As balmy seas help fuel abnormally hot weather across New Zealand this summer, scientists report that climate change has pushed the planet's oceans to record-breaking temperatures.
A joint study just published in the journal Advances in Atmospheric Sciences, co-authored by a New Zealand scientist, found global ocean temperatures continued a three-year record streak in 2021.
"The ocean heat content is relentlessly increasing, globally, and this is a primary indicator of human-induced climate change," said Dr Kevin Trenberth, honorary academic at the University of Auckland and distinguished scholar at the National Centre for Atmospheric Research in Colorado.
"In this most recent report, we updated observations of the ocean through 2021, while also revisiting and reprocessing earlier data."
For the past year, the researchers found that the upper 2000m in all oceans absorbed at least 14 to 16 more zettajoules (ZJ) - a measure of thermal energy - than in 2020. They absorbed 20 more ZJs than in 2019.
"As well as absorbing heat, currently, the ocean absorbs 25 to 30 per cent of human carbon dioxide emissions, leading to ocean acidification; however, ocean warming reduces the efficiency of oceanic carbon uptake and leaves more carbon dioxide in the air," said study lead author, Dr Lijing Cheng.
"Monitoring and understanding the heat and carbon coupling in the future are important to track climate change mitigation goals."
The researchers also assessed the role of various natural variations, such as the warming and cooling phases known as El Nino and La Nina, which greatly affect regional temperature changes.
According to Cheng, the regional analyses show that the robust and significant ocean warming since the late 1950s occurred everywhere.
Nevertheless, regional "marine heatwaves" were a consequence, with huge impacts on marine life.
Another such heatwave has been unfolding around New Zealand in tandem with a La Nina this summer, pushing seas around the north of the country to conditions more severe than even 2017-18's unprecedented event.
Niwa has singled out unusually warm coastal waters as one factor behind New Zealand's just-confirmed record-hot mean 2021 temperature of 13.56C.
"Our previous work showed that scientists need less than four years of ocean heat measurements to detect a human-induced warming signal from natural variations," said co-author Professor John Abraham, of the University of St Thomas in the US.
"This is much shorter than the nearly three decades of measurements required to detect global warming using temperatures of air near the Earth's surface."
Indeed, although in the top 10 warmest years, global surface temperatures for 2021 were not the highest on record because of La Nina conditions in the tropical Pacific, among other factors.
"Ocean heat content is one of the best indicators of climate change."
During La Nina, the ocean actually takes up but buries extra heat below the surface.
"With model experiments, our study shows that the pattern of ocean warming is a result of human-related changes in atmospheric composition," said Cheng, of China's International Centre for Climate and Environment Sciences.
"As oceans warm, the water expands and sea level rises. Warmer oceans also supercharge weather systems, creating more powerful storms and hurricanes, as well as increasing precipitation and flood risk."
Co-author and prominent US climate scientist Distinguished Professor Michael Mann added that, until we reached net-zero emissions, ocean heating would 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."