· 4 min read
The ocean has always absorbed excess heat, drawn down atmospheric carbon, and softened the chaos of our emissions – acting as Earth’s quiet guardian. But that balance is breaking. Beneath the surface, the ocean is becoming more sharply layered as its internal balance is shifting. A new comprehensive review of these aspects, led by Lijing Cheng, has been published in Nature Reviews.
Figure 1: A schematic representation of the processes and features of ocean stratification. Ocean stratification arises from many dynamic and thermodynamic processes, creating stable ocean conditions that limit vertical mixing (from Cheng et al, 2025).
Across the globe, scientists are tracking a sharp rise in ocean stratification. The separation between warm surface waters and the colder, denser layers below is becoming more severe. Since the 1960s, the layering has strengthened by nearly one percent per decade. The change is subtle in measure but sweeping in impact. If global warming continues as emissions of carbon dioxide into the atmosphere remain high, that rate could almost quadruple by 2100.
A system losing its breath
The ocean has always interacted with the atmosphere as part of the climate. As the surface warms and freshens through global warming and changes in rainfall and melting ice, it becomes lighter and less able to mix with the depths. The result is a thicker density barrier, a lid that traps heat and carbon near the top while starving deeper waters of oxygen and nutrients. In effect, the ocean’s natural “breathing” is slowing.
This breakdown in vertical exchanges weakens one of Earth’s key stabilising forces. The upper ocean overheats; the depths suffocate, and the ocean becomes more acidic. What was once a global buffer is turning into a feedback loop.
The buffer is breaking
For decades, more than 90% of the excess heat from greenhouse gases has gone into the ocean, increasing sea levels, but that generosity is running out. Reduced mixing intensifies marine heatwaves that bleach corals, jeopardise kelp forests, and destabilise fisheries. Changes in the ocean also affect methane and nitrous oxide, that are stronger than CO2. In this way, the ocean is beginning to amplify the warming it once absorbed.
Even under strong mitigation, ocean stratification will keep rising through mid-century before stabilising. In higher-emission scenarios, the divide between surface and deep waters could grow by up to 3% per decade by 2100. The largest shifts are already appearing in the tropical Pacific, Indian, and North Atlantic Oceans – altering rainfall, storm tracks, and sea-surface temperatures.
Why it matters beyond the waterline
A stratified ocean means:
• Hotter heatwaves and stronger storms, as trapped heat and increased evaporation fuels atmospheric extremes
• Collapsing marine productivity, from oxygen loss, acidification, and nutrient starvation
• Rising seas, through thermal expansion and ice melt
• Increasing rainfall intensity, reshaping food and water security
• Fueling tropical cyclones and hurricanes
What were once distant fears are now reshaping economies, migration, and geopolitics.
Figure 2: Ocean stratification impacts on physical and biogeochemical ocean systems. The observed mean stratification trend for 0–2,000 m (upper), meridional trend within 20° S to 20° N (front), and zonal mean trend (right), with icons demonstrating their importance in the global climate system. Ocean stratification change is a pervasive feature and has important impacts on the climate system (from Cheng et al, 2025).
The limits of the ocean’s generosity
Once heat enters its depths, it lingers for centuries. Even if emissions stopped tomorrow, warming would continue. Mitigation and adaptation must therefore move together: rapid decarbonisation to slow the driver, and building coastal resilience, climate-smart fisheries, and early-warning systems to face what’s inevitable.
What comes next
Tracking this transformation demands sustained investment in observations – global coverage, deep-ocean floats, long-term monitoring, and satellite systems capable of seeing changes in real time. But data alone will not solve the problem.
The science is clear: the ocean’s capacity to protect us is eroding. Whether the ocean changes even out or spiral further from normal will depend on the choices made this decade.
The ocean has carried our excess for generations. It will not carry it forever.
This article is based on Cheng et al., “Ocean Stratification in a Warming Climate,” Nature Reviews Earth & Environment, 2025; doi:10.1038/s43017-025-00715-5. illuminem Voices is a democratic space presenting the thoughts and opinions of leading Sustainability & Energy writers, their opinions do not necessarily represent those of illuminem.
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