Extreme heat is the “new normal” in the ocean

The old story goes that if you pop a frog in a pot of boiling water, it will quite rightly leap straight back out again. But if you start with a pot of cold water and raise the temperature in barely noticeable increments, the frog will fail to register the creeping disaster and will sedately submit to being boiled alive.

While science has debunked the literal interpretation of this tale, the story holds within it a familiarity around humanity’s perception of climate change. A new study published in PLOS Climate outlines how marine temperatures that once would have been perceived as extreme have been adopted into the new normal, with increasingly frequent marine heatwaves failing to garner the widespread concern needed to address the threat that they pose to marine life and ecosystem services.

Researchers from the Monterey Bay Aquarium in California mapped 150 years of sea surface temperature data to establish a historical benchmark for marine heat extremes. They identified the most dramatic ocean warming that occurred over the period spanning 1870 to 1919, defining the top two percent of increases as “extreme heat”.  They then looked at how often the world’s oceans surpassed this point.

The picture that emerged from their analysis was worrying, with heat trends worsening through time.

The first year on record in which more than half of the ocean experienced heat extremes was 2014. By 2019, this had increased to 57% of the ocean. In comparison, just 2% of the ocean was experiencing extremely warm temperatures at the end of the 19th century.

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Fraction of the ocean surface annually experiencing extreme heat, grouped by a, Northern Hemisphere and b, Southern Hemisphere and Indian ocean basins. The Point of No Return (PoNR) occurs when each series surpasses and remains above 50% (dashed grey line), or when the historical baseline of extreme heat becomes ‘normal’. This first occurs in 1998 in the South Atlantic basin and for the global ocean occurs in 2014. Credit: Tanaka and Van Houtan.

The researchers believe this is a clear indicator of the real and immediate threat posed by global warming.

“Climate change is not a future event,” says Dr Kyle Van Houtan, who headed the research team during his tenure as chief scientist for the aquarium. “The reality is that it’s been affecting us for a while. Our research shows that for the last seven years more than half of the ocean has experienced extreme heat.

“These dramatic changes we’ve recorded in the ocean are yet another piece of evidence that should be a wake-up call to act on climate change,” he adds. “We are experiencing it now, and it is speeding up.”

Van Houtan believes that, much like the fabled frog in the slowly boiling pot, we are collectively failing to register the peril of this warming trend, which can only be curbed by a drastic reduction of emissions from the burning of fossil fuels.

“Today, the majority of the ocean’s surface has warmed to temperatures that only a century ago occurred as rare, once-in-50-year extreme warming events,” he says.

The implications of this warming are likely to be far-reaching.

“When marine ecosystems near the tropics experience intolerably high temperatures, key organisms such as corals, seagrass meadows, or kelp forests can collapse,” Van Houtan explains.

“Altering ecosystem structure and function threatens their capacity to provide life-sustaining services to human communities like supporting healthy and sustainable fisheries, buffering low-lying coastal regions from extreme weather events, and serving as a carbon sink to store the excess carbon put in the atmosphere from human-generated greenhouse emissions.”

The question remains, how hot will the water have to get for us to fear the change?

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