What are the scenarios from the Sea Level Rise Interagency Task Force and how do they compare to the projections from the IPCC AR6?

The Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Interagency Task Force was jointly convened at the direction of the White House Resilience Council, in 2015, under the U.S. Global Change Research Program (USGCRP), the Subcommittee on Ocean Sciences and Technology (SOST), and the National Ocean Council (NOC). This was in recognition of the strong need and demand for authoritative, consistent, accessible sea level rise and associated coastal hazard information for the entire U.S. coastline, coordinated across the relevant Federal agencies, to serve as a starting point for on-the-ground coastal preparedness planning and risk management activities. The goal of the Task Force, since its inception, has been to develop the necessary products through sustained and coordinated participation of key agencies, based on the best available science, including regional science and expertise when possible and appropriate. A goal has also been to incorporate those products into user-friendly mapping, visualization, and analysis tools made easily accessible through existing agency portals serving specific partners and stakeholders, as well as interagency venues such as the National Climate Assessment (NCA), the U.S. Climate Resilience Toolkit, and others. One of the main outputs of the Task Force are Technical reports providing future sea level information for the coastlines of the U.S. The first of these technical reports was released in 2017 with a recent update and new Technical Report in 2022.

In 2022, the Sea Level Rise and Coastal Flood Hazard Scenarios and Tools Interagency Task Force produced a Technical Report titled "Global and Regional Sea Level Rise Scenarios for the United States: Updated Mean Projections and Extreme Water Level Probabilities Along U.S. Coastlines". This report provides the most up-to-date sea level rise scenarios, available for all U.S. states and territories, out to the year 2150. It is the latest product from the Task Force, which includes the National Aeronautics and Space Administration, the National Oceanic Atmospheric Administration, Environmental Protection Agency, U.S. Geological Survey, and U.S. Army Corps of Engineer, along with partners in academia. The information in the report is intended to inform coastal communities and others about current and future sea level rise to help contextualize its effects for decision making purposes.
There are three main components in the 2022 Technical Report:

• updated scenarios of global and regional sea levels out to the year 2150,

• a focus on the near-term time period using observation-based trajectories out to the year 2050

• a new national gridded set of Extreme Water Level (EWL) probabilities that illustrate how the frequency of NOAA minor, moderate, and major high tide flooding events will change in frequency out to year 2050 (See the Section on Observation-Based Extrapolations and Extreme Water Levels).

Sea level rise scenarios, also called Global Mean Sea Level (GMSL) scenarios, represent possible future sea level changes in response to increasing greenhouse gas emissions and ocean and atmospheric warming, allowing people to consider future impacts and responses and ask ‘what if?’ questions about the future to support planning and decision-making. Sea level rise scenarios are used to communicate how much sea level rise could occur, under what circumstances, and by when. They also show how sea level rise might occur globally and locally.

Sea level rise scenarios are generally based upon climate model outputs. These climate models allow scientists to simulate different responses, such as how the ocean might continue to warm, where ice melts and major ice sheets dynamically respond, and where and how the additional water disperses around the world’s ocean and affects circulation patterns. These responses differ under models that use different bounding conditions associated with various amounts of greenhouse emissions and ocean and atmospheric warming projections for future sea level rise. Thus sea level rise scenarios help us plan in the face of uncertainty by providing a range of possible futures that help represent a) potential future human-driven greenhouse gas emissions, and b) how earth’s physical processes will respond to increased temperatures. These scenarios are different than the projections provided by the IPCC 6th Assessment Report. As opposed to constructing a projection around a particular emissions pathway, the scenarios specify a targeted amount of sea level rise at a time in the future. The trajectory for getting to that target value does rely on the same science and projection framework from the IPCC AR6.

A goal of the 2022 Technical Report is to examine the full range of plausible amounts of future global sea level rise, not just those rise amounts considered ‘likely’. Quantifying the ‘unlikely but possible’ sea level rise response can be critical to help bound certain risk planning exercises. For GMSL, these hinge on potential physical changes in the major ice sheets, where possible rapid collapses would contribute to very large amounts of future sea level rise. These ‘unlikely but possible’ outcomes must be acknowledged and accounted for in some types of planning (e.g., mission critical; major infrastructure upgrades that have very long operational life cycles and/or are highly vulnerable etc.). For the 2017 report, these considerations led to the development of a set of GMSL scenarios that span the plausible range of sea level rise and are defined by a target value of rise in 2100. In the 2022 report, the same framework is adopted, and the following 2100 target values of sea level rise are used to differentiate the five scenarios: Low (1 ft; 0.3 m), Intermediate-Low (1.6 ft; 0.5 m), Intermediate (3.3 ft; 1 m), Intermediate-High (4.9 ft; 1.5 m) and High (6.6 ft; 2 m).

To create the GMSL scenarios and associated regional relative sea level values, the 2022 technical report uses the last report in 2017 (Sweet et al.) as a starting point. The scenarios from that report are updated with the most recent science and adopted analytical methods from the Intergovernmental Panel on Climate Change’s (IPCC) Sixth Assessment Report (AR6), completed in 2021. The IPCC regularly convenes top researchers from around the world to synthesize the best available climate science. Both the technical report and the IPCC AR6 draw upon the latest sea-level rise science. Because both reports depend upon the same underlying science, they are similar and consistent, but the GMSL scenarios lead to a different framing and structure. To generate the scenarios used in this report, the ensemble – or set - of projections in the AR6 that are tied to specific shared societal pathways (SSPs) are filtered to identify subsets of pathways that are consistent with the scenario target values in 2100 (i.e 0.3 m, 0.5 m, 1 m, 1.5 m and 2 m). As in the AR6, these scenarios are regionalized and then provided at individual tide gauge locations. The median, 17th and 83rd percentile values are provided for each scenario at each region and location.

For the first time, the IPCC 6th Assessment Report (AR6) provided an additional low confidence range of future sea level rise that is intended to represent the potential contribution from uncertain physical processes under high-emissions scenarios. These additional processes are associated with the Greenland and Antarctic Ice Sheet (GIS and AIS). As detailed in AR6, examples of low confidence processes include earlier-than-projected disintegration of marine ice shelves and the abrupt, widespread onset of Marine Ice Sheet Instability (MISI) and Marine Ice Cliff Instability (MICI) around Antarctica, as well as faster-than-projected changes in the surface mass balance and dynamical ice loss from Greenland.

To produce the low confidence range, ice-sheet projections based on both a structured expert judgment (SEJ) study and an AIS model that includes MICI were incorporated into the projection framework along with projections of the other processes causing sea level rise. Because of lack of consensus across and the limited availability of literature sources representing these processes, low confidence projections were produced for only two emissions scenarios: SSP1-2.6 (low emissions) and SSP5-8.5 (high emissions). The addition of the low confidence projections to the high emissions scenario resulted in much larger changes when compared to the addition to the low emissions scenario. This reflects the fact that low confidence processes are likely to play a more important role in a world with more extreme climate change and greater warming.

• There are several reasons the contributions from these processes were only included in this low confidence range. These are as follows:
• There is limited agreement among models regarding the low confidence processes. This includes whether these processes will happen in the future.
• There is limited evidence available to constrain these low confidence processes within models.
• It is unknown exactly how much warming may be needed to trigger the relevant physical processes.
• It is unknown how quickly ice sheets will respond if they are triggered.
• There are possible negative feedbacks that would affect or even limit the ice mass loss and resultant sea level rise if these processes are triggered.

For the Technical Report, low confidence processes contribute significantly to the Intermediate, Intermediate-High and High scenarios. In fact, under the assumption of high emissions and warming, the range between those three scenarios is most reflective of the relative importance and possible contributions of these processes to future sea level.

Observation-based extrapolations, or sea level rise “trajectories”, are estimates of relative sea level rise out to 2050 built from analyzing regional sets of tide gauge data. To create them, the rate and acceleration of sea level rise from 1970 to 2020 is calculated from sea level rise observations from regional sets of tide gauges after and filtering to remove some effects of natural variability, like large scale natural cycles such as El-Nino/La-Nina cycles. For the global sea level rise extrapolation, satellite-altimeter measurements of sea level were also included.

Included in the 2022 Technical report for the first time, observation-based extrapolations are provided for global sea level and eleven coastal regions (the Contiguous U.S., Northeast, Southeast, Eastern Gulf, Western Gulf, Southwest, Northwest, Hawaiian Islands, Northern Alaska, Southern Alaska, and the Caribbean). In the scenario tool (https://sealevel.nasa.gov/task-force-scenario-tool), extrapolations of individual tide gauges are also provided, although these are not found in the 2022 Technical Report. These observation-based extrapolations can be viewed as future sea levels that would result if sea level keeps following the same trend and acceleration as it has shown over the last five decades. Around the US coastlines, the trajectories at the regional level are very similar to the model-based projections through 2050, and therefore serve as a further line of evidence for the confidence in the near-term trajectory of sea level rise. On the local level, there can be differences - sometimes substantial - between the extrapolations and scenarios. Such disagreements could point to the influence of unresolved local variability impacting the extrapolation or to a local process that is not being correctly captured in the model-based scenarios.