Carbon dioxide (CO2) emissions are still high. From energy production alone, 36.3 billion tons were released worldwide in 2021. Too much to slow down global warming. Ways to remove CO2 from the atmosphere are all the more important. One leads across the seas: The water can absorb the greenhouse gas, it is dissolved in it. Uptake is increased when certain minerals are dissolved in the sea and change the chemical environment.

Something similar has been happening on land since ancient times with the weathering of rocks, but very slowly. Researchers now want to accelerate the carbon dioxide uptake of seawater. However, it is questionable how effective the procedures are and whether there are disadvantages.

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This should show tests in Raunefjord, south of Bergen in Norway, which will last until mid-July. The scientists brought mesocosms into the water there: Large plastic tubes, two meters in diameter and 20 meters deep, hanging in the fjord like giant test tubes.

Herring larvae were placed in the mesocosms, filled with the natural community of the Raunefjord, in order to follow their development in the climate experiment. The team adds slaked lime, representative of calcium-based minerals, and magnesium silicate, representative of siliceous minerals. “We will work with different amounts and observe what changes are associated with this for the habitat and how efficient the method is for absorbing carbon dioxide,” says research director Ulf Riebesell from the Geomar Helmholtz Center for Ocean Research in Kiel.

According to the scientist, model calculations have confirmed that the processes have enormous global climate protection potential. But it was calculated with an area-wide increase in the pH value. “In reality, the minerals are introduced selectively, so you have to look closely.” For example, ground basalt or limestone could cloud the water or sink quickly – the water near the surface, which is crucial for the absorption of carbon dioxide from the air, would remain unchanged and there would be no effect.

Even if the rock flour were dissolved, which is currently favored in research and possible industrial applications, the solution could disappear too quickly underground. Or the water could become supersaturated. Then carbonates fall out and CO2 is released again. Water temperature and natural carbonate content also greatly affect how much of the gas is absorbed.

The difficulties do not only lie in biology and seawater chemistry. It is also unclear where the huge amounts of rock are supposed to come from. Hardly any contributions can be expected, at least from Germany, where every quarry expansion is fought over. This is different in other countries, especially since substances from the cement industry are also viewed as potential acid buffers.

Whether they are allowed in the water is another question. The trials are taking place in Norway for a reason. “In Germany, you are not even allowed to put homeopathic amounts in seawater for research purposes,” says Riebesell. Legal requirements would have to be created for use with large quantities of ground rock or concrete.

“Governance will take at least as much work as laying the scientific foundations,” says the researcher. There is still a lot to be clarified: In addition to the immediate effects, the scientists are interested in whether the CO2 absorption is really long-term and is not reversed by other processes. “It is also unclear how the monitoring will take place,” says the marine researcher. How, for example, a company that receives CO2 credits for its performance can prove that it has really made a contribution. “The changes in alkalinity in the water after application are very small and are likely to be masked by natural variability.”

Several start-ups are active in this field. Ebb Carbon, for example, would like to use green electricity to extract hydrochloric acid from seawater for industrial processes and return the remaining caustic soda for pH correction. Instead, Planetary Technologies wants to treat wastewater from mining and discharge it into the sea to buffer the acidity. SeaChange, in turn, wants to use CO2 from the sea to extract synthetic limestone and magnesite for the construction industry. According to “”, these three companies have already sold climate compensation vouchers to Internet companies such as Shopify and Stripe.

This shows how quickly a new market is developing here. “I’m concerned that academic research will not be able to cope with this dynamic,” says Riebesell. That the pressure from the private sector is so great that individual procedures are already being approved in some regions, although it is not yet clear whether they will really bring anything.

Other experts are also concerned. In December, the US National Academy of Sciences recommended funding research on ocean-based CO2 removal with $125 million. “If we’re going to make informed decisions about the future of our oceans and our climate,” says Scott Doney of the University of Virginia, “we need to complete some very important research over the next decade.”