Extreme heat in the ocean is out of control

Ocean depth plays an important role in how regions are affected. While the depths of the central Pacific allow cooler waters to rise, the shallower areas around tropical islands do not receive such relief. Island nations face an additional risk because the water gets hotter and hotter, a phenomenon known as thermal expansion. “The same mass of water takes up more volume, so that’s it — sea level rises,” Rudnick said.

In fact, about half of the human-caused sea level rise comes from melting glacier, the other half comes from warmer waters and just takes up more space.But more localized, almost all regional variability Sea level rise is due to thermal expansion, Rudnick added. The hotter the coastal water, the more sea level rises. This can happen rapidly during extreme heat events, while sea level rise from ice melt occurs at a more glacial rate.

The ecological consequences of rising temperatures—both overall warming and peaks caused by extreme heat—are both obvious and subtle. Species with escape capabilities, such as fish, are moving towards the poles. “Lobster, for example. We’ve seen a huge shift in the geographic distribution of fisheries near the Northeast U.S.,” Van Houtan said. “New York and New Jersey used to have fisheries, but they basically didn’t exist. Now Maine is booming, but 10 years from now Maine may be behind, it may just be Canadian fisheries moving forward.” Likewise, with the entire Subsistence fishermen in the tropics may lose their livelihoods due to the migration of fish stocks.

But species that are anchored in place, such as sponges and corals, cannot move their legs (or fins) to cooler areas. “Fixed heats may see absolute heat thresholds where they can’t coexist, so you’re seeing a dip in those heats,” Van Houtan said.

Extreme heat poses additional dangers, even for species that are already gradually migrating to cooler regions. “When you have a sudden event like a marine heatwave, they don’t give any time to adapt,” Seggs said. “So they cause very high mortality. This happens in ecosystems from coral reefs to kelp forests, and they can kill seabirds.”

“It can take years for the system to recover,” Siggs added, “because if you have a lot of mortality, it doesn’t necessarily go back to normal right away.”

To complicate matters, these extreme high temperatures often coincide with low winds. Wind plays an important role in the marine food chain because it mixes water and carries nutrients from the depths. Tiny photosynthetic organisms called phytoplankton depend on these nutrients, just as garden plants depend on fertilizers. These phytoplankton feed animal species called zooplankton, which feed fish, and feed marine mammals and seabirds. Then, exposing the phytoplankton to extreme heat attacks the bottom of the food web.