Why does this matter? The science of how climate change will impact the ocean – and conversely how ocean dynamics will affect climate change – is complex. Recent studies have pointed out the rate of warming in the ocean, which absorbs 93% of the heat accumulated in the atmosphere, is increasing – even in the relatively understudied deep ocean. These changes are thus likely to disrupt ecosystems as species are forced to relocate.
This latest study in Science looks at the heat tolerance of 700 species of fish across their entire lifecycles, which previous research has not typically focused on. Looking across fish life-stages, the researchers found that both spawning adult fish and embryos have a narrower temperature tolerance range (of around 8C) than larvae and non-reproductive adults (around 27C).
This is partly due to their greater oxygen needs, and oxygen is less soluble in warmer waters. Separate studies have pointed more generally to the issue of oxygen depletion in oceans as they warm, with 700 ocean sites experiencing low oxygen levels compared to 45 in the 1960s.
The German researchers’ findings are significant. Earlier research on adult fish alone had suggested just 5% would struggle with temperature rises of 5C. This study suggests that even if the Paris Agreement on climate change’s preferred goal of restricting warming to 1.5C is met, temperatures will still be too high for 10% of fish in their current habitats.
While one might think species may be able to migrate to different areas, one of the researchers says warming will occur too quickly for species to adapt and that adequate spawning sites in cooler locations are not always available.
The analysis could have wide implications for both ocean ecosystems and fisheries, which have seen their productivity decline on average 4.1% since 1930 and at up to 35% in some regions.
The new findings also highlight the potential disruption to marine ecosystems in the shorter term, with ocean heatwaves such as 2015-16’s “the blob” off the west coast of the US potentially being more detrimental to fish breeding than previously considered – on top of its already significant consequences. The blob was also linked to the death of a million North Pacific seabirds.
Ocean warming is likely to continue even if global emissions are significantly reduced, laying bare the need to quickly address climate change. Alongside this, environmental groups including Greenpeace have highlighted that boosting ocean health can help combat climate change in itself by allowing the ocean to effectively absorb more CO2.