In 2019, the oceans reached higher temperatures than at any other point in recorded human history, according to a new analysis published on 13 January in Advances in Atmospheric Sciences (1). The new record demonstrates, unequivocally, the reality of global warming.
To paint a clearer picture of ocean warming, the team of scientists from around the world collected data from 3,800 free-drifting Argo floats dispersed across the ocean. Data were compiled to a depth of 2,000 metres over several decades by China’s Institute of Atmospheric Physics (IAP). Statistical methods were used to interpolate heat levels where no data is available, for example, under the Arctic ice cap.
The latest analysis provides evidence that global warming is “irrefutable and accelerating”. Hotter ocean temperatures were recorded every year compared to the previous for the past decade, bar one. Over the last five years, ocean temperatures have peaked, and in 2019 were the hottest on record: 0.075 Celsius hotter than the historical average between 1981 and 2010.
To reach this record-breaking temperature, the oceans would have absorbed 228 sextillion Joules (or 228 billion trillion joules zeroes) of heat, according to lead author Dr Lijing Cheng.
In a statement, Cheng said: “The amount of heat we have put in the world’s oceans in the past 25 years equals to 3.6 billion Hiroshima atom-bomb explosions”.
“This measured ocean warming is irrefutable and is further proof of global warming. There are no reasonable alternatives aside from the human emissions of heat-trapping gases to explain this heating.”
The researchers also showed that ocean warming is accelerating. The rate of warming was four and a half times faster from 1987 to 2019 compared to 1955 to 1986. Increases were observed in the majority of oceans regions.
Why do ocean temperatures matter?
Despite widespread recognition of the climate crisis, emissions are still on the rise, which means more heat trapped by greenhouse gases. And since water absorbs a lot more heat than air, most of this is absorbed by the oceans: Two-thirds of the planet is covered by oceans, which absorb around 90 per cent of the heat compared to just 4 per cent by land and air.
As co-author Prof John Abraham of the University of St. Thomas explained: “It is critical to understand how fast things are changing”.
“The key to answering this question is in the oceans – that’s where the vast majority of heat ends up. If you want to understand global warming, you have to measure ocean warming.”
Consequences of warming oceans
Warmer oceans lead to increased evaporation, which will likely mean more flooding, droughts, and wildfires. Furthermore, a dramatic increase in the number of ocean heatwaves poses a serious threat to marine life. And persistent warming will also lead to unstoppable sea-level rise, mainly owing to rapidly expanding and melting ice sheets.
The authors warn that ocean warming is so severe that it will likely continue over the next few years even if the global community meets the targets of the Paris Agreement, which looks less and less likely.
However, Cheng also said: “the more we reduce greenhouse gases, the less the ocean will warm”. We just need to start taking meaningful action.
(1) Cheng, L. et al. Record-Setting Ocean Warmth Continued in 2019. Advances in Atmospheric Sciences (2020). DOI: 10.1007/s00376-020-9283-7