What helps the climate is not automatically good for the ocean
GEOMAR study analyses the impact of marine carbon dioxide removal methods on global ocean oxygen levels
Helmholtz Centre for Ocean Research Kiel (GEOMAR)
Global warming is the primary cause of the dramatic loss of oxygen in the ocean — approximately two percent of the ocean’s oxygen inventory has been lost over the past decades, with serious ecological consequences already today. Any additional warming will lead to additional oxygen loss. One might therefore expect that climate mitigation measures would help to counteract oxygen decline. Yet a new study reveals that many proposed marine carbon dioxide removal (mCDR) methods – especially those based on biological processes – could in fact intensify oxygen loss in the ocean.
“What helps the climate is not automatically good for the ocean,” says Prof. Dr Andreas Oschlies, lead author of the study and head of the Biogeochemical Modelling research division at GEOMAR. Together with an international team that is part of the UNESCO Global Ocean Oxygen Network (GO2NE), he conducted a comprehensive assessment using idealised global model simulations to analyse both the direct impacts of various mCDR approaches on ocean oxygen and their indirect effects through climate mitigation. The results have now been published in Environmental Research Letters.
Ocean fertilisation and seaweed sinking among the most critical approaches
The study identifies several biotic mCDR methods as particularly critical — including ocean fertilisation, large-scale macroalgae farming followed by sinking of the biomass, and artificial upwelling of nutrient-rich deep water. These approaches involve the enhancement of photosynthetic biomass production, followed by its decomposition in the ocean interior. This remineralisation process consumes oxygen — at levels comparable to the current rate of global deoxygenation caused by ocean warming.
“Methods that increase biomass production in the ocean, and subsequently lead to oxygen-consuming decomposition, cannot be considered harmless climate solutions,” says Oschlies. “Our model simulations show that such approaches could cause a decrease in dissolved oxygen that is four to 40 times greater than the oxygen gain expected from reduced global warming.”
By contrast, geochemical mCDR approaches that do not involve nutrient input – such as ocean alkalinity enhancement through the addition of alkaline substances based on limestone – appear to have minimal effects on ocean oxygen levels and are comparable to simply reducing CO₂ emissions.
Among all methods examined, only large-scale macroalgae farming with biomass harvesting (i.e. removal from the ocean) resulted in an overall increase in oceanic oxygen levels. In this case, no additional oxygen is consumed within the marine environment, and the removal of nutrients limits oxygen consumption elsewhere. Model results suggest that if deployed at sufficient scale, this approach could even reverse past oxygen losses — providing up to ten times more oxygen than has been lost due to climate change within a century. However, here it is the removal of nutrients that would negatively impact biological productivity in the ocean.
Call for systematic monitoring of ocean oxygen
Given these findings, the authors advocate for mandatory inclusion of oxygen measurements in all future mCDR research and deployment efforts.
“The ocean is a complex system which is already heavily under pressure,” says Oschlies. “If we intervene with large-scale measures, we must ensure that, no matter how good our intentions are, we are not further threatening marine environmental conditions that marine life depends on.”
Background: Carbon dioxide removal as part of climate strategy
Even with ambitious climate policy, Germany is expected to emit 10 to 20 percent of today’s greenhouse gas levels in three decades’ time — continuing to drive global warming. Carbon dioxide removal (CDR) is therefore being considered to help reach net-zero emissions. The ocean is the key player in the global carbon cycle due to its natural CO2 uptake and its huge storage capacity. However, these processes typically occur over long timescales. Marine Carbon Dioxide Removal (mCDR) approaches aim to accelerate these natural processes, thereby increasing the ocean’s carbon uptake capacity.
Journal
Environmental Research Letters
Article Title
Potential impacts of marine carbon dioxide removal on ocean oxygen
Article Publication Date
12-Jun-2025
Increased forest fires due to climate change could alter oceanic CO2 absorption, according to a BSC study
According to an article published in the prestigious journal Nature Climate Change, atmospheric particle emissions generated by forest fires could double current projections by the end of the 21st century due to climate change
Forest fires are a fundamental force in Earth's dynamics with a direct impact on human health, food security, and biodiversity. From air quality to landscape configuration and resource availability, the consequences of fire have influenced the development of society throughout history. Their effects on the oceans, though less known, are equally significant.
Fires release particles and nutrients into the atmosphere that travel long distances and are deposited in ocean waters, influencing the development of phytoplankton, aquatic photosynthetic microorganisms that absorb CO₂ from the atmosphere. This phenomenon, similar to when agricultural land is fertilized to increase production, influences Earth's carbon cycle and, therefore, has consequences for global climate balance.
A new study led by researchers from the Barcelona Supercomputing Center – Centro Nacional de Supercomputación (BSC-CNS) and ICREA, published in the journal Nature Climate Change, provides new information on the link between forest fires and marine ecosystems. The work shows that climate change could significantly increase fires, especially in boreal areas, and, therefore, associated iron emissions, as well as the supply of this micronutrient to the ocean, boosting phytoplankton productivity.
"Climate-driven fires arise from more favorable weather conditions for fire, such as low humidity and high temperatures, which in turn are influenced by anthropogenic climate change. Understanding these fires and their impact on the fertilization of key ocean regions like the North Atlantic is essential for more accurately predicting future atmospheric CO2 levels," states ICREA and AXA Professor Carlos Pérez García-Pando, co-leader of the BSC's Atmospheric Composition group and senior co-author of the study.
Researchers have used advanced climate models to project the increase in iron emissions from fires, especially at high latitudes of the Northern Hemisphere. In areas of the North Atlantic characterized by iron scarcity, the deposition of these nutrients could increase the productivity of phytoplankton, which is not only the base of the marine food chain but also fundamental in the carbon cycle, as these microorganisms absorb large amounts of CO2 from the atmosphere through photosynthesis.
Increase in marine productivity in the North Atlantic
The study concludes that this increase in iron emissions from climate-driven fires, projected to be between 1.7 and 1.8 times higher than current projections that only consider the direct effect of human activity on their future evolution, could increase marine productivity in the North Atlantic due to atmospheric deposition by up to 40% in the summer months by the end of the 21st century.
However, the research also considers the projected decrease in other essential nutrients in vast oceanic areas due to climate change, which could diminish the ocean's capacity to absorb CO2 and attenuate the positive effects of increased iron deposition.
"Quantifying this nutrient source for phytoplankton is important for gaining a more precise idea of how much CO2 will remain in the atmosphere in the coming decades. By determining how climate-driven fires will increase the supply of iron to the ocean, we are revealing a crucial feedback loop in the Earth system that we must understand to address climate change," indicates Elisa Bergas-Massó, BSC researcher and lead author of the study.
The work points to the need for a multidisciplinary approach to understanding the role of fire in the Earth system, encompassing atmospheric science, oceanography, and climate policy, as well as the importance of improving observations and models to better quantify these effects and their final impact on CO2 absorption.
"The results of this study are crucial for improving projections of the carbon cycle and ocean health under a changing climate, paving the way for more accurate climate models and better-informed future climate change adaptation policies," concludes Maria Gonçalves Ageitos, BSC and UPC researcher and senior co-author of the study.
Reference: Bergas-Masso, E., Hamilton, D.S., Myriokefalitakis, S., Rathod, S., Gonçalves Ageitos, M., and Pérez García-Pando, C. (2025). Future climate-driven fires may boost ocean productivity in the iron-limited North Atlantic. Nature Climate Change, DOI:10.1038/s41558-025-02356-4.
Journal
Nature Climate Change
Article Title
Future climate-driven fires may boost ocean productivity in the iron-limited North Atlantic
Article Publication Date
13-Jun-2025
