How Will Sea Levels Change in the Future?

IPCC AR6 Sea Level ProjectionsSea Level Projections From the IPCC 6th Assessment Report

The IPCC 6^th Assessment Report (AR6) report released in 2021 summarized our latest understanding of sea level rise and produced new projections of future sea level rise. The updated IPCC AR6 sea level projections are formed by combining different projections of individual processes that cause sea level change within a consistent framework. The AR6 uses shared socioeconomic pathways (SSP) scenarios, which predict global socioeconomic changes, to model sea level rise. There are five main scenarios based on processes we understand well and two additional ones (one low and one high emissions) that include uncertain ice sheet processes. These uncertain processes involve unexpected ice-shelf disintegration in Antarctica, sudden marine ice-sheet and ice-cliff instability, and rapid changes in Greenland’s ice balance. We have limited scientific understanding of these processes, making it hard to project their impact on sea level rise. Therefore, the scenarios that include these processes are considered uncertain.

Despite the uncertainty, these processes are important for understanding the full range of possible sea level rise. The AR6 said that these high-impact processes are less likely to occur soon but could become significant by the end of the 21st century and beyond. At lower warming levels (below 2°C by 2100), these processes are not expected to significantly contribute to sea level rise until after 2100, if at all. As a result, there is less sea level rise acceleration before 2050 and the possibility of more acceleration only towards the end of the 21st century and beyond.

Sea Level ScenariosSea Level Scenarios From the 2022 Interagency Technical Report

The 2022 Interagency Technical Report uses the foundational knowledge and projections from the IPCC AR6 to develop specific sea level rise scenarios tailored for practical use in the United States. This report is produced by multiple U.S. federal agencies and aims to provide actionable information for policy-makers, planners, and stakeholders. The 2022 Interagency Technical Report adapts these projections to a more localized context, providing detailed scenarios for different regions and communities in the U.S. It includes site-specific vertical land motion and other localized factors to create relevant sea level scenarios. While the IPCC AR6 provides a global scientific foundation, the 2022 Interagency Technical Report translates this knowledge into practical scientific information for U.S. policy-makers and planners. This helps bridge the gap between global scientific assessments and local implementation needs.

Creating Sea Level ScenariosCreating the Sea Level Scenarios

The five sea level scenarios are created following a specific process:

1. An assessment of the projections and science contained in IPCC AR6 is used to determine a plausible range of global mean sea level rise of between 1 ft (0.3 m) and 6.6 ft (2.0 m) in 2100. The low end – 1 ft by 2100 - represents the best possible case for future sea level rise, while the high end of 6.6 ft by 2100 represents a very pessimistic case.
2. The IPCC AR6 created medium confidence sea level projections for multiple SSPs. In addition, low confidence projections allowing for potential contributions from rapid ice-sheet loss processes with unknown likelihood were also created for a high and low SSP. For each SSP, an ensemble of thousands of “samples” of the trajectory of sea level rise from 2020 to 2150 was produced. These samples cover a range of possibilities, and the associated distribution of these samples for each SSP is shown graphically on the left of the schematic below.
3. Five global mean sea leve targets or “gates” in 2100 that span the plausible range from 1 ft (0.3 m) to 6.6 ft (2 m) are defined. The full set of samples is filtered to find the ones that fit through each of these five gates +/- 2 cm. The SSPs with the lowest emissions primarily build the lowest sea level scenarios. The SSPs with the highest emissions plus the scenario that takes into account rapid ice sheet loss primarily build the higher scenarios.
4. Finally, five time-varying Sea Level Scenarios of GMSL and associated local relative sea level rise are constructed by collecting all the samples from across SSPs that fit through the defined gates. Logically, the Intermediate Low and Intermediate scenarios will include the highest number of samples, while the other sea level scenarios will have comparatively less. The individual samples of sea level projections provide important contextual information for each of the Sea Level Scenarios, carrying with them a range of warming levels and emissions pathways. While it is not possible to directly assign probabilities to each of the sea level scenarios, additional assumptions about the future (e.g. warming level) allow for the assignment of a probability of exceeding a particular Sea Level Scenario in that assumed future.

Storylines for Sea Level ScenariosStorylines for Sea Level Scenarios

• Low (0.3 m or 1.0 ft by 2100): The target of 1 foot of increase in global sea level rise by 2100 is set under the assumption of the current rate of sea level rise continuing on into the future. This assumption is inconsistent with current observations of an acceleration in sea level rise, but could still be considered plausible under the most aggressive emission reduction scenarios. This scenario is on the lower bounding edge of plausibility given current warming and sea level trajectories.
• Intermediate Low (0.5 m or 1.6 ft by 2100): This scenario arises under a range of future emissions pathways, associated with a range of future warming levels and socioeconomic development pathways. Given current sea level observations and estimates of future warming, this scenario provides a reasonable estimate of the lower bound for the most likely sea level rise by 2100. Since low confidence processes (e.g., rapid ice sheet melt) are not important to this scenario, the range of possible sea level rise after 2100 does not expand significantly.
• Intermediate (1.0 m or 3.3 ft by 2100): This scenario is driven dominantly by high emissions scenarios, and thus higher warming levels. Projections including contributions from low confidence processes provide about 25% of the pathways for reaching the scenario target by 2100. This scenario provides a reasonable upper bound for the most likely range of sea level rise by 2100.
• Intermediate High (1.5 m or 4.9 ft by 2100): This scenario is heavily reflective of a world where rapid ice sheet loss processes are contributing to sea level rise, associated with intermediate to high future emissions, and high warming. The amount of sea level rise by 2100 corresponds to other scientific estimates of plausible high-end projections.
• High (2.0 m or 6.6 ft by 2100): This scenario only arises with high future emissions and high warming with large potential contributions from rapid ice sheet loss processes. Deep uncertainties and ambiguity embedded in this scenario frame a worst case beyond 2100 as we currently understand it, and a statement about the likelihood of reaching this scenario is not possible.

Contribution of WarmingHigher Scenarios are More Likely with Higher Warming

Given that sea level rise is highly dependent on if and how fast the world’s nations reduce global emissions and mitigate warming trends, there are no probabilities that can be assigned directly to each of the Sea Level Scenarios. Instead each scenario combines information on a potential future pathway for warming levels and emissions. Prossible future warming levels can be translated into the probability of exceeding a particular Sea Level Scenario in that assumed future. For higher future warming levels, it is virtually certain we will pass the Low and Intermediate Low Scenarios. On the other hand, if we keep future warming low, the likelihood of surpassing the higher scenarios is greatly reduced.

Next: What are the impacts of sea level change?