North Pacific marine zones are a hotbed for marine heat waves due to climate change.

Systematic warming pool: Scientists have found a three million square kilometer sea area in the North Pacific that has developed into a breeding ground for marine heat waves.

While extreme heat waves are 4.5 times more often, three times more powerful, and nine times longer than everywhere else, the ocean has warmed up here more than the norm.

The research has established that 99 percent of this is attributable to human-caused climate change.

Despite their role as a climate system buffer, the oceans are now feeling the effects of climate change: Nearly every year, sea temperatures set new records, and maritime heat waves are happening more frequently and lasting longer. These heat waves are especially noticeable in the Northeast Pacific, where there have been numerous large zones of abnormally warm water in recent years that lasted up to three years.

According to Armineh Barkhordarian of the University of Hamburg and colleagues, these blob-like temperature anomalies have disastrous repercussions on marine ecosystems. 

Ocean productivity plummeted, huge seabird extinctions occurred, and strong algae bloom produced hazardous toxins. 

The first heat wave started in 2014 and lasted 600 days; the second one lasted three years, from 2019 to 2021.

What’s the cause?

But what is causing these anomalies? Why do they consistently occur in the Northeast Pacific? The initial heat wave, at least, may have been closely related to the Pacific climate phenomena El Nino, according to measurement data. However, it is still unknown whether anthropogenic climate change is also involved. This was clarified by an attribution analysis that Barkhordarian and colleagues carried out.

The researchers reproduced the occurrences and associated circumstances in two distinct coupled ocean climate models for their study. In all cases, simulations were run dozens of times with the same fundamental physical characteristics, but with varied greenhouse gas concentrations and global temperatures. By comparing them to the real values that have been measured, it is possible to figure out how much of what has been seen is due to climate change.

The researchers also used historical measurement data to look at changes in marine heat wave frequency, intensity, and duration from 1982 to the present.

99% of climate change is human-induced

The end result: Of the 40 marine heat waves that have occurred in the Northeast Pacific since 1898, 31 of them have happened in the past 20 years. In comparison to the first half of the research period, they were 4.5 times more frequent, nine times longer, and three times more intense. As reported by the authors of the study, there have been three such instances since 2014 alone, with sea temperatures in this particular region being five to six degrees higher than average.

Yet why? The team claims that the result of the attribution analysis was that “increased anthropogenic greenhouse-gas emissions were directly responsible for the extreme event. The probability of such a heatwave arising without human influences is less than one percent; there is a 99-percent probability that increased greenhouse-gas emissions were also required.”

In other words, the growing frequency of heat waves is 99% caused by climate change.

The newly found heat pool is always warmer

Additionally, it became clear that the Northeast Pacific is experiencing an increase in heat waves for no random reason. The research team also found a long-term anomaly in this area: the seawater is warmer than usual even outside of heat waves in a marine area of about three million square kilometers. Winters are eleven days shorter and summers are 38 days longer than average in this region, which has warmed by an average of 0.4 degrees each decade.

Natural temperature variations also cannot account for this Pacific heat pool entirely. As the researchers report, it probably wouldn’t exist in the absence of anthropogenic climate change. At the same time, this ongoing anomaly explains why marine heat waves are more common in this region of the Northeast Pacific.

According to Barkhordarian, “The discovery of the long-term warming pool will now provide us with crucial information on the likelihood of such extreme events in the future.”

Future heatwave hotspot

The researchers predict that the heat pool they found in the Pacific would one day serve as a “hotbed” for marine heat waves.  

Additionally, “this not only poses a tremendous threat to biodiversity; it can also push these marine ecosystems past a tipping point, after which they can no longer recover.”

Image Credit: Barkhordarian et al.

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