Wave extremes diverge from mean conditions under climate change

The ocean is never still. Even on a calm day, wind pushes against the water, creating a restless surface of swells and chop that travels thousands of miles. For centuries, scientists have tracked these waves to understand the climate, focusing mostly on the average height and energy of the sea. This average state is like a baseline, a steady hum that tells us what the ocean is doing most of the time. However, the true danger to our coastlines rarely comes from this average hum. It comes from the outliers: the rare, massive storms that generate towering walls of water capable of flooding towns, eroding beaches, and crushing sea walls. As the planet warms, a critical question has emerged: do these dangerous, extreme waves simply get bigger because the average ocean gets rougher, or do they behave in a completely different way?

A new global study by an international team of researchers suggests that the answer is far more complex than a simple "yes." By analyzing a massive collection of computer simulations that model the Earth's climate and ocean waves, the scientists discovered that the most dangerous waves are not following the same rules as the average waves. In fact, in some of the world's most populated coastal regions, the average ocean is predicted to become calmer, yet the rare, extreme storms are expected to become more violent. This divergence means that relying on average conditions to predict future coastal hazards could lead to a dangerous underestimation of risk.

The researchers, working through a coordinated international effort known as COWCLIP-3, brought together thirty different computer models to look at how waves might change by the end of this century. They examined three different future scenarios, ranging from a world where greenhouse gas emissions are drastically cut to one where they continue to rise unchecked. The team focused on two main things: the average state of the ocean and the most extreme events, defined as the largest waves that occur only once every few decades. They looked at wave height, the time between waves, the direction they travel, and the energy they carry.

The results painted a clear picture of a shifting ocean. In the middle latitudes of the Northern Hemisphere, particularly across the North Atlantic and North Pacific, the average waves are projected to become smaller. This is largely because the storm tracks that drive these waves are shifting toward the poles, leaving the mid-latitudes with slightly calmer seas on average. However, the story changes completely when looking at the extremes. Despite the average ocean becoming calmer, the most severe storms in these same regions are predicted to grow stronger. In the North Atlantic and North Pacific, the largest waves are expected to increase in height and intensity, even as the day-to-day sea state becomes more gentle.

This finding is a significant departure from previous assumptions. For a long time, it was often assumed that if the average wave conditions were weakening, the extreme events would weaken too. This study shows that logic does not hold up. The researchers found that while the frequency of large waves might decrease in these regions—meaning there are fewer days with big swells—the few big waves that do occur will be more powerful. It is as if the ocean is taking a deeper breath before a single, more forceful exhale. This pattern is particularly strong during the winter months in the Northern Hemisphere.

In the Southern Hemisphere, the story is different but equally dramatic. Around the Southern Ocean, which circles Antarctica, both the average waves and the extreme waves are projected to grow larger and more energetic. This is consistent with a strengthening of the westerly winds in that region. The study also found that in the tropics, the picture is more mixed and less certain, with some areas seeing decreases in wave energy and others seeing increases, often driven by tropical cyclones.

The implications for human safety are profound. The researchers translated these open-ocean changes into potential impacts on the world's coastlines. They found that about 62% of the global coastline is sensitive to these changing wave patterns. This sensitivity is not uniform; it depends on what kind of impact is being considered. Changes in wave direction could accelerate beach erosion along western continental margins, while changes in wave height and energy could increase the risk of flooding in tropical regions and parts of the Southern Hemisphere. Perhaps most critically, the study highlights that coastal infrastructure, such as ports and sea walls, which is often designed based on historical data and average conditions, may face a higher risk of damage. If engineers design a sea wall for the average wave conditions of the future, they might be unprepared for the more violent, albeit less frequent, extremes that are actually coming.

The study underscores a vital lesson for climate adaptation: we cannot simply look at the average weather to understand the future risks. The most dangerous events are decoupling from the mean, behaving in ways that standard averages cannot predict. By using a large ensemble of models and two different methods to analyze the data—one looking at how often waves exceed a certain height and another looking at the statistical probability of rare, century-scale events—the team confirmed that these extreme changes are real and robust. They found that in the North Atlantic and North Pacific, the historical "50-year storm," a massive wave event expected to happen once every half-century, could occur as often as once every 20 to 40 years by the end of the century. In some parts of the Southern Hemisphere, that same historical extreme could return in less than a decade.

This research does not just tell us what the ocean will look like; it tells us how we must rethink our approach to coastal safety. The divergence between average conditions and extreme events means that adaptation strategies based on mean trends are likely to fail. Coastal planners and engineers must now account for a future where the ocean can be deceptively calm on average, yet capable of delivering sudden, intensified blows. The study concludes that dedicated assessments of these extremes are no longer optional but essential for protecting the billions of people and trillions of dollars in assets that line the world's shores. The ocean is changing in ways that are subtle in the daily average but dramatic in the extremes, and understanding that difference is the key to surviving the coming century.