As our planet continues to warm, there is a growing consensus that we are likely to surpass the critical 1.5℃ warming threshold. In fact, research suggests that we may even temporarily exceed the 2℃ threshold if atmospheric carbon dioxide (CO₂) levels peak higher than anticipated. This exceeding of emissions targets is known as a climate overshoot and could lead to irreversible changes in our lifetime. These changes include sea-level rise, loss of functional ecosystems, increased risk of species extinction, and the loss of glaciers and permafrost.
In a recent study published in Communications Earth & Environment, researchers examined the implications of a climate overshoot specifically for the world’s oceans. Through analysis of simulations using Earth system models as part of the Coupled Model Intercomparison Project (CMIP6), the researchers found that increased water temperatures and reduced oxygen levels associated with a climate overshoot would have long-lasting detrimental effects on ocean habitats. The impacts would persist for centuries, long after atmospheric CO₂ levels have peaked and declined.
One of the key findings of the study was the link between changes in ocean temperature and oxygen levels. Warmer water has a reduced capacity to hold dissolved oxygen, leading to deoxygenation. As a result, ecosystems can become less viable as the supply of oxygen becomes closer to the demand. Under global warming, this decrease in supply and increase in metabolic demand has already been observed in the ocean.
To quantify the impacts of these changes, the researchers used a metabolic index, which describes the aerobic energy balance of individual organisms. In viable ecosystems, the supply of oxygen needs to exceed the demand. However, as ocean temperatures rise and deoxygenation occurs, the closer the supply gets to demand. Eventually, demand may exceed supply, rendering these ecosystems non-viable.
Using the metabolic index, the researchers assessed the long-term viability of 72 marine species and their habitats under changing conditions. Across all climate overshoot experiments and models, the findings showed a decrease in water volumes that can provide viable habitats. This decrease persisted for centuries, even after global average temperature recovered from the overshoot.
This decline in viable habitats raises concerns about the future of marine species. For example, species like tuna, which rely on well-oxygenated surface waters, may be compressed towards the surface for hundreds of years. This has significant implications for fisheries that depend on such species, as changes in their distribution could affect fishing grounds and productivity.
The combination of temperature and deoxygenation studied in this research reveals that warming can have long-term impacts on marine ecosystems, even after global mean temperatures have peaked. It is crucial to consider resource management to avoid compromising species abundance and food security in the face of these changes.
Climate overshoots not only matter in terms of their peak value but also in terms of how long temperatures remain above the target. While it is better to return from an overshoot, staying at the higher level for a prolonged period can have irreversible climate change impacts. Therefore, it is imperative to drastically reduce emissions now to avoid a significant climate overshoot. This includes reaching net-zero emissions by mid-century and keeping warming “well below” the 2℃ target set by the Paris Agreement.
The findings of this study heavily rely on Earth system models, and it is essential to continue improving these models to better understand key questions surrounding climate overshoots and the reversibility of the climate system. This improvement should include sustained observations to validate the models’ findings. Additionally, new experimental frameworks need to be developed to explore strategies that can minimize the long-term impact of a climate overshoot if one were to occur.
A climate overshoot could have significant and long-lasting impacts on ocean habitats. The combination of warmer waters and deoxygenation can reduce viable habitats for marine species, leading to detrimental effects on ecosystems, fisheries, and food security. Drastically reducing emissions and implementing sustainable resource management strategies are crucial to avoid a major climate overshoot and protect the health and viability of our oceans for future generations. Further improvements in models and observations are also needed to enhance our understanding of these complex processes and develop effective mitigation and adaptation measures.
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