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

There are two types of uncertainty that are important to consider when thinking about future sea level changes: 1) uncertainty in representing or modeling the physical processes that cause sea level change known as process uncertainty, and 2) uncertainty in how human behavior will drive future emissions and ensuing warming known as emissions uncertainty. The suite of projections in this report captures both process uncertainty and emissions uncertainty.

Process uncertainty is associated with how well we currently understand why sea level has changed in the past and how it will change in the future at specific times and locations. To capture process uncertainty in sea level rise projections, there is a range of uncertainty around each individual scenario (i.e., the low/17th%, median/50% and high/83rd% values for each particular scenario). The farther forward in time we move, the greater the uncertainty around each projection.

Emissions uncertainty is captured in the range between the five global mean sea level rise scenarios: Low (1 foot; 0.3 meters), Intermediate Low (1.6 feet; 0.5 meters), Intermediate (3.3 feet; 1.0 meter), Intermediate High (4.9 feet; 1.5 meters), and High (6.6 feet; 2.0 meters). In other words, the range between the five sea level scenarios is closely connected to emissions uncertainty, while the range around a given scenario is associated with process uncertainty.

In addition to process and emissions uncertainty, there is still scientific discussion and investigation underway on the potential for rapid ice sheet melt and collapse, sometimes referred to as low confidence processes. Currently there is no scientific consensus on whether rapid melt will occur and, if it does, what that process will look like. Given that it is possible, those processes are included in international and federal assessments. The possibility of rapid ice sheet melt is a significant driver in reaching the highest scenarios in the 2022 technical report.