There is a growing consensus among scientists that our planet is on track to surpass the 1.5℃ warming threshold. In fact, research suggests that global warming may temporarily exceed the 2℃ threshold if atmospheric carbon dioxide (CO₂) levels peak beyond what was initially anticipated. This phenomenon, known as climate overshoot, poses significant risks and may lead to irreversible changes that will impact our planet for generations to come.
Recent research published in Communications Earth & Environment explores the implications of a climate overshoot specifically on ocean habitats. The study indicates that increased atmospheric CO₂ levels will result in changes in water temperatures and oxygen levels, leading to the decline of viable ocean habitats. These changes are projected to persist for centuries, even after CO₂ levels have peaked and declined.
The researchers conducted simulations using Earth system models as part of the Coupled Model Intercomparison Project (CMIP6), which underpins the latest assessment reports by the Intergovernmental Panel on Climate Change (IPCC). They analyzed multi-model results from two CMIP6 experiments that simulated a climate overshoot. One experiment considered a climate scenario with an overshoot occurring within this century, while the other focused on the reversibility of a climate overshoot.
The study specifically examined the combined effects of changes in ocean temperature and oxygen levels. Warmer water holds less dissolved oxygen, which has a direct impact on the long-term viability of marine ecosystems. The researchers used a metabolic index to quantify these impacts, which measures the energy balance of individual organisms. A viable ecosystem requires oxygen supply to exceed demand, and as the balance shifts, ecosystems become increasingly precarious.
Under global warming conditions, the study found a significant increase in metabolic demand and a reduction in oxygen supply due to deoxygenation. This negatively affects the long-term viability of marine species and their habitats. The findings indicate that water volumes capable of providing viable habitats will decrease across all climate overshoot experiments and models. This decrease in viable habitats is projected to persist for centuries, even after global average temperature recovers from the overshoot.
Some specific species, such as tuna, which rely on well-oxygenated surface waters, will face significant challenges. Their habitats will be compressed toward the surface for hundreds of years, according to the research. This poses concerns for fisheries dependent on these species, as changes in their distribution will impact fishing grounds and productivity. The study emphasizes that ecosystems must adapt to these changes, or risk collapse with severe environmental, societal, and economic implications.
Until now, most research has primarily focused on ocean warming. However, the combination of temperature and deoxygenation, as examined in this study, indicates that warming may harm marine ecosystems for an extended period, even after global mean temperatures have peaked. In light of this, resource management strategies need to be reevaluated to avoid compromising species abundance and food security.
The study underscores the importance of avoiding significant climate overshoots and highlights the urgency to reduce emissions promptly. It is crucial to adhere to the temperature targets established in the Paris Agreement, aiming for net-zero emissions by mid-century and keeping warming “well below” 2℃. Climate overshoots not only matter in terms of their peak value but also in terms of how long temperatures remain above the target. Returning from an overshoot is preferable to remaining at a higher level, but it is still far from ideal.
It is essential to note that the study’s assessment of potential future changes heavily relies on Earth system models. Therefore, it is vital to continue improving these models to gain a better understanding of climate overshoots and the reversibility of the climate system. This includes ongoing observations to validate the models’ accuracy. Additionally, new experimental frameworks should be developed to explore strategies that can minimize the long-term impact of a climate overshoot.
The implications of a climate overshoot on ocean habitats are significant and far-reaching. The research highlights the urgent need to address climate change to ensure the long-term viability of marine ecosystems. Shrinking habitats and changes in species distribution pose challenges for fisheries, environmental stability, and global food security. By taking immediate action to reduce emissions and avoid climate overshoots, we can mitigate these risks and safeguard the health of our oceans for future generations.