Scientists alarmed by extreme marine heat wave in North Atlantic

The ocean waters surrounding the United Kingdom and much of Europe are baking in an unprecedented marine heat wave that scientists say is being intensified by human-caused climate change. Scientists are astounded not only by how much the waters have warmed during the past month but also how early in the year the heat wave is occurring. The warm waters are a threat to marine life and could worsen heat waves over land this summer, they say.

Sea surface temperatures are running as high as 5 degrees Celsius (9 degrees Fahrenheit) above normal, the warmest in more than 170 years, and are more typical of August and September when the waters are usually warmest. The event has registered as a Category 4 on the National Oceanic and Atmospheric Administration’s marine heat wave scale with localized areas reaching Category 5, the two highest categories on the scale.

NOAA defines a marine heat wave as a period with persistent and unusually warm ocean temperatures, “which can have significant impacts on marine life as well as coastal communities and economies.” The agency describes Category 4 as “extreme” and Category 5 as “beyond extreme.”

Last month was the warmest May since 1850 for the Atlantic Ocean around the United Kingdom and the warmest compared to average for any month, the country’s Met Office reported. And that was before water temperatures soared in early June, in part because of abundant sunshine and warm breezes from the southwest, Met Office meteorologist Aidan McGivern said in a video update Tuesday.

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The average sea surface temperature near the United Kingdom and Ireland is closing in on 15 degrees Celsius (59 degrees Fahrenheit), which has only happened once before in June, tweeted Ben Noll, a meteorologist at the National Institute of Water and Atmospheric Research in New Zealand. “It seems very likely that June 2023 will set a new record in the region as sea temperatures continue to rise,” he said.

Record ocean warmth spans the globe

The North Atlantic heat wave is part of a rapid warming of ocean waters globally since March that has scientists confused about the cause and concerned about its impacts.

Global ocean surface temperatures reached a record high in May for the second consecutive month, NOAA said in a report last week, and appears to have continued on a record pace during June. The chance of seeing such warm sea surface temperatures is 1-in-256,000, said Brian McNoldy, a hurricane researcher at the University of Miami, adding “this is beyond extraordinary” in a recent tweet.

NOAA forecasters say the marine heat wave conditions in the North Atlantic have a 90 to 100 percent chance to continue through August and a 70 to 80 percent chance to last through the end of the year, although the intensity of the heat is predicted to decrease. Most of the world’s oceans have at least a 70 percent chance of marine heat wave conditions continuing at least through the summer, NOAA predicts.

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The unusually warm waters in the tropical Atlantic are already influencing the hurricane season, having helped Tropical Storm Bret form the farthest east of any storm on record so early in the season. Longer-term impacts of warming oceans could include higher sea levels, more intense storms with heavier rain and more frequent regional marine heat waves like the one surrounding Europe now.

Possible causes range from cleaner air to climate change

What’s causing such extreme ocean warmth is debatable.

Some scientists have pointed to a reduction in air pollution from ships, which starting in 2020 were required to use fuel containing less sulfur. Sulfur degrades air quality but also cools the Earth’s surface by reflecting sunlight back into space.

Other warming influences might include a weaker-than-normal area of high pressure in the North Atlantic, weaker winds carrying less sun-blocking Saharan dust into the Atlantic and a developing El Nino, which tends to warm ocean waters in certain areas. In addition to natural variation in weather and ocean patterns, scientists say that human-caused climate change has increased the chances of heat waves both on land and in the oceans.

“Every spike in the variability between warmer and cooler events tends to be greater than the previous one,” Thomas Smith, a professor of environmental geography at the London School of Economics and Political Science, said in an email. “That underlying trend is caused by fossil fuel greenhouse gas emissions.”

Potential impacts on marine life and local weather

Experts say while the impacts on marine life depend on how long the heat wave persists, they could be deadly because warming waters deplete the oxygen that marine animals need to survive.

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“The temperatures are not yet lethal for most sensitive species, although they will be stressed,” Smith said. “However, if temperatures remain at 4 to 5 degrees Celsius above normal through to September, we could witness a significant die-off in critical species for the marine ecosystems that surround the UK, such as kelp and seagrass, as well as oysters and various fish species that are important for regional economies.”

Potential weather impacts for the United Kingdom and elsewhere in Europe this summer and beyond range from heat waves to higher chances for heavy rain.

Smith notes that because the United Kingdom and Ireland are surrounded by an unusually warm North Atlantic Ocean to the west and an equally warm North Sea to the east, “whichever way the winds blow, they will pass over warmer waters than we’ve ever experienced in the observational record for this time of year.”

That could lead to “a more turbulent atmosphere, and the associated storms and heavy rainfall,” Smith said. “If the winds don’t blow and we sit under a high pressure system, the surrounding heat has the potential to form a heat dome that might exacerbate summer heatwaves.”

Heat domes are sprawling zones of high pressure that trap heat and can lead to extreme and extended heat waves, such as the one that has scorched Texas since last week. Summer is already off to an abnormally warm start in Europe, and temperatures are forecast to remain well above normal through at least much of the next week.


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