As Earth’s atmosphere traps more heat under the influence of greenhouse gases, the oceans become warmer. This means that, ideally, if we wish to understand how much Earth’s climate has changed, we need to look at the warming of the oceans. The oceans have a massive thermal inertia (are slow to heat up) and have been able to absorb around 90 percent of the Earth’s increasing heat load, but they have been warming inexorably for many decades. The implications of this change in ocean heat content, which is unlikely to stop any time soon, are likely to be devastating to natural systems and to all living creatures.
In a new review published in Nature Reviews Earth & Environment, an international team of scientists has analyzed temperature data from the world’s oceans in order to put together a clear picture of the extent of global ocean heat change since the 1950s. Using this information, the researchers are able to predict likely trends in ocean warming going forward, and to assess the potential consequences. This study is important because it motivates us to take actions to mitigate and respond to climate change. It shows what will happen if we don’t take urgent action to slow global warming.
The study involved scientists from leading institutions in France, USA, Australia, the UK, and China, including the Institute of Atmospheric Physics. The team collected temperature information from past studies of the world’s oceans that made use of a variety of different temperature sensors. Some of these sensors are manually placed in the oceans by researchers, others are deployed by cargo vessels, and many are autonomous devices that float in the ocean or are tethered, like a buoy or mooring. By combining many thousands of temperature measurements spread across the globe and over the years, the scientists were able to piece together a clear picture of global ocean heat content change back to the 1950s.
Analysis of the data showed that the upper 2000m of ocean began warming, unequivocally, at least in the 1950s. The warming has continued since then and has been driven, mostly by the heat uptake at the ocean surface, and the transport of this warmed water by currents, mixing and stirring in the ocean interior. This has meant that heat has not been equally distributed between the world’s oceans; some, such as the southern Atlantic and Southern Oceans, have warmed more than others due to the effects of current movements.
Now, the upper 2000m of ocean is warming at a rapidly accelerating rate that has more than doubled (from <5 to 10 ZJ per year. A ZJ is 1021 Joules of energy, or 1,000,000,000,000,000,000,000 Joules) between the 1960s and the 2010s. To put this in perspective, the annual energy consumption for the USA is approximately 0.1 ZJ. In addition, this overall warming pattern is now driven less by ocean current mixing, and more by the passive uptake of added heat by surface waters, as the Earth becomes warmer and warmer.
A massive temperature data set like the one used in the current review is key to understanding both our past and our future, in terms of ocean heat content. The researchers describe the heat content changes between 1950 and 2019 in all the world’s oceans and this enables them to predict what will happen to the oceans in the coming decades. The most important finding is that the future of the oceans is in our hands.
The authors stress that if significant cuts in greenhouse gas emissions are made (i.e., if the target of the Paris Agreement of limiting global surface temperature increase to 2°C above the pre-industrial level can be achieved), the acceleration of ocean warming will stop at around 2030. However, if we don’t take action, the predictions are quite dire. The rate of warming will skyrocket throughout the 21st century: by the 2090s, the rate of ocean warming is projected to be four times larger than the current level.
Some of the potential consequences of this unabated ocean warming are described by the researchers. By 2100, all regions of the ocean will exhibit warming, although the Southern and Atlantic Oceans will have warmed the most. This will lead to continued melting of ice sheets and glaciers and a consequent increase in sea levels worldwide. This, in turn, will cause coastal erosion, displacement of people, flooding and damage to coastal infrastructures. Ocean warming also affects the cycling of carbon, oxygen and nutrients in the ocean, affecting food chains that support the ocean’s living creatures.
In addition, ocean warming intensifies tropical cyclones and changes the paths of storms. It leads to marine heat waves that bleach corals, change phytoplankton blooms and kill kelp forests and sea grasses. This can lead to the mass mortality of benthic organisms, declines in fish and seabird populations and a shortage of food for humans. Weather patterns also change as a consequence of a warm ocean, with some areas receiving more rain and flooding while others become drier with more heat waves and droughts.
Continued monitoring of ocean temperatures is critical, say the authors, along with enhanced understanding and adaptation to the effects of warmer waters. However, they stress that the future of ocean warming is in our hands. Low greenhouse gas emissions would be likely to lead to a detectable and lasting reduction in ocean warming rate, with noticeable reductions in climate-change impacts. But if these emissions continue unabated, the consequences will be very severe. We have already experienced a wide range of climate disasters during 2022: heat waves, flooding, heavy rainfalls, and strong hurricanes. If the current weather is wild, just wait for the coming decades … we haven’t seen anything yet.
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