In March last year the temperature curve rose, every day was a new heat record in the North Atlantic. Its average surface temperature was at its highest level since measurements began around 40 years ago – usually well ahead of the previous daily record. This emerges from data from the “Climate Reanalyzer” platform at the University of Maine, which is based, among other things, on satellite measurements. A week later, on March 14, daily record temperatures in the world’s oceans also began to rise. “If you look at what the temperature trends in the oceans have been over the other 40 years, you can see that the current warming is really far outside of natural fluctuations,” said Anders Levermann from the Potsdam Institute for Climate Impact Research to the DPA news agency. “It is also outside of what we have observed in terms of warming in recent years and decades.” In addition to the constant man-made warming, there must be other dynamic effects, explained Levermann. “How many of these are actually caused or aggravated by global warming is not yet certain.” For example, the climate phenomenon El Niño is currently pumping heat up from the ocean depths in the Pacific.
Mojib Latif from the Geomar Helmholtz Center for Ocean Research in Kiel also assumes that El Niño has increased the warming – previously the opposite phenomenon La Niña had rather dampened the warming. However, both emphasize the effect of man-made climate change: “The main reason why the oceans are so warm is of course humans. That should not be forgotten,” said Latif.
Latif is not surprised by the records. “Oceans are a damn good indicator of global warming,” he told dpa. The oceans absorb over 90 percent of the heat that remains in the atmosphere due to the increase in greenhouse gases. “It is abundantly clear that the oceans will warm as the Earth continues to warm.”
“What is currently causing a certain amount of astonishment is this particularly warm Atlantic,” explained Helge Gößling, climate physicist at the Alfred Wegener Institute in Bremerhaven. “By the way, the South Atlantic has now been added. That too has been unusually warm since around December.” This needs to be investigated in more detail.
The future course of the temperature curve will be interesting, Gößling told the DPA. “The forecasts actually all assume that El Niño will disappear over the course of the spring and probably even reverse into its counterpart La Niña later.” La Niña is the most likely situation from July or August. The World Weather Organization (WMO) has already registered a weakening of El Niño. However, the effects on the global climate continue to be felt.
According to Gößling, there are various possible explanations for the warming of the North Atlantic. Ship emissions have been subject to stricter regulations since 2020. “This means that the sulfur compounds that are emitted have now been reduced.” The compounds have a cooling effect on the climate. However, it is unlikely that this can explain the entire anomaly in the Atlantic. According to Gößling, weak trade winds in the spring of 2023, which were blamed for its warming, are also excluded as an explanation: This development should have subsided over the course of the winter, which did not happen.
“So this anomaly that we are now seeing globally, and that we are seeing above all in the North Atlantic, I still have no idea how we can really explain it,” emphasized Levermann. “This is indeed extraordinary.”
According to Gößling, in addition to the high values in the oceans, there are other factors that are currently affecting global air temperature: At the beginning of 2022, an underwater volcano erupted, sending large amounts of water vapor into the stratosphere. Roughly estimated, this contributes to a twentieth of a degree of warming. In addition, the sun’s radiation fluctuates in a cycle of eleven years. Since it is currently on the way to a strong phase, another twentieth of a degree could be added. “I’m talking about small effects, but they can add up.”
However, he is also certain that “climate change caused by man-made greenhouse gases is the main cause in the long term.” According to the World Weather Organization, the concentrations of the main greenhouse gases carbon dioxide, methane and nitrous oxide in the atmosphere reached record levels in 2022 since records began. According to Gößling, ocean data clearly shows a relatively continuous increase in the amount of heat absorbed by the oceans.
“We have observed a global average of 1.2 degrees of warming and the continents have already warmed on average by more than two degrees,” said Levermann. As the oceans warm, the water in them expands. Together with the melting ice, this is causing sea levels to rise more and more quickly, says Levermann: “At the beginning of the last century we had sea level rises of around one centimeter per decade, at the beginning of this century around three and now around five.”
The expert emphasized that warming also has fatal consequences for marine ecosystems. The warming of the oceans is disrupting countless food chains and networks. “This has consequences not only for marine life, but also for fishing and therefore also for people’s nutrition.” More destructive extreme weather events are also expected: Ocean scientist Latif pointed out that heavy rain could become more frequent because more water evaporates and warmer air can hold more water vapor, which eventually comes down as precipitation.
