Satellites, ships and buoys monitoring the Earth have captured a significant rise in global ocean temperatures sincethe early 1980s.
The data in this chart shows daily sea surface temperature records forall of Earth's oceans, except the poles.
Recent years have seen ocean temperatures reach recordhighs, well above the long-term average.
But of all these records, nothing looks like 2023. Since March 16, the world's oceans have been running a fever. The average temperature of oceans across the globe is the highest on record, and has beenfor months.
The ocean plays an integral role in shaping our climate and weather patterns.
The warmer the ocean temperatures, the more power they give to weather systems — fuelling heavy rainfall and extreme bouts of heat by increasing evaporation and rearranging weather patterns.
CSIRO honorary fellow Dr Wenju Cai, who specialisesin global climate variability and change, says it is only a matter of time before humans feel that power.
"I think we are definitely going to see a lot of extremes in the upcoming years simply because the system is more powerful," he said.
"There is a great deal of energy in that ocean warming, and it could power so much more extreme events."
Many climate scientists believe it already has.
A look at daily sea surface temperature anomalies from around the world shows most of the northern hemisphere and much of the southern hemisphere is currently flashing red.
Georgia Institute of Technology climate scientist Annalisa Bracco said what made this year's ocean temperatures especially "peculiar" werehow widespread the heat was.
"It's been nearly everywhere," she said.
During July, sea surface temperatures across much of the Mediterranean Sea were as much as 3 degrees Celsius higher than normal, with pockets up to 5.5C above average along the coasts of Italy, Greece and North Africa.
Dr Bracco said it was "very plausible" these warm ocean temperatures aided extreme weather events in2023, although formal studies would be required to confirm it.
She said theocean heat may have played a role in the extreme heatwaves and wildfires that raged across Greece and surrounding countries during July.
"The land tends to warm up more than the ocean, but if the ocean is so warm, you essentially start having very high temperatures and dry conditionsbecause the ocean is evaporating and raining on itself," Dr Bracco said.
She said the same could be said for the intense heat baking central America, with ocean temperatures in the eastern Pacificcurrently 3-5C above normal amida developing El Niño climate pattern.
The south-westUnited States city of Phoenix endured a record 31-daystreak of temperatures above 43C during July.
In China during July, record-breaking rainfall triggered by powerful Typhoon Doksurikilled dozens andleft a trail of destruction as it stretched as far north as the Chinese capital Beijing and the surrounding province of Hebei.
Both Dr Cai and Dr Bracco believethe warm ocean temperatures likely contributed to its power.
'Anaesthetic'has worn off
Scientists believe climate change has played a clear role in the ocean temperature spike.
The difference this year, they say, is that it has been superimposed over myriad natural climate drivers that are no longer disguising its full impacts as they have in the past.
For decades, human activity has been emitting increasing levels of greenhouse gasses that have trapped more of the sun's energy in the Earth's system, warming the atmosphere, ocean and land.
Of this extra heat energy, about 90 per cent is stored in the oceans, according to the World Meteorological Organization (WMO).
Research has found once carbon dioxide is added to the atmosphere, it hangs about for 300 to 1,000 years.
Data from the National Oceanic and Atmospheric Administration (NOAA)shows a general upward trend of sea surface temperatures since 1901, which have acceleratedfrom the 1970s onwards.
But Dr Cai said the true "pain" of this energy imbalance had been masked through many individual years by a countering cooling influence from various natural climate drivers, something that was missing this year.
"It's like an anaestheticto the pain caused by global warming," he said.
"That anaesthetic is wearing out and we are now feeling it."
El Niño and La Niña effects
Dr Cai said the most significant of these so-called anaestheticswas the La Niña climate pattern, the "cool" phase of the El Niño Southern Oscillation (ENSO), which occurred for three years in a row from 2020-2022.
The major climate driver featurescooler-than-normal sea surface temperatures in the central and eastern equatorial Pacific, and warmer waters on the western side of the Pacific, in turn affecting rainfall and temperatures across the world.
The event has been linked to a significant reduction in average global surface temperatures, with the surface primarily made up of oceans.
The extra heat fostered by greenhouse gasses does not suddenly go missing during a La Niña climate pattern, it is just hidden below the sea's surface.
With the triple-dip La Niña event now finished, Dr Cai said it was no longer there to act as a buffer.
"There is no break on global warming anymore, and so global warming will power up," he said.
Instead, the world is staring down the barrel of El Niño — its opposite phase —which is known to raise global surface temperatures.
The last El Niño event was in 2015 to 2016, which also happened to be the warmest year on record for global average surface temperatures on land and sea.
But Dr Cai said the Earth this year was "eight years of background warming" worse off than in 2016.
Melting ice fostering heat
Another part of the overall story is the melting of ice, Dr Cai says.
The bright, smooth surface of the ice reflects a lot of sunlight back into space, meaning its heatis not absorbed into the ocean.
The more it melts, the hotter the oceans get, and vice versa.
For this reason, Dr Cai said ice was the "most powerful atmospheric positive feedback in our Earth's system".
Antarctic sea ice extent this year has fallen to record lows and is struggling to recover substantially during the winter months like normal.
Data shows sea ice deficitfor July was 2.1 million square kilometres below average — an area bigger than the size of Queensland.
Arcticsea ice, and Greenland and Antarctic ice sheets, are also in decline and have been for decades.
Less pollution, less 'shade'
Developed countries in the northern hemisphere have been slowly reducing their pollution for decades, as has the shipping industry in recent years.
New international regulations of sulphur particles in shipping fuels were introduced in 2020, leading to a global reduction in sulphur dioxide emissions —a health-damaging air pollutant.
University of Exeter climate change professor Mat Collins said the overall reductionhad positive impacts for human health, but it could be playing a role in increased global heat in the short term because of the reduction in aerosols that came with it.
Aerosols actlike a "shade" to incoming sunlight, reflecting it back into space.
"We've been reducing emissions, which is good for air quality, but it means now the carbon dioxide signal can punch through," Professor Collins said.
"How long that lasts depends on how quickly we reduce the pollution, so if we stop polluting, we could have a clean atmosphere within a year."
A lack of dust from the Sahara Desert — a natural aerosol —may have also played arole in the localised warming in the North Atlantic during June, according to Dr Bracco.
What's to come?
These are just some of the factors lining up this year, with many more likely to be contributing.
Dr Collins said the exact anatomy of what had caused this year to see such extremes was a "puzzle that was going to occupy scientists for a few years".
But the broader message from climate scientists isthat, even though the background warming has this year been emphasisedby natural drivers, it would not be the last year like it.
"There will be other years in the future where this happens, because there will be other years where the natural variability tips us into a warm year and that will be on top of an even warmer background trend [underclimate change]," Dr Collinssaid.
Dr Bracco said this year'srecord was indicative of a level of background global warming that could not be undone for hundreds of years, and urgent action was needed tostop it getting worse.
"It's getting worrisome,"she said.
"I hope this is a situation that gets people, politicians and administrators together to act on it."
As for this year,Dr Cai said withEl Niño in its infancy, there waslikely more heat to come and it would be harshest for those countries entering summer, including Australia.
El Niño hadalso been linked to increasedglobal mean temperatures in the following year, according to the WMO.
But climate scientists have previously said Australia will not see a mirror of what was experienced in the northern hemispherebecause of the local weather systems at play.
Reporting: Tyne Logan
Developer: Katia Shatoba
Charts and data: Mark Doman
Photographs: Reuters/Costas Baltas, Cameron Shwartz, Associated Press/Andy Wong