Photo by Luis Graterol
An international study led by the Helmholtz-Zentrum Hereon has found that marine heatwaves increase the amount of CO₂ absorbed by coastal and shelf seas worldwide, even as the same extreme events weaken carbon uptake in the open ocean. The research, published in Nature Communications, found that CO₂ uptake in coastal and shelf seas increased by an average of 11% during heatwaves, while open-ocean uptake fell by 8% over the same events.
A counterintuitive result
The finding runs against a long-standing assumption in ocean science: because gases dissolve less easily in warmer water, scientists have generally assumed that warmer seawater absorbs less CO₂, a pattern that previous studies of the open ocean have consistently confirmed. Zhentao Hu, lead author of the study and a doctoral researcher at Hereon’s Institute of Coastal Systems – Analysis and Modeling, said: “At first glance, the issue seems simple: warmer water dissolves less CO₂. However, our results show that temperature is only one factor among many.”
The effect is concentrated in polar and subpolar shelf seas of the Northern Hemisphere – shallow waters along continental margins such as the North Atlantic and Northwest Pacific that together make up only about 7% of the global ocean’s surface area. Researchers define a marine heatwave as a period in which sea temperature exceeds the local 90th-percentile threshold, measured against a 30-year reference period, for at least five consecutive days, with events sometimes lasting several weeks.
Why shelf seas behave differently
In these ice-affected regions, heatwaves can shrink sea ice cover so sharply that the area of open water increases by as much as 30%. A thick layer of sea ice normally acts as a barrier to gas exchange between ocean and atmosphere; when it melts, more CO₂ can be absorbed by the exposed liquid water beneath. Losing that ice cover also lets more sunlight reach the surface, which promotes the growth of phytoplankton – the microscopic algae that form the base of the ocean food web. As phytoplankton photosynthesise, they draw carbon out of the water and lower its CO₂ concentration, further encouraging the ocean to pull in additional CO₂ from the air.
Analysing datasets on CO₂ exchange in global coastal and shelf seas from 1985 to 2020 using ocean models, the researchers showed that this combination of expanded ice-free water and enhanced biological carbon fixation can partially or even completely offset the reduction in CO₂ solubility that warming would otherwise cause. Co-author Dr Wenyan Zhang said: “Our findings provide an important building block for a better understanding of the complex developments of the Earth’s climate.”
A caveat on the finding
Zhang was explicit that the result should not be read as good news about marine heatwaves overall: “The fact that we observe increased CO₂ uptake in coastal and shelf seas does not necessarily mean that marine heat waves have positive effects on the ocean and the climate. Marine heat waves 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.” She added that it remains unclear whether the additional uptake in shelf seas will persist as marine heatwaves become more frequent, longer-lasting and more intense in the future.
What comes next
The authors say their findings should help refine how future climate models represent the interaction of sea ice, ocean circulation, nutrient cycling and biological productivity in coastal carbon budgets – factors that models often treat separately or simplify. The study does not propose or evaluate any technology for deliberately exploiting this effect to remove atmospheric CO₂; its stated purpose is to improve scientific understanding of a natural process that affects how the wider climate system is assessed, not to offer an applied climate solution.