Marine heat: Heat waves have become normal for more than half of our oceans. The temperature of these sea areas is now permanently in ranges that were considered rare heat extremes there just 100 years ago, as researchers have determined. This marine heat is particularly pronounced in the Indian Ocean, the South Atlantic and the Arctic Ocean. This poses an acute threat to many marine ecosystems.
It is not surprising that the oceans are becoming warmer as a result of climate change. As an important climate buffer, they absorb a large part of the heat caused by anthropogenic climate change. As a result, global sea temperatures have repeatedly reached record values for several years. But marine heat waves are also increasing, causing severe coral bleaching and other mass extinctions.
Marine heat extremes in focus
Kisei Tanaka from the Monterey Bay Aquarium in California and Kyle Van Houtan from Duke University have now examined in more detail how frequent the extreme heat phases in the oceans have become. To do this, they first evaluated historical measurement data to determine the most extreme temperature values for marine areas worldwide between 1870 and 1919. The extreme values were considered to be the top two percent of the measured values.
In the second step, the researchers used the measurement data from 1920 to 2019 to examine the long-term trend for these extreme values. They determined when and how often these heat limits were exceeded for each of the seven seas and for each sub-region right down to national territorial waters. “We have also determined from which year at least half of a sea has remained permanently above these limits,” explains the team.
Former extremes are now the norm
The result: Temperatures that were once considered rare extremes are now the new normal across much of the ocean—they have become the norm. “57 percent of the global sea surface now has temperatures that were considered rare heat events that only occur once every 50 years more than a hundred years ago,” reports Van Houtan. In many economically and ecologically important marine areas, temperatures have exceeded their historical limits.
The first year all oceans crossed this threshold was 2014, the researchers determined. In some marine areas, however, the former extreme temperatures became the norm much earlier – in the South Atlantic since 1998 and in the Indian Ocean since 2007. To this day, they are the hardest hit by warming. But cooler sea areas such as the Barents Sea and the sea area off Norway now have temperatures that they once only reached during extreme heat waves.
“Beyond the limits of tolerance”
According to the researchers, these results demonstrate once again how much climate change is affecting the oceans in particular. “These drastic changes are another wake-up call,” says Van Houtan. “This change is happening now and is accelerating more and more.” The increasing “hot flashes” in the oceans are problematic, especially for the marine ecosystems.
“This condition and further increases in heat extremes could push many ecosystems beyond their tolerance limits,” warns the scientist. As early as 2021, a study showed that up to 95 percent of today’s marine climate niches could disappear by the year 2100.
Corals are losing their sanctuaries
Another current study shows what this means in concrete terms for sea creatures. In it, a team led by Adele Dixon from the University of Leeds investigated how warming oceans affect coral thermal refugia – areas where cooler currents and other balancing factors buffer ocean warming. Currently, 84 percent of the world’s coral reefs are still in such refuges.
But that will soon change: Even with global warming of 1.5 degrees compared to pre-industrial conditions, only 0.2 percent of coral reefs worldwide will have enough recovery time between the heat phases, as the team determined. 90.6 percent of reefs will experience heat stress that they cannot tolerate. At two degrees warming, even the last thermal refuges would disappear.
Humans are also affected
But humans would also be affected by these changes: “If the structure of such marine ecosystems changes, important ecological services also suffer,” explains Van Houtan. These include protecting coastal areas from high tides and waves, producing fish and seafood for food and acting as a carbon sink and climate buffer. (PLOS Climate, 2022; doi: 10.1371/journal.pclm.0000007; doi: 10.1371/journal.pclm.0000004)
Quelle: Monterey Bay Aquarium