What does it mean to have “uncertainty” in sea level rise projections?

Uncertainty in sea level rise projections from the IPCC AR6 refers to the range of possible future outcomes due to various factors that affect the precision and reliability of predictions. It reflects the complexity of the climate system and the limitations of current scientific understanding and modeling capabilities.

Uncertainty arises from several sources:

• Climate Forcing Uncertainty: Variability in future greenhouse gas emissions and the effectiveness of climate policies influence how much the climate will change. Different socioeconomic pathways, such as the Shared Socioeconomic Pathways (SSPs), represent this uncertainty.
• Model Uncertainty: Differences in climate models, which use various assumptions and methods to simulate processes like ice sheet dynamics, ocean circulation, and thermal expansion, contribute to varying results.
• Ice Sheet and Glacier Dynamics: The behavior of ice sheets and glaciers, especially in Antarctica and Greenland, is one of the largest sources of uncertainty. These processes are complex and not yet fully understood, particularly the potential for rapid and irreversible ice loss.

Additionally, deep uncertainty refers to scenarios where experts cannot agree or confidently predict key processes or future developments, such as the potential for abrupt ice sheet collapse or unpredictable socio-political actions affecting climate policies. This type of uncertainty highlights the challenges in making precise forecasts when our scientific understanding is incomplete, or the future is inherently unpredictable. As a result, decision-making under deep uncertainty often involves preparing for a wide range of possible outcomes and developing robust, adaptive strategies to manage risks associated with sea level rise.