Alkalinity affects the uptake of CO2 and contributes to the long-term buffering capacity of the ocean.
Photo: © Hereon/ Nele Lehmann

Stones for the climate

If climate change causes more rain, this promotes the weathering of rocks and thus the erosion of the soil. The dissolved substances reach the sea via rivers. A new model from the Helmholtz-Zentrum Hereon shows that the process has an impact on CO2 storage there.

If global emissions of greenhouse gases increase sharply, as they have in the past, this increases the ability to bind them. When emissions are low, the opposite happens. In a recent study researchers from the German Helmholtz-Zentrum Hereon, in Geesthacht, looked at the factors that favour sequestration capacity and their effects. The study appeared in the journal Nature Communications last March.

If a cook alters the amount of ingredients when preparing a dish, a completely new taste is created. It is exactly the same with the binding of CO2 in the sea – a change in the substances in the water changes everything else. Alkalinity, i.e. the acid binding capacity, is created by the weathering of rocks and their entry into the ocean.

Increased erosion on land causes an increase in weathering of silicates and carbonates. The researchers identified the factors for more alkalinity using a model incorporating  the degree of erosion, area fraction of carbonate, temperatures, catchment size and soil thickness.

Method and influencing factors

"The model we used is a statistical model, not a mechanistic one. We applied it to identify the factors influencing alkalinity based on our compiled data set and to describe their interdependencies," says lead author of the study Nele Lehmann of the Hereon Institute for Carbon Cycles. The study was an international collaboration with the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) and funding from the German Academic Exchange Service (DAAD).

If warming were to continue slowly, alkalinity would drop by up to 68 per cent by 2100, depending on the watersheds. That means the ocean's ability to sequester CO2 would decrease significantly.

Rapidly progressing warming, on the other hand, would lead to higher temperatures and thus more precipitation in temperate climate zones. This would increase alkalinity by up to 33 per cent. "But that doesn't mean that more emissions are good for the climate. The impact of alkalinity is small compared to the amounts of man-made CO2 emitted around the world. The process of weathering unfolds its effects over much longer periods of time," Lehmann said.

Climate change is greatly accelerating the interplay of carbon cycling and weathering, which is fundamental to the development of life. The team first looked for existing data. The goal was to find as many alkalinity measurements as possible in the immediate vicinity of erosion measurement sites. To do this, the researchers searched databases and publications, and took samples themselves.

They conducted the investigation of the alkalinity factors using their new model. The biggest limitation is that the erosion rate measurements the researchers used had often only been taken over 20 years, and are complex and expensive. This made it difficult to produce the data set. Especially in the higher latitudes, there are hardly any measurements, so the study is limited to the mid-latitudes.

New questions in the Arctic

Next, Lehmann would like to investigate alkalinity and the erosion rate in the Arctic. There, the data situation is patchy. And climate change is clearly becoming noticeable, so potentially the biggest change in alkalinity flux could also occur. Whether erosion itself is changing as a result of climate change is of particular importance.



Nele Lehmann, Tobias Stacke, Sebastian Lehmann, Hugues Lantuit, John Gosse, Chantal Mears, Jens Hartmann & Helmuth Thomas: Alkalinity responses to climate warming destabilise the Earth’s thermostat, Nature communications,14, March 2023.


More information:

Website Institute of Carbon Cycles 

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