Scientists say the increase in greenhouse gases in the atmosphere has caused the oceans to trap more energy from the sun, absorbing more heat and causing surface temperatures to rise.
As the world works to avert the effects of ocean warming, misinformed social media users are claiming that the oceans are warming because of heat coming from the ocean floor, and that surface temperatures and CO2 cannot explain it.
The claim was posted by a Twitter user on 10 May 2023, who said: "The oceans are apparently warming though.... but the heat is coming from the bottom of the ocean. Surface temperatures, let alone atmospheric CO2, cannot explain it. We have little idea of how or what relative factors are driving the temperatures."
The first claim, that the heat is coming from the ocean floor, is incorrect.
However, scientists have recorded marine heatwaves at the bottom of the ocean, which they attribute to increased greenhouse gas emissions and "prolonged periods of extreme ocean warming".
According to the International Union for Conservation of Nature, "extended periods of extreme ocean warming have increased in frequency by 50 percent in the last 10 years and are becoming more intense".
"The ocean is absorbing large amounts of heat as a result of increased concentrations of greenhouse gases in the atmosphere, mainly from the use of fossil fuels. The Intergovernmental Panel on Climate Change's (IPCC) Fifth Assessment Report, published in 2013, found that the ocean has absorbed more than 93 percent of the excess heat from greenhouse gas emissions since the 1970s. This is causing ocean temperatures to rise," says IUCN.
"The distribution of excess heat in the ocean is not uniform, with the greatest ocean warming occurring in the Southern Hemisphere and contributing to the sub-surface melting of Antarctic ice shelves. The ocean's ability to absorb excess heat has protected humans from even more rapid climate change. Without this oceanic buffer, global temperatures would have risen much more than they have," the IUCN adds.
The second claim, that atmospheric CO2 is not responsible, is false. The Union of Concerned Scientists (UCS) states that the ocean does absorb C02, and in fact has absorbed almost 29 per cent of global carbon dioxide emissions since the end of the pre-industrial era.
"When carbon dioxide enters the ocean, it dissolves in salt water. First it forms carbonic acid. Then this carbonic acid breaks down to form bicarbonate ions and hydrogen ions. Ocean acidification results from an increase in the concentration of hydrogen ions and a decrease in carbonate ions due to the absorption of increased amounts of carbon dioxide," says UCS.
"Over the past 150 years, humans have greatly increased the amount of carbon dioxide in the atmosphere by burning fossil fuels and changing land use. This has had a significant impact on the ocean. Surface waters are now 30 per cent more acidic than they were at the start of the industrial era. Ocean acidification is now occurring at a faster rate than at any time in the last 66 million years, and possibly in the last 300 million years. Projections show that by the end of this century, ocean surface waters could be more than twice as acidic as they were at the end of the last century if we do not reduce our carbon emissions," adds UCS.
According to Climate.gov, "more than 90 percent of the Earth's warming over the past 50 years has occurred in the ocean".
"Recent studies estimate that warming of the upper oceans accounts for about 63 percent of the total increase in the amount of heat stored in the climate system from 1971 to 2010, and warming from 700 metres down to the seafloor adds about another 30 percent," it says.
"In the present, warming ocean water is raising global sea levels because water expands as it warms. Combined with water from melting glaciers on land, the rising sea threatens natural ecosystems and human structures near coastlines around the world. Warming ocean waters are also linked to the thinning of ice shelves and sea ice, both of which have further consequences for the Earth's climate system, threatening marine ecosystems and human livelihoods.
According to the Woods Hole Oceanographic Institution (WHOI), ocean warming has also exacerbated some extreme weather events by intensifying storms and changing global weather patterns.
Higher sea surface temperatures increase evaporation, adding extra moisture to the atmosphere over the oceans. This extra water vapour increases the precipitation that rainstorms and blizzards bring to coastal and inland areas. While wetter areas will experience more precipitation, the world's drier regions are likely to become drier. These drier conditions will cause areas such as the southwestern United States to experience prolonged droughts and increased wildfire risk," WHOI adds.
This fact check was produced by Nation with support from Code for Africa's Pesa Check, the International Fact-Checking Network and the African Fact-Checking Alliance Network.