The Long-Term Impacts of Climate Overshoot on the Oceans

The effects of climate change are becoming increasingly evident, and there is growing consensus among scientists that our planet is likely to pass the 1.5°C warming threshold. In fact, research suggests that global warming may temporarily exceed the 2°C threshold due to unexpected levels of atmospheric carbon dioxide (CO₂) emissions. This phenomenon, known as a climate overshoot, could have irreversible consequences for our planet, including sea-level rise, extinction of species, and loss of important ecosystems such as glaciers and permafrost.

A newly published study in Communications Earth & Environment explores the implications of a climate overshoot specifically for the oceans. The researchers conducted simulations using Earth system models as part of the Coupled Model Intercomparison Project (CMIP6), which is crucial for the Intergovernmental Panel on Climate Change (IPCC) assessment reports.

They analyzed multi-model results from two different CMIP6-developed experiments simulating a climate overshoot. The experiments focused on changes in ocean temperature and oxygen levels, as these factors are closely linked. Warmer water holds less dissolved oxygen, which can impact the viability of marine ecosystems in the long term.

Decrease in Viable Ocean Habitats

The study found that across all climate overshoot experiments and models, there was a consistent decrease in water temperatures and oxygen levels, resulting in a decline in viable ocean habitats. This decrease persisted for centuries, even after atmospheric CO₂ levels had peaked and declined. The researchers used a metabolic index to assess the energy balance of individual organisms in different ecosystems. In viable ecosystems, the supply of oxygen should exceed the demand. However, under global warming, there is an increase in metabolic demand and a reduction in oxygen supply due to deoxygenation.

The study’s findings raise concerns about shrinking habitats for marine species. For instance, species like tuna, which rely on well-oxygenated surface waters, may face significant challenges as their habitat is compressed toward the surface for hundreds of years. This has implications for fisheries that depend on these species, as changes in distribution could affect fishing grounds and productivity.

Furthermore, the combination of temperature and deoxygenation studied in this research indicates that warming could harm marine ecosystems long after global mean temperatures have peaked. This highlights the need for resource management strategies to prevent compromising species abundance and food security.

The study emphasizes the critical importance of avoiding climate overshoots. While it may be better to return from an overshoot than to remain at higher temperatures, overshooting temperature targets set by the Paris Agreement can have irreversible consequences. Therefore, it is crucial to make significant efforts to reduce emissions immediately, aiming to achieve net-zero emissions by mid-century and keep warming “well below” the 2°C target.

It is essential to continuously improve Earth system models to better understand climate overshoots and the reversibility of the climate system. Sustained observations are needed to validate these models and ensure their accuracy. Additionally, new experimental frameworks must be developed to explore effective strategies in the event of a climate overshoot, minimizing its long-term impact.

The study’s findings highlight the long-term impacts of climate overshoot on the oceans. As global temperatures continue to rise, the decrease in viable ocean habitats poses a significant threat to marine ecosystems and the species that depend on them. Urgent action is required to reduce greenhouse gas emissions and mitigate the effects of climate change. By prioritizing sustainable resource management and improving our models and experimental frameworks, we can work towards minimizing the long-term consequences of climate overshoots and securing a healthier future for our oceans.