Brett Monroe Garner
In recent years, more than half of Earth’s oceans have surpassed extreme heat thresholds on a regular basis, a trend linked to human-caused climate change that has sobering implications for marine ecosystems and the many species that rely on them, including our own.
Scientists have been tracking the rise of marine heat waves, which are short-lived pulses of intense heat in specific regions, but less is known about the general long-term pattern of extreme marine heat across the entire ocean surface. To bridge this knowledge gap, a pair of marine ecologists built a map of sea surface temperatures that dates back 150 years, which revealed that a “new normal” of extreme heat in the global oceans began in 2014, according to a study published on Tuesday in PLOS Climate.
The results are a “wake-up call,” said study co-author Kyle Van Houtan, who conducted the research while he was chief scientist at Monterey Bay Aquarium, in a call.
“You cannot overestimate the stabilizing and significant force of the ocean in making our planet habitable for all of our lives,” added Van Houtan, who is now president and CEO of Loggerhead Marinelife Center in Florida. “It's telling us, right now, that it is out of balance. It is extremely hot.”
Van Houtan and first author Kisei Tanaka, who is a research marine biologist at the National Oceanic and Atmospheric Administration (NOAA) in Hawaii, were inspired to develop the new study at Monterey Bay Aquarium after examining the decline of kelp forests off the coast of California. While combing through historical records of these marine forests, the team realized that there was no coordinated index or map of extreme heat data, based on sea surface temperatures, that extended back in time to the late 19th century.
To create this comprehensive historical benchmark, Tanaka and Van Houtan drew on two datasets that date to 1870. The researchers reviewed temperatures logged between 1870 and 1919, and recorded what the peak temperature was for each month to create a map of extreme heat during that 50-year period.
“These are rigorous contemporary indices,” said Takana in an email. “We didn't make these indices, but we apply them in a unique way. We used the 1870-1919 baseline to highlight the centennial-scale of these global datasets, but anyone can come up with different baselines to define their new normals.”
The historical baseline that the team established showed that extreme surface temperatures occurred in about two percent of the world’s ocean around the turn of the 20th century. By contrast, more than 50 percent of the global ocean have crossed this extreme heat threshold every consecutive year since 2014—with the number reaching 57 percent in 2019, the last year covered in the study—marking what Tanaka and Van Houtan called “a point of no return.”
Moreover, the team discovered that some regions have long since crossed this point where extreme heat is a new normal: For instance, the South Atlantic reached this threshold in 1998 and the Indian Ocean crossed it in 2007.
“This is important because if you take a step back and you look at the public discourse on climate change, it's about future uncertainty,” Van Houtan said. “What our study finds is this is not some hypothetical future event or phenomena that may or may not occur. This is something that's a historical fact. It's already happened. We're living in an extremely hot world in the ocean as of 2014, and in some regions of the ocean, a couple of decades earlier.”
Given the alarming acceleration of extreme heat in such huge swaths of the ocean, it’s not surprising that marine ecosystems are struggling to adjust to a much hotter environment. In some cases, rising temperatures are driving migrating species to new territories in cooler waters at higher latitudes. Other vital ecosystems, such as coral reefs, are dying off as temperatures exceed their survival thresholds, causing once-vibrant biological hotspots to become inhospitable to many species.
“All organisms have a physiological limit,” Van Houtan said. “They have constraints, and extremes hit on those constraints. The more often they occur, the more constrained organisms and species—plants and animals in the ocean and on land—are going to be adversely affected. And that's what we're seeing.”
This mass disruption of marine environments is, of course, a troubling trend in its own right, but these changes also have major implications for human lives and livelihoods. Tanaka and Van Houtan found that extreme heat is intruding into many ocean regions that are economically important, which will restrict production from fisheries and other industries. Tanaka has already had a front-row seat to one of these disruptions, during his PhD at the University of Maine.
“The Gulf of Maine was experiencing one of the fastest-warming rates globally, and lobster was the biggest fishery in the US by value,” he said. “Naturally, I did my dissertation on climate change and US lobster fisheries.”
“The 2012 Gulf of Maine heatwave gave us a really eye-opening case study of how climate-driven changes in the commercially valuable species biogeography threw off the entire lobster fisheries system,” Tanaka continued. “This is just one case study, but we can expect similar outcomes from future extreme heat events.”
In this way, the new study adds to the overwhelming evidence that human-driven climate change is not only wreaking havoc on marine species, it is also a major threat to human societies across the globe.
“Any discussion of climate change that doesn't begin and end with the ocean probably needs to be reframed,” Van Houtan said. “It's 97 percent of the water on our planet. It generates the primary source of protein for two to three billion people every day. It provides medicine and good stable jobs.”
“It is the beating heart of our climate system, and it needs to be at the forefront of any climate discussion,” he concluded.
Update: This article has been updated to include comments from study lead author Kisei Tanaka.