It has been another season of extreme weather, with everything from record temperatures to devastating floods hammering countries around the world. But one of the most worrying trends has been what scientists have called an unprecedented planet-wide ocean heatwave.
June saw the highest ever average global ocean surface temperatures, with local records being set from Ireland to Antarctica. In Florida, waters reached 38°C. The North Atlantic recorded its hottest ever temperature in July. And the Mediterranean Sea hit 28.7°C, an all-time high.
So, why are marine temperatures reaching unprecedented levels? Experts say it is a combination of factors, from the climate crisis to flagging winds over the Sahara Desert. They also say the record temperatures have deep implications for both ocean life and humanity.
“The effects of ocean heatwaves are varied and extremely damaging,” said Leticia Carvalho, the Head of Freshwater and Marine Ecosystems at the United Nations Environment Programme (UNEP). “We know severe temperature rises can lead to mass mortalities of sea life, increase ocean acidification and disrupt the currents that influence our weather patterns, potentially causing hundreds of millions of dollars in economic losses and posing a severe risk to global food security.”
Marine heatwaves doubled in frequency between 1982 and 2016 and have become longer and more intense since the 1980s, found a 2021 study by the UN’s Intergovernmental Panel on Climate Change.
The latest heatwave is being driven by climate change, said Carvalho. As humanity burns fossil fuels, massive amounts of planet-warming greenhouse gasses are flooding into the atmosphere. Much of that heat is being absorbed by the ocean.
This not only results in higher marine temperatures, it also makes the ocean less effective at absorbing carbon dioxide, leaving more of the greenhouse gas to percolate in the atmosphere.
The climate crisis is being compounded by the natural El Niño climate pattern now underway, which is driving up sea temperatures in the Pacific Ocean.
There are other causes of the ocean heatwave, too. Weaker-than-average winds have reduced the amount of Saharan dust in the atmosphere, which usually reduces sea temperatures by blocking some of the sun’s energy.
Hotter marine temperatures can have a devastating effect on marine life, and there have been multiple mass mortalities of marine animals and plants due to ocean heatwaves.
One example is the mass die off of Alaskan Snow Crabs in the Bering Sea, where the population dropped from approximately 11 billion to less than 2 billion in four years. As a result, the American state of Alaska closed the snow crab season for the first time in October 2022.
Heatwaves can also cause damaging algal blooms, coral bleaching, the displacement of marine species – as they search for colder water – and the disruption of food chains. UNEP estimates that 25 to 50 per cent of the world’s coral reefs have been destroyed, and that all coral reefs will be dead by 2100 if greenhouse gas emissions are not drastically cut.
Increased marine temperatures can be harmful on land too, causing extreme weather, such as storms and hurricanes.
As water temperatures rise – and average ocean temperatures have risen 1.5°C in the past century – marine ecosystems’ ability to absorb local temperature rises is reduced, further increasing the likelihood of more marine heatwaves.
“The ocean is a vital carbon sink,” Carvalho says. “It absorbs 90 per cent of the excess heat generated by carbon dioxide emissions and generates 50 per cent of the oxygen we need. It is the lungs of the planet, and regulates our climate. However, it is in serious peril and we need to immediately prioritize protection and restoration efforts.”
The worst may not be over, with America’s National Oceanic and Atmospheric Administration warning that half of the world’s oceans may experience marine heatwaves by September. And with no sign of global emissions reducing, these often catastrophic events may become far more common in the years to come.
One part of the world increasingly vulnerable to marine heatwaves is Antarctica, which plays a vital role in regulating the Earth’s climate system.
Since 1992, the Antarctic has been losing around 100 billion tonnes of ice per year, and this year saw the lowest winter ice extent on record, about 2.6 million square kilometres below the 1981-2010 average.
Less sea ice not only means possible future sea level rises, it means that there’s less ice to reflect the sun’s energy, resulting in higher sea temperatures and more frequent marine heatwaves, a vicious cycle that could have global ramifications.
“This summer really needs to be a wake-up call that the effects of climate change are being seen in all ecosystems in every part of the world,” says Carvalho. “There needs to be a reduction in emissions, but also more investment in nature-based solutions as well as more research into monitoring marine heatwaves.”
The Six-Sector Solution to the climate crisis
UNEP is at the forefront of supporting the Paris Agreement goal of keeping global temperature rise well below 2°C, and aiming for 1.5°C, compared to pre-industrial levels. To do this, UNEP has developed the Six-Sector Solution, a roadmap to reducing emissions across sectors in line with the Paris Agreement commitments and in pursuit of climate stability. The six sectors identified are: energy; industry; agriculture and food; forests and land use; transport; and buildings and cities.
