Adding Alkalinity to European Seas Could Turn Summer Carbon Emissions Into Year-Round Ocean Uptake

Every summer, the coastal seas of Europe quietly exhale carbon dioxide. Warm water holds less of the gas than cold water, so as the sun climbs and the surface warms, a vast stretch of ocean from the Bay of Biscay to the Norwegian shelf flips from absorbing atmospheric CO2 to releasing it. For climate planners hoping to enlist the ocean in the fight against warming, that seasonal reversal has always been an awkward complication. Now a team of European modellers suggests a way to erase it: dissolve enormous quantities of alkaline material into the water, and the summer exhale becomes a summer inhale.

The new study, published in the journal Earth System Dynamics, is one of the first to examine how ocean alkalinity enhancement, or OAE, would reshape the seasonal rhythm of the carbon cycle rather than just the annual bottom line. Led by Chiara Ciscato of the Euro-Mediterranean Center on Climate Change in Bologna, with colleagues at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, the work used a full Earth system model to simulate a continuous, decades-long dose of alkalinity along the European coastline under two very different futures: a low-emission pathway called SSP1-2.6 and a high-emission pathway called SSP3-7.0. The results reveal a carbon cycle that is stranger, and in some ways more productive, than anyone had mapped before.

OAE is built on a simple piece of chemistry. When alkaline substances such as quicklime or crushed olivine dissolve in seawater, they consume dissolved CO2 and convert it into bicarbonate and carbonate ions, stable forms of inorganic carbon that can persist in the ocean for millennia. This conversion lowers the partial pressure of CO2 at the ocean surface, opening a chemical gap between sea and air. The atmosphere rushes to fill it, and the ocean draws down more CO2. In theory, the approach mimics and accelerates the natural weathering of rocks that has regulated Earth’s climate over geological time, compressed into a human timescale.

In the simulations, the team injected alkalinity continuously into the surface layer of coastal grid cells ringing the European continent, excluding the Mediterranean and the Baltic. Starting in 2025 and ramping up over a decade, the addition reached the equivalent of one gigatonne of fast-reacting calcium hydroxide per year, held constant until 2100. That is an industrial-scale intervention, comparable in ambition to the largest proposed carbon removal schemes, and the model tracked its consequences through every season of the century’s final decades.

The most striking finding concerns summer. Under natural conditions, surface alkalinity in European waters peaks in winter, when deep mixing dredges alkalinity-rich water up from below, and bottoms out in summer, when a warm, stratified cap isolates the surface. But the model showed the opposite happening once alkalinity was added. Because summer stratification traps the added alkalinity in the top layer rather than letting it mix away, surface concentrations climbed highest in the warmest months. Along the coastline itself, the seasonal swing in alkalinity exploded from roughly 19 to 109 micromoles per kilogram under low emissions, and from 24 to 138 under high emissions. The seasonal cycle, in effect, had been turned upside down.

That reversal has cascading consequences. With alkalinity peaking in summer, the partial pressure of CO2 in surface waters falls most sharply precisely when it would naturally be highest, during the months of peak outgassing. Fast gas exchange between ocean and atmosphere then does the rest: the summer release of CO2 flips into summer uptake, and the region becomes a carbon sink all year round. In the broader European domain, the seasonal amplitude of ocean pCO2 shrank by 41 percent under the low-emission scenario, a damping of the natural cycle that the authors attribute to alkalinity addition working hardest against the season of greatest chemical resistance.

Yet the biggest carbon gains did not come in summer. The air-sea CO2 flux remained locked to the rhythm of the mixed layer, deepening in winter as storms and cooling stir the upper ocean and shoaling in summer as the water stratifies. Winter, with its cold water and vigorous mixing, is when CO2 dissolves most readily, and it is winter that saw the largest enhancement of uptake once alkalinity was added. In the European region, the seasonal amplitude of the CO2 flux grew by about 51 percent under low emissions and 52 percent under high emissions. Along the coastline, the effect was more dramatic still: the winter-to-summer swing of the flux nearly tripled under SSP1-2.6 and grew more than two and a half fold under SSP3-7.0.

Perhaps the most counterintuitive result is that the dirtier emissions scenario produced the bigger carbon sink. Under SSP3-7.0, atmospheric CO2 concentrations are far higher, which raises the partial pressure of CO2 in surface waters and weakens the ocean’s buffering capacity, its chemical resistance to absorbing more of the gas. The result is that each unit of added alkalinity pulls down more carbon. The study’s measure of seasonal carbon sequestration potential, the ratio of extra CO2 uptake to the alkalinity added, was consistently higher under high emissions, both along the coast and across the wider European region. A hotter, more carbon-polluted world, in other words, makes this particular removal technology more efficient, even as it makes the underlying climate problem worse.

The findings carry practical weight for the emerging industry of marine carbon removal. Because the excess CO2 uptake extended well beyond the strip of coastline where alkalinity was injected, with some of the additional flux appearing over the open North Atlantic, monitoring schemes confined to national waters or deployment sites could systematically underestimate how much carbon a project actually removes. The authors also flag a caution: summer surface alkalinity along the coast rose by more than 500 micromoles per kilogram in the simulations, levels that could push pH and aragonite saturation past thresholds harmful to local marine life. Choosing the right alkaline feedstock, and tailoring the rate and timing of addition to each site, may be essential to keeping the ecological risks in check.

The study is not without limits. The Earth system model, called FOCI, resolves the ocean at half a degree, which smooths over the fine coastal dynamics where alkalinity is actually added, and the model carries known biases in the North Atlantic, including a cold patch and a low-alkalinity shelf region near Norway. Riverine alkalinity inputs, which follow a strong seasonal cycle, were not represented. The authors argue that because their analysis rests on differences between paired simulations, such systematic errors largely cancel out, but they acknowledge that real-world deployment would demand finer tools. They also note that their continuous injection scheme is idealised; future projects are more likely to use pulsed additions, which earlier work suggests behave differently. Still, as coastal OAE moves from whiteboard to pilot project, the message of this study is clear: the seasons matter. Whoever deploys alkalinity in European waters will be rewriting not just the ocean’s carbon budget, but its calendar.

Subject of Research: Effects of coastal ocean alkalinity enhancement on the seasonal carbon cycle in European waters under low- and high-emission scenarios

Article Title: Impacts of simulated coastal ocean alkalinity enhancement on the seasonal carbon cycle in European waters under a low- and a high-emission scenario

Article References: Impacts of simulated coastal ocean alkalinity enhancement on the seasonal carbon cycle in European waters under a low- and a high-emission scenario. (n.d.). https://doi.org/10.5194/esd-17-1513-2026

Image Credits: AI Generated

DOI: 10.5194/esd-17-1513-2026

Keywords: ocean alkalinity enhancement, carbon dioxide removal, seasonal carbon cycle, European coastal waters, Earth system modelling, ocean pCO2, marine carbon sink, SSP1-2.6, SSP3-7.0, ocean stratification, air-sea CO2 flux, climate mitigation