which are emitted when fossil fuels are burned.
Hotter oceans are causing drastic changes to marine life, sea levels and weather patterns.
Some of the most visible casualties of ocean warming have been coral reefs. When ocean temperatures rise too much, corals can bleach and die. About 84 percent of reefs worldwide experienced bleaching-level heat stress at some point between January 2023 and March 2025, according to a recent report.
Last year, the warmest on record, sea levels rose faster than scientists expected. Research showed that most of that rise in sea levels came from ocean water expanding as it warms, which is known as thermal expansion, not from melting glaciers and ice sheets, which in past years were the biggest contributors to rising seas.
Excess heat in the oceans can also affect weather patterns, making hurricanes more likely to rapidly intensify and become more destructive. In the southwest Pacific, last year’s ocean heat contributed to a record-breaking streak of tropical cyclones hitting the Philippines.
“If we understand how global warming is affecting extreme events, that is essential information to try to anticipate what’s going on, what’s next,” said Marta Marcos, a physicist at the University of the Balearic Islands in Spain.
Dr. Marcos was the lead author of a recent study published in Proceedings of the National Academy of Sciences that found that climate change has been responsible for the overwhelming majority of marine heat waves in recent decades.