Ocean temperatures the world over are building at a relentless rate as humans continue to alter the atmosphere around them.
In 2021, according to a new summary of two international datasets, the wave of warmth in our oceans hit a new peak, eclipsing the influence of cooler regional episodes.
While last year's ocean warming is unprecedented, it was no exception. This is the sixth year in a row that the world's ocean temperatures have exceeded anything we've measured before.
Since the late 1950s, when reliable recordings of climate change first began, each decade has hosted warmer oceans than the last. Since the 1980s, the authors of the summary say there has been an "unambiguous" increase in marine temperatures.
The heat is being felt right across the board. Last year, the North Pacific Ocean, the North Atlantic Ocean, and the Mediterranean Sea all experienced the hottest marine temperatures on record.
Altogether, the upper 2,000 meters in our oceans absorbed 14 more Zettajoules (ZJ) in 2021 than in 2020.
The difference is equivalent to dropping about seven more Hiroshima bombs into the ocean per second. In 2019, scientists calculated human warming of the ocean was equivalent to dropping the equivalent of five Hiroshima bombs into the ocean per second.
"The oceans are absorbing most of the heating from human carbon emissions," says climate scientist Michael Mann from Pennsylvania State University.
"Until we reach net zero emissions, that heating will continue, and we'll continue to break ocean heat content records, as we did this year. Better awareness and understanding of the oceans are a basis for the actions to combat climate change."
Without cutting our emissions, not even short-term, regional fluctuations in temperature and circulation can change our current trajectory.
During an episode of La Niña, for instance, when winds and ocean currents change, the northern parts of the Pacific Ocean tend to become cooler, while southern waters above Australia grow slightly warmer.
Despite this minor cooling effect swinging into action in late 2021, warming in the North Pacific ocean has remained "broad and deep-reaching".
Last year, warming anomalies in the middle of the North Pacific were measured at ~2°C near the surface and 1°C ~300 meters deep.
"The relentless increases in [ocean heat content] have direct implications for the frequency, intensity, and extent of marine heat waves and other 'hot spots' within the ocean," the authors write.
Take, for example, the notorious 'Blob' that keeps popping up off the coast of the pacific northwest in the United States and southwest Canada. In 2014, this massive ocean heat wave, possibly the largest on record, grew and spread for years, devastating food webs along the way.
In 2019, the Blob had returned, and in 2021 the swirl of hot ocean persisted beneath a "high-pressure heat dome" that at times exceeded 40°C. The cool La Niña period was not enough to stop it, although it did reduce the impact somewhat.
"Indeed, although in the top 10 warmest years, global surface temperatures for 2021 are not the highest on record because of La Niña conditions in the tropical Pacific, among other things," explains mechanical engineer John Abraham from the University of St. Thomas.
Long-term ocean trends suggest the Atlantic and Southern Oceans are absorbing the greatest amount of heat from our greenhouse gas emissions.
When an ocean absorbs heat, sea water expands, leading to rising sea levels. If the marine heat in our southern oceans chips away at enough of the Antarctic ice sheet, it could destabilize the structure, adding more water to the ocean and sinking even more of our coastlines.
"Warmer oceans also supercharge weather systems, creating more powerful storms and hurricanes, as well as increasing precipitation and flood risk," says atmospheric scientist Lijing Cheng from the Chinese Academy of Sciences (CAS).
We need to prepare for these disasters, and the best way to do that is to incorporate ocean temperatures into our climate models.
Unfortunately, however, there are still major "uncertainties and knowledge gaps in monitoring ocean warming", researchers say. If we don't improve awareness and understanding of these dynamics, we will be missing out on a fundamental part of climate change.
"[O]cean warming reduces the efficiency of oceanic carbon uptake and leaves more carbon dioxide in the air," says Cheng.
"Monitoring and understanding the heat and carbon coupling in the future are important to track climate change mitigation goals."
The study was published in Advances in Atmospheric Sciences.