How Will Sea Levels Change in the Future?

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

The AR6 (Sixth Assessment Report) sea level projections by the Intergovernmental Panel on Climate Change (IPCC) are built using comprehensive assessments of a wide range of physical processes contributing to future sea level changes. The report integrates results from global climate models and specialized research on individual components affecting sea level, such as ice sheet dynamics, ocean thermal expansion, glacier melt, and land water storage changes. By combining these processes, AR6 provides a probabilistic framework that accounts for both well-understood mechanisms and more uncertain aspects of future sea level rise.

Models play a crucial role in the AR6 sea level projections, providing a scientific basis for estimating how different processes will contribute to future sea level rise. The AR6 report uses a combination of models to simulate the physical behavior of the climate system and its impact on sea level. Each model type focuses on different components of the Earth system and employs both observational data and theoretical physics to project future outcomes under various greenhouse gas emission scenarios.

Climate models, such as those used in the Coupled Model Intercomparison Project (CMIP6), simulate how the Earth’s atmosphere, oceans, and land surfaces interact under different levels of greenhouse gas concentrations. These models are used to project the sterodynamic sea level response, which is one of the primary contributors to global sea level rise. By modeling how heat is distributed and absorbed by the oceans, these simulations can estimate how much and how quickly sea levels will rise due to thermal expansion and shifts in ocean patterns.

Ice sheet models are used to project mass loss from the Greenland and Antarctic ice sheets. These models incorporate physical processes such as surface melt, ice flow dynamics, and interactions with the ocean and atmosphere. Because of the complex and uncertain nature of ice sheet behavior—particularly the potential for rapid ice sheet collapse or destabilization—different modeling approaches are used to bracket the possible outcomes. Some models use simplified physics to explore extreme scenarios, while others are more detailed and focus on understanding the mechanisms driving ice loss.

Glacier models estimate the contribution of smaller glaciers to sea level rise. These models typically use a combination of empirical data and climate model outputs to simulate glacier mass balance over time. They take into account factors such as regional temperature and precipitation patterns, which influence how glaciers accumulate and lose ice. Since glaciers respond relatively quickly to climate changes, these models provide a clearer, though regionally variable, picture of glacier-driven sea level rise.

Finally, models of land water storage and vertical land motion help to complete the picture. For example, models of human water use and land subsidence are integrated with climate projections to understand how local and regional processes affect sea level. The combined output from all these models is then used to generate probabilistic projections that reflect both the uncertainties in each individual process and the overall range of possible futures. This approach allows AR6 to provide comprehensive and nuanced predictions of sea level rise, which are crucial for informing policymakers and communities about potential risks and adaptation strategies.

Sea Level Projections Vary Across the GlobeSea Level Projections Vary Across the Globe

Regional and locally specific sea level projections are derived from the global sea level rise estimates presented in AR6 but are adjusted to account for a variety of regional factors that influence how sea level changes are experienced differently around the world. These factors include ocean dynamics, gravitational and rotational effects, land subsidence or uplift, and regional climate patterns. The combination of these influences is what makes regional and local projections more complex than global averages.

First, ocean dynamics play a significant role in regional projections. The AR6 uses high-resolution ocean models to simulate how global warming will impact ocean circulation and sea level patterns. For instance, warming may lead to changes in major ocean currents like the Gulf Stream, which can alter sea levels along coastlines near these currents. Regional variations in the expansion of the ocean due to heat absorption are also important, as some regions of the world’s oceans warm faster and expand more than others.

Gravitational, rotational, and deformational effects are critical for understanding how ice melt impacts sea level differently across regions. When large ice sheets like those in Greenland or Antarctica lose mass, the gravitational pull they exert on nearby ocean water decreases, causing sea levels to drop locally near the melting ice but to rise more significantly farther away. This effect, combined with the redistribution of mass around the Earth, also affects the planet’s rotation and causes a phenomenon called "rotational feedback," which further influences sea level patterns. As a result, areas far from melting ice sheets can experience greater sea level rise, while areas closer may see less rise or even a temporary drop.

Vertical land motion (VLM) is another crucial factor in regional projections. Land can be subsiding (sinking) or uplifting (rising) due to processes such as tectonic activity, sediment compaction, groundwater extraction, or post-glacial rebound. For example, regions like the Mississippi River Delta or parts of Southeast Asia experience significant land subsidence, exacerbating relative sea level rise. In contrast, areas like Scandinavia, which are still rebounding from the melting of ancient ice sheets, experience land uplift, partially offsetting sea level rise. AR6 projections incorporate VLM data to adjust global sea level rise estimates to reflect local conditions more accurately.

Next: What are the impacts of sea level change?