Heatwaves change carbon uptake in the ocean

Marine heat waves are causing a decline in sea ice in polar regions. Source: Hereon/Hanna Joerss

Researchers define marine heatwaves as periods during which seawater temperatures remain higher for at least five days than 90 percent of the temperature values measured for the same region during a 30-year reference period. They can last for several weeks. Until now, scientists assumed that warmer seawater can absorb less CO₂ because the physical solubility of gases decreases with increasing temperature. Previous studies from the open ocean clearly show a corresponding weakening of natural CO₂ uptake during heatwaves. In polar and subpolar shelf seas, however, the Hereon study shows the opposite effect.

Sea ice is melting, primary production is rising

Shelf seas are shallow marine areas along the edges of continents, such as regions of the Northwest Pacific and North Atlantic. They account for about 7 percent of the global ocean area. In polar regions, marine heatwaves lead to a reduction in sea ice. As a result, the area of open water can increase by up to 30 percent. A continuous exchange of gases takes place between the atmosphere and the ocean. Water absorbs, among other gases, CO₂ from the air. A thick layer of sea ice acts as a barrier and inhibits this exchange. When the ice melts, more CO₂ can be absorbed by the liquid water.

At the same time, the retreat of sea ice allows more light to penetrate the surface water, promoting the growth of phytoplankton. These microscopic algae form the basis of all life in the ocean food web. Through photosynthesis they produce biomass and remove carbon from the water, reducing the CO₂ concentration in surface waters.

The researchers analyzed extensive datasets on CO₂ exchange in global coastal and shelf seas from 1985 to 2020 using advanced ocean models and compared them with data from the open ocean. The result: during heatwaves, CO₂ uptake in coastal and shelf seas worldwide increased by an average of 11 percent, while it decreased by 8 percent in the open ocean. The team was able to show that the combination of an expanded ice-free water surface and enhanced CO₂ uptake can partially or even completely offset the reduction in CO₂ solubility caused by warming.

Important insights for climate projections

“At first glance, the issue seems simple: warmer water dissolves less CO₂. However, our results show that temperature is only one factor among many,” says Zhentao Hu, lead author of the study and doctoral researcher at the Hereon Institute of Coastal Systems – Analysis and Modeling.

“Our findings provide an important building block for a better understanding of the complex developments of the Earth's climate,” says co-author and Hereon researcher Dr. Wenyan Zhang. Zhang emphasizes that the study highlights one aspect of a highly complex system. “The fact that we observe increased CO₂ uptake in coastal and shelf seas does not necessarily mean that marine heatwaves have positive effects on the ocean and the climate. Marine heatwaves cause substantial changes worldwide. The melting of shelf ice contributes to sea-level rise, habitats, biodiversity and food webs are being altered, ocean salinity decreases, and the Earth warms more rapidly because less sunlight is reflected by ice.”

Today, the ocean absorbs a significant share of human-caused CO₂ emissions and is therefore an important factor in the Earth’s climate system. “It remains unclear whether the effect of additional CO₂ uptake in shelf seas will persist in a future in which marine heatwaves may become more frequent, longer-lasting and more intense,” says Zhang. The study results may help improve the assessment and calculation of future climate scenarios. The interaction of all factors within the highly complex climate system must be investigated in greater detail in future studies.

Cutting-edge research for a changing world

Helmholtz-Zentrum Hereon’s scientific research aims at preserving a world worth living in. To this end, around 1000 employees generate knowledge and research new technologies for greater resilience and sustainability - for the benefit of the climate, the coast and people. The path from idea to innovation leads through a continuous interplay between experimental studies, modeling and AI to digital twins that map the diverse parameters of climate and coast or human biology in the computer. This is an interdisciplinary approach that spans from the fundamental scientific understanding of complex systems to scenarios and practical applications. As an active member of national and international research networks and the Helmholtz Association, Hereon supports politics, business and society in shaping a sustainable future by transferring the expertise it has gained.