The surface temperature of the oceans has reached new records. According to preliminary data from the US platform “Climate Reanalyzer”, the global average has been 21.1 degrees for around two weeks – a value that has never been reached in the 40 years of recording until 2022. The temperature is thus consistently well above the usual values ​​for the month of August.

The oceans have been extraordinarily warm for almost six months now, and since March the surface of the oceans have had record temperatures for the respective month worldwide. At the beginning of April, the temperatures had already been at 21.1 degrees for several days and were thus higher than at any time since the start of the evaluation. Before that, a record 21 degrees was recorded in March 2016 and again in late March 2023.

Greenhouse gases drive up temperatures

Man-made greenhouse gases are the main reason for the increase. According to experts, more than 90 percent of the heat generated by them is absorbed by the oceans. As tiny as changes of tenths of a degree may sound, behind this is the warming of incredibly large masses of water, as Anders Levermann from the Potsdam Institute for Climate Impact Research (PIK) explains. A liter of water can absorb three thousand times more heat than a liter of air.

There are two peaks in global ocean temperature over the course of the year: one in March as southern summer draws to a close, and one in August as summer draws to a close in the north. “The south has a lot more ocean, so its summer effect usually dominates,” explains Levermann. The fact that there are such high values ​​this time in August is due to the heat in the North Atlantic, which has been unprecedented for months.

On August 1, for example, the water there was 23.6 degrees on average over the past decades – but on August 1, 2023 it was 25.0 degrees, almost one and a half degrees more. “That’s massive.” The climate phenomenon El Niño does not currently play a major direct role. “It’s just building.”

Fatal development for ecosystems

Levermann has developed a theory as to what – along with other factors such as heat waves in the atmosphere – could have caused the high North Atlantic temperatures of recent months. The Gulf Stream system has been weakening for decades due to global warming. Actually, a cooling in the North Atlantic is to be expected as a result. However, there may be a build-up of heat because one of the two cooperating conveyor belts that bring warm water flowing from the US East Coast out into the North Atlantic further north is failing.

The first band is still working, albeit to a lesser extent, but the second could be in the process of failing, explains the climate researcher about his theory. “It could be that the heat is only transported through the southern band to the south of Iceland. If the deep water formation in the far north is greatly weakened, then the heat is no longer carried and accumulates off the Spanish and French coasts, as we do experience it at the moment. That’s at least one possibility,” says Levermann.

The development is fatal for the ecosystems in the sea. “They are used to stability, much more so than land habitats.” Many of them reacted correspondingly sensitively, says the PIK researcher. This in turn has consequences for the fisheries. “There are countless food chains and networks that we’re messing with.”

Risk of extreme weather events

The additional flow change in the North Atlantic, like El Niño, also brings more heat into the atmosphere – with a further increasing risk of extreme weather events as a result, as Levermann explains. The warming brings more movement into the system, the jet stream system, which actually extends circularly around the earth, begins to bulge – which in turn causes heat waves or heavy rain.

The rate at which oceans are warming has at least tripled since the late 1980s, according to a study presented earlier this year. The amount of heat in ocean layers down to a depth of 2000 meters peaked in 2022, the research team reported in the journal Advances in Atmospheric Sciences.

2023 should bring new record values. “Until we reach carbon neutrality, the warming trend will continue, and we’ll be seeing new ocean heat records every year,” said co-author Michael Mann of the University of Pennsylvania.

Due to the heat storage in the ocean, the climate system also has a long memory, emphasizes Levermann. “We must stop burning gas, oil and especially coal, because temperatures in the atmosphere will not come down long after we stop emitting CO2.”