Global assessment of marine CO2 removal efficiency in the coastal ocean

The ocean is a vast, natural sponge that has absorbed much of the carbon dioxide humans have pumped into the atmosphere, helping to slow the pace of global warming. However, to keep the planet's temperature rise below a critical threshold, scientists know that simply stopping emissions will not be enough; we must also actively remove carbon from the air. One promising idea is marine carbon dioxide removal, a process where we alter seawater to make it hungry for carbon, encouraging the ocean to pull more from the atmosphere. The catch is that this process is not instant. Once the water is altered, it takes months, years, or even decades for the carbon to fully move from the air into the sea. Because this exchange happens over such huge distances and long times, it is impossible to measure directly in the real world. Instead, scientists rely on computer models to predict how well this method would work in different places, but running these detailed simulations for every coastline on Earth is so expensive and slow that it has been out of reach.

A team of researchers has now found a way to bypass these computational limits, offering a clear, high-resolution map of where this ocean-based carbon removal would actually work best. By developing a new method that tracks virtual particles moving through the ocean, they were able to simulate the efficiency of carbon removal along coastlines worldwide with a level of detail previously impossible. Their work reveals that while some tropical shores are ideal for this technology, other regions, particularly near the poles and in specific temperate zones, are far less effective. Crucially, they discovered that efficiency can change dramatically over very short distances, meaning that a broad, general assessment of a country's coastline could be dangerously misleading.

The researchers, led by Yinghuan Xie at the University of Tasmania, created a system of "virtual absorbers." Imagine releasing thousands of tiny, invisible barrels into the ocean at specific coastal points. Each barrel starts empty, representing a patch of seawater that has been treated to have a deficit of carbon. As the ocean currents carry these barrels along, they act as sensors, absorbing carbon from the air whenever they float near the surface. If a barrel is pushed deep underwater, it stops absorbing because the air cannot reach it. By tracking the journey of these virtual barrels over a decade, the team could calculate exactly how much carbon each patch of water would eventually pull from the atmosphere.

This approach allowed them to test the method against existing, more expensive computer models. They found that their virtual barrels could reproduce the global patterns of carbon removal efficiency with remarkable accuracy, matching the results of complex, high-resolution models while using a fraction of the computing power. In fact, their method was up to a million times cheaper to run than the traditional simulations. This efficiency breakthrough meant they could finally run the experiment across the entire global coastline, testing nearly 150,000 specific locations every few kilometers, rather than just a handful of broad regions.

The results painted a detailed picture of the world's coastal potential. In the warm, tropical, and subtropical waters of the Indo-Pacific and the Atlantic, the virtual barrels showed that the ocean could absorb carbon very efficiently, often reaching near-complete uptake within ten years. These areas are characterized by conditions that keep the water near the surface, where the air can reach it, and where the water chemistry and wind speeds help speed up the exchange. However, the map also highlighted significant limitations. Around Antarctica, in parts of the North Atlantic, the Mediterranean Sea, and along the coasts of Japan and Australia, the efficiency dropped considerably. In these colder or more turbulent regions, the water often sinks deep below the surface or gets trapped under sea ice, cutting off the connection to the atmosphere and leaving the carbon deficit unaddressed.

Perhaps the most striking finding was how much the efficiency varied over short distances. The researchers discovered that just tens or hundreds of kilometers along a single coastline, the potential for carbon removal could swing by 30 to 50 percent. A broad, low-resolution map might suggest a country's entire coast is a good candidate for this technology, but a closer look reveals that the most effective spots are often isolated pockets surrounded by less effective waters. For instance, while parts of Japan's coast showed lower efficiency, specific nearshore areas still offered high potential, and vice versa. This level of detail is vital because real-world projects will likely be built on small scales, where local currents and depth matter far more than regional averages.

The study also explained why these differences exist. In the high-efficiency zones, the water tends to stay in the upper layer where it can breathe the atmosphere, aided by warm temperatures and strong winds. In the low-efficiency zones, the water is often pushed down into the deep ocean or covered by ice, preventing the carbon from being exchanged. The researchers noted that in some of these difficult areas, the process doesn't just slow down; it can stop entirely after a certain point, meaning that waiting longer might not yield better results.

This new tool does more than just map the ocean; it provides a way to understand the physical reasons behind the numbers. By linking the efficiency of carbon removal to specific factors like wind speed, water temperature, and how deep the water mixes, the researchers can tell policymakers exactly why a certain location is suitable or unsuitable. They have made their data and an interactive website available to the public, allowing anyone to explore these coastal conditions in detail. While the study is based on simulations and does not account for every biological reaction in the ocean, it offers a robust and affordable way to screen the world's coastlines. It suggests that for marine carbon removal to be a viable tool in the fight against climate change, we must look beyond broad generalizations and focus on the specific, high-resolution realities of our local shores.