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. 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 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 Andreas Oschlies, lead author of the study and researcher GEOMAR.
Ocean fertilisation and seaweed sinking
The study identifies several biotic methods as particularly critical — including ocean fertilisation 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 4 to 44 times greater than the oxygen gain expected from reduced global warming.’
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 marine carbon dioxide removal research and deployment efforts.
Microbiologist Caroline Slomp: ‘Oceans are complex and are already heavily under pressure. If we intervene with large-scale measures, we must ensure that we are not further threatening marine life.”