
CHENNAI: Say ‘global warming’ and most of us would think of wildfires, droughts or glaciers melting. Seas and oceans are rarely the first things that come to mind.
But, as NASA puts it: 90% of global warming is occurring in the oceans. They absorb about 90% of the heat trapped in the Earth’s atmosphere by greenhouse gases. That is because water has a high “specific heat capacity”, which means it can absorb a lot of heat without getting too hot.
While global sea surface temperatures have risen significantly over the past several decades, the rate of increase in warming has not been uniform. A new study has found that the sea surface temperatures have risen more sharply in recent years compared to the 1980s.
The study by researchers from the University of Reading showed that ocean temperatures rose at a pace of 0.27°C per decade during 2019-2023, compared to 0.06°C per decade during 1985-1989.
“Global mean sea surface temperature (GMSST) is a fundamental diagnostic of ongoing climate change, yet there is incomplete understanding of multi-decadal changes in warming rates and year-to-year variability,” the researchers wrote.
The study, titled Quantifying the Acceleration of Multidecadal Global Sea Surface Warming Driven by Earth's Energy Imbalance, was published in the journal Environmental Research Letters.
Explaining the study’s core finding through an interesting analogy, Christopher Merchant, its lead author, said: “If the oceans were a bathtub of water, then in the 1980s, the hot tap was running slowly, warming up the water by just a fraction of a degree each decade. But now the hot tap is running much faster, and the warming has picked up speed.”
Global ocean temperatures are known to have hit record highs for 450 consecutive days in 2023 and 2024. “For 450 days during April 2023-July 2024, near-global mean sea surface temperatures exceeded previously observed seasonal maxima by up to 0.31 K and by 0.18 K on average,” the study noted.
It said while some of the warmth is due to the El Nino of 2023-24, that alone can’t explain the acceleration in the rise of global sea surface temperatures between April 2023 and July 2024. They stated that at least 44% of the record warmth was due to the oceans absorbing heat at an accelerating rate.
“While variability associated with the El Nino Southern Oscillation triggered the exceptionally high GMSSTs of 2023 and early 2024, 44% (90% confidence interval: 35%–52%) of the +0.22 K difference in GMSST between the peak of the 2023/24 event and that of the 2015/16 event is unexplained unless the acceleration of the GMSST trend is accounted for,” the study said.
Ideally, there must be a balance between the amount of energy the Earth receives from the Sun and the amount of energy the Earth radiates back into space. But in reality, the energy from the Sun being absorbed by Earth is more than that escaping back into space. This imbalance has roughly doubled since 2010, which is why the oceans are undergoing accelerated warming. “This accelerating ocean surface warming is physically linked to an upward trend in Earth's energy imbalance,” the study noted.
“The solution is cutting global carbon emissions and moving toward net-zero,” Merchant, who is a professor of Ocean and Earth Observation, said. The study warned that the ocean temperature increase seen over the last 40 years will likely be exceeded in just the next 20 years. “Policymakers and wider society should be aware that the rate of global warming over recent decades is a poor guide to the faster change that is likely over the decades to come, underscoring the urgency of deep reductions in fossil-fuel burning.”
Apart from the threat to marine life, warmer oceans cause sea levels to rise due to thermal expansion, endangering coastal areas. While the surface of the seas has warmed faster, recent research has shown that the warmth is percolating to the deeper layers of the ocean as well.