When the Sea Changes Quietly

When we talk about climate change and the ocean, we usually think of the surface. We talk about record-breaking sea surface temperatures, longer swimming seasons, and milder winters.

And that is not wrong. The surface ocean is warming, and we can feel it. I can feel it too.

Having grown up on an island, the sea has always been a familiar presence in my life, which makes these changes impossible to ignore.  But as a physical oceanographer, I know that this is only the first layer of the story - quite literally.

Looking Beyond the Surface

The ocean is a three-dimensional system. It has depth, internal dynamics, and memory, all hidden from the naked eye. Climate change does not affect only what we see at the surface. It slowly changes how the ocean mixes, cools, sinks, and renews itself. 

Step by step, it alters the sea from within.

Most of the extra heat in the climate system does not remain in the atmosphere. Instead, it is absorbed by the ocean. This helps slow down atmospheric warming, but it also means that the sea is storing more heat. This warming is not uniform. The surface responds first and warms fastest, while deeper layers warm more slowly, but steadily. Recent observations in the Adriatic Sea show that even deep waters, at depths of around 1000 meters, are now warming and becoming saltier. This tells us that the impact of climate change is no longer limited to the surface, it is reaching deep into the interior of the sea.

Temperature alone does not control how the ocean behaves. Salinity, although much less visible, is just as important. It rarely makes headlines, but it plays a major role in ocean dynamics. As the climate warms, evaporation increases, leading to a gradual rise in salinity. These changes are not the same everywhere, but the long-term trend is becoming clearer. 

Together, temperature and salinity determine how dense seawater is, and density controls how the ocean circulates and renews itself.

When the Adriatic Stops Breathing as It Used To

This brings us to one of the most important physical processes in the Adriatic Sea: winter dense water formation. Under cold and windy conditions, especially during strong bora events, the sea loses heat rapidly. Surface water becomes colder, saltier, and denser, and sinks toward the bottom. This process renews deep waters, supplies oxygen, and drives Adriatic thermohaline circulation helping maintain the balance of the system. 

Today, warmer winters and stronger stratification mean that dense water formation happens less often and less efficiently, depending on short periods of intense atmospheric forcing rather than occurring regularly every winter. The result is that the Adriatic thermohaline circulation has not stopped, but it has slowed down and changed its rhythm. Deep waters are renewed less efficiently, allowing heat and salt to remain stored in the sea for longer periods. The changes are gradual and easy to miss, but they mark a fundamental shift in how the Adriatic Sea functions. 

A Changing Relationship Between Sea and Atmosphere

If we return briefly to the surface, where the ocean meets the atmosphere, another link becomes clear. Very warm sea surface temperatures can supply heat and moisture to the lower atmosphere and, under certain conditions, support strong local convection. This does not mean that the sea directly causes storms, but it shows that a warmer ocean can influence weather near the coast. Even though these events are local and short-lived, they remind us how tightly connected the ocean and atmosphere are.

 Overall, the ocean is moving toward a state with stronger stratification, weaker vertical mixing, and longer heat retention at depth. These changes affect oxygen supply, biogeochemical processes, and marine ecosystems, but they also shape how the ocean responds to future extremes.

One other important feature of the ocean is its memory. Heat stored at depth can remain there for years or even decades. The changes we observe today are not short-term anomalies, but part of a long-term adjustment of the ocean to a warming climate. As someone who studies the physics of the sea, I am often reminded that many of the most important changes happen quietly, below the surface.

Climate change in the ocean is not only about records and extremes. It is about slow, structural changes in how the system works. 

Physics is not a tool for fear, but for understanding – it helps us connect processes, see cause and effect, and realize that these changes are happening within a system that is close to us and deeply intertwined with our lives.

The sea is changing from within, layer by layer.

If we want to adapt to what lies ahead, we must learn how to read these subtle signals – before they become impossible to ignore.

Interview with Natalija

Can you tell us a bit about yourself and your work?

I am an associate researcher at the Institute of Oceanography and Fisheries in Split, Croatia, where my work focuses on the Adriatic Sea and the impacts of climate change on marine systems. With a background in physical oceanography, my research encompasses thermohaline properties, marine heatwaves, climate modelling, and their broader environmental implications.

Born and raised on an island, my lifelong connection to the sea naturally evolved into a scientific career dedicated to understanding marine processes. My work extends beyond theoretical and modelling approaches to extensive field expeditions, where I collect and analyze in situ data to better understand marine dynamics and long-term change.

I am strongly committed to bridging science and society through science communication and outreach. I am one of the initiators of the “Oceanographers on the Island” initiative, which introduces children to ocean science and raises awareness of climate change and ocean conservation. I actively disseminate scientific knowledge through media engagement and public communication, advocating for marine protection and fostering informed dialogue on ocean health and climate resilience.

What does adapting to climate change mean to you?

For me, adapting to climate change means learning to live with change while reducing the vulnerability of natural systems and society. As a scientist, I seek to understand how the ocean is responding to warming and extreme events such as marine heatwaves, and to translate that knowledge into information that can support better decisions.

It also means accepting that some changes are unavoidable. For that, we need to protect marine ecosystems, adjust how we use and manage the sea, and prepare coastal communities for a warmer future. Adaptation means responsibility. I use science not only to observe and describe change, but also to help society prepare for it.

How do you experience climate change in your daily life?

I feel it in longer and more intense heat, in warmer summers (especially at night), and in the uncertainty of seasons that no longer follow familiar patterns. Working with ocean data, these changes are not something I only read about; they are something I see and experience directly, year after year.

I think society can address these problems by first acknowledging that climate change is already part of our everyday reality. We need to listen more to science, but also to local experience, and make choices that protect people, nature, and, most importantly, future generations. Small changes in how we live, together with responsible decisions at the community and policy level, can really make a difference. Adaptation, for me, is not only about technology or plans, but also about care, responsibility, and solidarity.

How can we involve young people and local communities in climate action?

Young people and local communities get involved when climate change feels real and close to their everyday lives. It helps when we talk about things that are close to them: heat, water, food, the sea, or the places where they live.

We can involve young people and local communities by giving them a chance to take part, not just listen. Simple activities, local projects, and learning by doing work much better than lessons. It is also important to listen to their ideas and show that their voices matter. When people feel included, they feel a sense of responsibility and are much more willing to act and help find solutions.