There is a mood of upheaval below the surface of the sea. Thousands and thousands of species migrate through the oceans in search of new habitats. Especially in the tropics, the animals flee from the heated sea water. And migration could accelerate significantly in the coming decades.
If global warming is not significantly curbed, half of all marine species could lose half their habitat each by 2100, as one recently did in the journal Global Change Biology published study shows. The research team led by Dorothee Hodapp from the Helmholtz Institute for Functional Marine Biodiversity at the University of Oldenburg examined the future distribution of more than 33,500 species under different emission scenarios. To do this, they predicted the extent to which properties such as the average water temperature or the oxygen and salinity of the current distribution areas would change and remain suitable for the respective species.
In a scenario with particularly high greenhouse gas emissions, the tropical seas will become uninhabitable for 88 percent of the animal species found there by 2100. In the low emissions scenario it is eight percent, in the middle scenario it is 24 percent. Rainer Froese from the Helmholtz Center for Ocean Research (Geomar) in Kiel assumes the worst if a change of course does not take place soon. “Meanwhile, the highest emissions scenario is also the most likely,” says the marine biologist and co-author of the study.
Sharks also migrate to colder regions. There they find easy prey
New research shows that maritime ecosystems suffer even more from climate change than those on land. Water is sluggish and shows little diurnal temperature changes. Accordingly, many marine animals are poorly adapted to large fluctuations and react sensitively. Species richness in the tropics is already declining and for that in middle latitudes. Tropical sea creatures migrate south and north in search of colder water. This not only robs coastal communities of their livelihoods, but also poses challenges to ecosystems worldwide.
According to the study, the migration movements under water lead to new species constellations and ecosystems. In the long term, these could stabilize, but the immediate invasion of new species endangers existing systems. Predatory fish, for example, open up new areas with unadapted prey. So far, sharks have only sporadically been in the cold waters of the Southern Ocean. However, the rise in sea temperature allows the predators to migrate further poleward each year. Native animal species have no protection from the unfamiliar predators. “We know this from islands with flightless birds. When cats or dogs were introduced there, it took a few years for these birds to be extinct,” says Froese.
And it’s not just the rise in temperature that is affecting the oceans. Hans-Otto Pörtner, climate researcher at the Alfred Wegener Institute and lead author of the IPCC Working Group II, speaks of a “deadly trio” in the sea: rise in temperature, shortage of oxygen and acidification. All three factors restrict the habitat of marine animals and interact in a complex manner.
Warmer water stores less oxygen. So while the water temperature increases and the metabolism and oxygen requirement of the animals increases as a result, this is exactly what is missing. This results in species-specific temperature limits. For most tropical fish, this is around 35 degrees. This is already being exceeded at times on some coasts and lagoons. For many coral ecosystems, the limit is already 29 degrees.
“We don’t know where the tipping point is, but we know it’s close.”
In addition, the oceans absorb large amounts of CO₂ from the atmosphere. The gas reacts with water and forms carbonic acid. Acidification of the water is particularly problematic for calcareous species and ecosystems such as mussels, sea urchins and corals. So far, there is little evidence that the falling pH is causing any major damage, says marine biologist Froese. But the water is getting more acidic every year. “We don’t know where the tipping point is, but we know it’s close.”
The acidification puts the marine animals under additional stress. Every species depends on a temperature window for its survival. “If the oxygen becomes scarce, this temperature window narrows and if CO₂ enrichment also takes place, it narrows even further,” explains climate expert Hans-Otto Pörtner.
How important it is to protect ecosystems and biodiversity is slowly coming to the fore in politics. At the UN Biodiversity Conference last December, the delegations decided to place 30 percent of the world’s land and sea under protection by 2030. An important milestone, says Pörtner. However, this global average value must be broken down into specific ecosystems. Putting 30 percent under protection is certainly not enough for some ecosystems to ensure their preservation, he says.
In addition to global warming, pollution and overfishing of the seas are also affecting many species. Stress and unnaturally small populations as a result of overfishing make affected species less adaptable. Overfishing in particular can be stopped quickly and easily, argues Froese. “We need to end fisheries subsidies. It doesn’t cost us anything and it would be a huge contribution to protecting our oceans and enabling their inhabitants to adapt to climate change.”