Maps of unusually warm sea surface temperatures are awash in reds and oranges this summer, illustrating a series of marine heat waves across much of the globe and raising fears for what the rest of the year could bring as the heat persists and even intensifies.
Roughly 40% of the world’s oceans are experiencing marine heat waves, the most since satellite tracking started in 1991, according to the National Oceanographic and Atmospheric Administration.
By September, that number is projected to climb to 50%, a number that is “kind of scary,” said Dillon Amaya, a research scientist with NOAA’s Physical Sciences Laboratory. The heat waves could linger through the end of the year.
The spike in ocean temperatures – fueled in part by global climate change – has set off alarms among scientists because of the devastating impact marine heat waves can have on ocean ecosystems, including fish and other marine life. The warmth could help usher in the hottest year on record this year or next and spin up more tropical cyclones than originally forecast in the Atlantic hurricane basin.
In the Antarctic meanwhile, sea ice coverage reached a record low for early winter in the Southern Hemisphere in June, international organizations including NOAA reported. Sea ice extent on June 27 was nearly 1 million square miles lower than the 1981-2010 average and nearly a half-million square miles below the previous lowest extent for the month, set in June 2022.
How hot is the ocean?
Sea surface temperatures in many areas outside the polar regions have been warmer than normal since March. In April and May, they were highest on record for those months in a series of data dating back to 1850, said the United Kingdom’s Met Office.
Anomalies in sea surface temperatures – the difference between the actual temperature and the average temperature – were a record high in May. In the eastern Atlantic, temperatures have been about 1.8 degrees higher than the 1961-1990 average, said Phil Klotzbach, a senior research scientist at Colorado State University.
What is a marine heat wave and why are they a concern?
A spike in a region’s ocean surface temperatures that ranks among the top 10% warmest for that time period in that region when compared with the 1991-2020 average. It could last from several days to months.
Normally, only about 10% of the world’s oceans would be warm enough to meet the criteria for a marine heat wave, Amaya said.
NOAA and the United Nations Intergovernmental Panel on Climate Change say marine heat waves have increased in frequency and become more severe in recent decades.
“We know that marine heat waves are getting warmer and warmer as a result of global warming,” Amaya said.
Heat waves can disrupt ecosystems, kill fish and other marine life and bleach corals. A massive heat wave off the U.S. West Coast – from Alaska to Mexico’s Baja California Peninsula – in 2014-2016, dubbed “the blob,” caused:
What is NOAA’s marine heat wave forecast and what does it show?
NOAA developed a system to forecast marine heat waves based on 30 years of satellite data, and is releasing a monthly experimental forecast. In the tropical Atlantic, the forecast shows marine heat wave conditions are likely to linger through August and could linger through the end of the year.
Heat wave conditions are forecast to persist through November in the Northeast Pacific. The forecast projects, with medium confidence, that marine heat wave conditions will form by next spring along the U.S. West Coast, a region where the forecast has the best skill at predicting long-term conditions, Amaya said. Along the equator in the Pacific, where El Niño is developing, heat wave conditions are forecast to persist and intensify.
What’s driving the heat wave in the Atlantic and what does it mean for hurricanes?
Overall, the world’s oceans are warming because they’ve absorbed the majority of the excess warming caused by an increase in greenhouse gas emissions over decades, according to NOAA. Global ocean temperatures were a record high in April and May.
However, natural climate patterns also influence temperatures. The record heat in the Atlantic this spring and summer may be attributed to a weakening in a high pressure system that expands and contracts over the North Atlantic, known as the Bermuda high or the Azores high, said Klotzbach and Amaya.
The high pressure controls the strength of the trade winds. When weaker winds blow over the ocean, it decreases evaporation at the surface and allows ocean temperatures to warm rapidly, Amaya said.
Also partly to blame may be a decrease in seasonal Saharan dust blowing over the Atlantic, which has allowed more sunlight to reach the water, said Albert Klein Tank, head of the Met Office’s Hadley Centre.
Sea surface temperatures in the Eastern Atlantic, where hurricanes are often born, are tracking well ahead of previous hyperactive hurricane seasons, Klotzbach said. And typically, warmer ocean temperatures in the Atlantic more fuel hurricanes.
However, El Niño usually creates vertical wind shear over the Atlantic, preventing hurricanes from building the tall, cloud formations that help give them their structure and wind speeds.
“It’s certainly a tug-of-war this year between a likely moderate/strong El Nino and the warmest Atlantic on record,” Klotzbach said.
What’s happening in the Antarctic and what does it mean?
It’s not unusual for sea ice extent to vary, but the current low in the Antarctic is unusual, NOAA said. Sea ice has set record lows since April.
While scientists know a lot about sea ice in the Arctic and its link to the warming climate, trends in Antarctic sea ice are more difficult to decipher, said Zach Labe, a postdoctoral research associate at Princeton University and NOAA’s geophysical fluid dynamics laboratory.
“It’s likely the current record low sea ice (in the Antarctic) is related to feedbacks from the atmosphere and ocean,” Lalbe said. For example, strong winds associated with storm systems can reduce sea ice growth, but areas of unusual ocean heat in the Southern Ocean also can reduce the formation of sea ice.
From the time satellite observations of the Antarctic started in 1979, until 2015, the observations showed sea ice extent increasing very slightly. A rapid change started in 2016, and since then sea ice has been mostly below the 1981-2010 average, NOAA reported.
The sea ice reached a record low at the end of the Southern Hemisphere’s summer in 2022, Labe said. That record was broken again in February 2023. Now, with winter just beginning, sea ice is expanding at a much slower rate than normal.
