Marine heat waves—periods when ocean temperatures rise above 90% of historical values for a region over a 30-year period, lasting at least five days—have been found to increase the absorption of carbon dioxide (CO₂) in coastal and shelf seas by 11%, according to a new study. The research, led by the Helmholtz-Zentrum Hereon and published in Nature Communications, highlights that this effect is most noticeable in polar and subpolar shelf seas in the Northern Hemisphere, where sea ice is rapidly disappearing due to global warming. Previously, scientists believed that warmer seawater would absorb less CO₂ because the solubility of gases decreases as temperature increases. In open oceans, this assumption has been supported by studies showing reduced CO₂ absorption during heat waves. However, the new study reveals a different pattern in coastal and shelf seas. As sea ice melts, it exposes more open water—up to 30% more in some areas—which allows greater CO₂ absorption. Additionally, the increased light penetration into the water promotes the growth of phytoplankton, microscopic plants that take in CO₂ through photosynthesis, further reducing CO₂ levels in surface waters. The researchers examined global data on CO₂ exchange in coastal and shelf seas from 1985 to 2020 using advanced ocean models, comparing them with data from the open ocean. They found that during marine heat waves, CO₂ absorption in coastal and shelf seas increased by 11% on average, while in open oceans, it decreased by 8%. The study suggests that the combination of increased open water and enhanced biological activity can counteract the reduced solubility of CO₂ due to warming. Zhentao Hu, the lead author and doctoral researcher at the Hereon Institute of Coastal Systems, explains that while the initial assumption was that warmer water absorbs less CO₂, the study shows that temperature is just one of many factors influencing this process. Dr. Wenyan Zhang, a co-author, emphasizes the complexity of the climate system, noting that while coastal areas may absorb more CO₂ during heat waves, these events also cause significant environmental changes, such as melting ice that contributes to sea level rise, altered habitats, and shifts in biodiversity and food webs. Additionally, the melting ice reduces ocean salinity and decreases the Earth's ability to reflect sunlight, leading to further warming. The findings may improve future climate models and assessments by highlighting the interplay of various factors in the ocean system. The study underscores the need for more detailed research into the complex interactions within the climate system to better predict future environmental changes.