Scientists analysed sea surface temperatures over the past 150 years. They found that extreme temperatures that occurred just 2% of the time a century ago have occurred at least 50% of the time in the global ocean since 2014, according to The Guardian.

In some hotspots, extreme temperatures occur 90% of the time, severely affecting wildlife. More than 90% of the heat captured by greenhouse gases is absorbed by the ocean, which plays a key role in maintaining a stable climate.

The study, published in the journal Plos Climate, examined monthly temperatures in each part of the ocean and set the highest temperature in 50 years as the benchmark for extreme heat. The scientists then examined temperature records from 1920 to 2019. They found that, by 2014, more than 50% of the monthly records across the ocean had exceeded the once-in-50-years extreme heat benchmark. The researchers called the year when the percentage exceeded 50% the "tipping point" and did not fall below it in subsequent years.

But extreme heat has been particularly severe in some parts of the ocean, with the South Atlantic exceeding the point of no return in 1998. "That was 24 years ago - it's amazing," he said.

The proportion of the ocean experiencing extreme heat in some large ecosystems is now 80%-90%, and the five worst affected areas include areas off the northeast coasts of the US and Canada, off Somalia and Indonesia, and in the Norwegian Sea.

"Using this measure of extremes, we've shown that climate change is not something that is uncertain and could happen in the distant future - it's something that is a historical fact and has already happened," said Kyle Van Houtan, of the Monterey Bay Aquarium, USA, and one of the members of the research team.

Other scientists reported in 2019 that the number of heat waves affecting the planet's oceans has risen sharply, killing vast areas of marine life like "wildfires destroying huge areas of forest".

The heat content in the top 2,000 metres of the ocean set a new record in 2021, the sixth in a row. Professor John Abraham of the University of St Thomas in Minnesota, one of the team members who conducted the assessment, said that ocean heat content is most relevant to global climate, while surface temperatures are most relevant to weather models as well as many ecosystems.

"The oceans are key to understanding climate change. They cover about 70% of the planet's surface and absorb more than 90% of the global warming heat," said Abraham. "The new study is useful because researchers are looking at surface temperatures. The study concludes that there has been a large increase in extreme heat at the surface of the oceans and that extremes are increasing over time."