The ocean is responding to mounting human threats in surprising ways

• Overfishing, pollution, record-shattering global sea surface temperatures and increasingly common marine heat waves are putting immense pressure on the world's largest ecosystem.
• Understanding how marine life can recover — and when it can't — can help prioritize and tailor conservation efforts.
• "This is an existential question about human survival on the planet," says Ben Halpern, a marine ecologist at the University of California, Santa Barbara. "Our relationship with the ocean is fragile."

Driving the news: Currently, nearly 50% of the global oceans meet the criteria for a marine heat wave, periods of anomalously warm ocean temperatures that are higher than 90% of the previous observations for a particular date.

• This ranks as the most extensive marine heat wave coverage on record for this time of year, dating back to 1991.
• These events can profoundly affect marine life, particularly tropical coral reefs, which have a narrow temperature range. When stressed, they expel the symbiotic algae that give them their brilliant colors and allow them to nurture unique fish species, leading to coral bleaching. If exposed to high temperatures for a long duration, they can die.
• Marine heat waves can also wreck coastal fisheries, hitting national economies.
• The latest NOAA forecast calls for marine heat waves to remain near record level through February 2024, partly as a result of a burgeoning El Niño, combined with long-term climate change.

How it works: Warmer ocean temperatures from heat waves can force fish, whales and other marine life to migrate to cooler waters.

• The impact of rising temperatures can be compounded by ocean acidification — a drop in ocean pH as seawater absorbs the ever-increasing amount of carbon dioxide being released into the atmosphere by human activities.
• Warmer waters — and pollution — also create low oxygen conditions that increase marine animals' metabolism — but decrease the amount of food available to them, leading to massive fish die-offs.
• But it's unclear if marine heatwaves have a universal impact on ecosystems.

Zoom in: A new study of more than 200 marine heat waves that occurred in the oceans around North America and Europe between 1993 and 2019 found more often they didn't have a significant effect on the abundance and diversity of bottom-dwelling fish communities.

• These ecosystems support some of the world's largest fisheries, the study notes.
• "I was entirely surprised. I thought it would be a clear-cut threshold — if you have a big heat wave, you have a really big impact," says Malin Pinsky, a marine biologist at the University of California, Santa Cruz and co-author of the paper published this week in the journal Nature.

But, but, but... Some major events did cause large declines in the amount of fish — like "The Blob" of warm water that emerged at the end of 2013 in the North Pacific and persisted for more than two years, decimating pollack and other fish populations in the Gulf of Alaska. New research suggests the movement of fish due to that heat wave persists.

• Also, numerous other studies have documented the dramatic effects of marine heat waves on different species.
• The new study's design "effectively excludes kelp beds and tropical ecosystems, two major ecosystems where the authors acknowledge that marine heatwaves have already had a huge toll," marine biologist Terry Hughes of James Cook University tells Axios via email.

The takeaway: "We're rolling the dice" with each one, Pinsky says. "It might be a bad one or it might be fine."

• It also suggests a single way to manage marine heat waves may not be successful.
• "Each one is its own story, and we have to unpack them individually because we don't detect a general trend," says Alexa Fredston, an ecologist at the University of California, Santa Cruz and co-author of the new study.

The bottom line Halpern harbors hope for the ocean — more so than land.

• The connectedness of the ocean and the ability of many species to move freely makes it "much more resilient," he says.
• Pinsky echoes that sentiment. "On one level [the study] is a good news finding — not every heat wave is a disaster," he says. "The ocean does seem to have more resilience than we thought."
• "On the other hand, the more global warming continues, the more we are pushing the ocean toward tipping points we don’t know about yet."

Go deeper: A crucial year for understanding how ocean warming affects marine life