The ocean plays a critical role in regulating Earth’s climate. The IUCN reported in 2016 that the oceans have absorbed more than 90 percent of the warming created by humans since the 1970s. If this heat had stayed within the atmosphere, global average temperatures would have increased by almost 100 degrees Fahrenheit. However, despite the vastness of the oceans, there is a limit to how much heat they can absorb, and they are beginning to show it. Although some effects of climate change on the ocean are well-known, such as warming ocean temperatures, rising sea-levels and melting of polar ice, a lesser known impact is the ongoing and increasing stratification of the ocean water.

The ocean is becoming more layered

It is normal for seawater to form layers, with lighter water closer to the surface and denser water found at deeper levels. That means that warm, fresher water is located above the more saline colder water. This stratified configuration of water inhibits water mixing, impacting the efficiency of vertical exchanges of oxygen, carbon, heat and other elements. However, heat is able to seep deeper into the ocean, and these layers mix due to current, winds and tides. However, the greater the difference in density between layers (due to salinity and temperature), the slower and more difficult the mixing becomes, and the more stable the ocean is.

According to a study done by scientists from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences, as well as collaborators from the United States, the ocean is becoming more stable and layered, and this is having an impact on life in the ocean, by decreasing oxygen and nutrients. This study was published in September 2020 in the Nature Climate Change journal, and funded by the Chinese Academy of Sciences and the National Key R&D Program of China.

Climate change and an increase in carbon dioxide and greenhouse gas emissions in the atmosphere is causing changes in water temperature, ocean acidification and deoxygenation, resulting in changes in oceanic circulation and chemistry. This more stable water layering inhibits the ability for heat, oxygen and carbon dioxide from the surface to be transported deeper into the ocean.

Previously it has been difficult to study stratification changes in the ocean and studies have neglected the spatial complexity of ocean density change. However, this new study overcame these key limitations and provided an estimate on ocean stratification for the upper 2000 m (1.2 miles). The research team used a recently produced data set of temperature and salinity that covered large ranges of the ocean, going back to 1960. The data was validated with the help of a detailed set of ocean observations by Argo floats since 2000, which profile conditions in the ocean down to 2000 m. The study also investigated and evaluated ocean temperature and salinity data, and adopted an improved metric of stratification to then provide a true estimate of ocean stratification and the ongoing changes.

The data collected by the study shows that the ocean has become more layered – by approximately 5.3% – since 1960 to 2018, for the upper 2000 m. An even stronger ocean stratification increase, up to 18%, was observed for the upper 150 m of water. The largest increase in stratification has been in the Southern Ocean (9.6%), the Pacific Ocean (5.9%), the Atlantic Ocean (4.6%) and the Indian Ocean (4.2%).

The cause for this increase in ocean stratification is contributed to an increase in temperatures of the upper layers of water, however changes in salinity also plays an important role. Increased global temperatures are melting more ice, which is adding freshwater to the ocean. Increased temperatures also expand the volume of water, therefore the top layer is becoming less dense and less saline, increasing stratification. This observed change in stratification indicates the extent to which humans are changing the ocean, and contributes to the growing body of evidence of human-induced climate change and increased atmospheric and ocean temperatures.

Why is this significant?

The ocean has played an important role in absorbing the majority of excess heat due to climate change. Increased stratification in the ocean, caused due to climate change, can in turn feedback to climate change. With increased layering, the water layers are not mixing as much, and heat from climate change cannot penetrate deeply into the ocean, which in turn results in increasing atmospheric and surface temperatures. It also influences the ocean’s ability to sequester carbon, as warmer water absorbs less carbon dioxide from the atmosphere, which again worsens climate change and global warming. This may result in intense and destructive hurricanes, which feed off an increasingly warm ocean surface.

Increases in the temperature of the upper layers of water in the ocean has a number of other negative effects. Warmer water absorbs less oxygen, and the oxygen that is absorbed does not mix easily with colder, deeper water layers, impacting marine life. Warmer oceans also result in increased coral reef bleaching events. If water layers do not mix, nutrients and oxygen from the surface do not penetrate deeper levels, which impacts the food supply of entire marine ecosystems.

Therefore, the impact of the changes in stratification will not be limited to the oceans, but will affect the entire Earth as well as the daily lives of humans.

What can be done to reduce the impacts of climate change on the ocean?

The sustainable management, conservation and restoration of marine ecosystems are important if we want the ocean to provide continued ecosystem services such as carbon sequestration.

Support for scientific research is key. This will result in continued monitoring and analyses of data and the observed impacts of climate change. If knowledge can be gained, it can be used to design and implement climate mitigation and adaptation strategies.

Governments and large companies need to commit to reduce the use of fossil fuels, and increase the use of renewable energy sources, as well as enhancing energy efficiency. This will reduce the impacts of carbon dioxide and other greenhouse gasses on the ocean.