More than half of the world’s oceans are experiencing extreme temperatures that were unheard of a century ago.

Between 2013 and 2016, the Blob, a mass of hot water in the eastern Pacific Ocean, wreaked havoc on marine ecosystems. The deepest red is 3 degrees Celsius over average, indicating how unusually hot the waters were in May 2015.  JEFFREY R. HALL/PO.DAAC/JPL/CHELLE GENTEMANN, CHARLES THOMPSON
Between 2013 and 2016, the Blob, a mass of hot water in the eastern Pacific Ocean, wreaked havoc on marine ecosystems. The deepest red is 3 degrees Celsius over average, indicating how unusually hot the waters were in May 2015. 
 JEFFREY R. HALL/PO.DAAC/JPL/CHELLE GENTEMANN, CHARLES THOMPSON
The scorching ocean extremes of yesterday have become the new normal. Researchers report in PLOS Climate on February 1 that a new analysis of surface ocean temperatures over the past 150 years finds that in 2019, 57 percent of the ocean’s surface encountered temperatures rarely seen a century ago.
Marine ecologists Kisei Tanaka, now at the National Oceanic and Atmospheric Administration in Honolulu, and Kyle Van Houtan, now at the Loggerhead Marinelife Center in Juno Beach, Fla., analyzed monthly sea-surface temperatures from 1870 to 2019, mapping where and when extreme heat events occurred decade by decade to provide context for the frequency and duration of modern extreme heat events.
Rather than looking at annual averages, monthly extremes showed new markers in how the ocean is changing. The study discovered that as time passes, more and larger patches of water are exposed to harsh temperatures. The ocean then reached a “point of no return” in 2014, according to Van Houtan. At least half of the ocean’s surface waters had temperatures hotter than the most extreme episodes from 1870 to 1919 starting that year.

In hot water

A new study of monthly sea surface temperatures across the globe finds that every year, more and more parts of the ocean experience heat extremes. Since 1980, temperature fluctuations can be seen in these four decadal snapshots. More than half of the ocean began to reach extremes hotter than even the most extreme episodes between 1870 and 1919 starting in 2014.
Changes in sea surface temperatures, 1980–2019
Changes in sea surface temperatures, 1980–2019

At least five days of extremely high temperatures for a stretch of the ocean are considered a marine heatwave. Heatwaves devastate ocean ecosystems, causing seabird malnutrition, coral bleaching, kelp forest death, and fish, whales, and turtle migration to cooler seas (SN: 1/15/20; SN: 8/10/20).
NOAA said in May 2021 that it was changing its “climate normals,” which it uses to place daily weather events in historical context, from average 1981–2010 values to higher 1991–2020 averages (SN: 5/26/21).
According to Van Houtan, this study shows that ocean heat extremes are becoming the norm. “Much of the current public debate on climate change is focused on future events and whether or not they will occur,” he says. “In 2014, extreme heat became common in our seas.” It’s a known historical reality, not a hypothetical possibility.”

CITATIONS

K.R. Tanaka and K.S. Van Houtan. The recent normalization of historical marine heat extremesPLOS Climate. Published February 1, 2022. DOI: 10.1371/journal.pclm.0000007.

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