Pda Technical Report 3

[Florene Pothoven]

Longterm sea level rise will affect the extent, frequency, and duration of coastal flooding events. High-tide flooding events that occur only a few times a year now may occur once a month, or once a week in the coming decades. These same water level changes may also increase coastal erosion and groundwater levels. Elevated groundwater levels can lead to increased rainfall runoff and compromised underground infrastructure, such as public utilities, septic systems, and structural foundations. Higher water levels also mean deadly and destructive storm surges, wave impacts, and rainwater are unable to drain away from homes and businesses.

Another reason for differences in regional sea level is vertical land motion. Across the U.S., land is sinking or rising at different rates and times, and this affects how high sea level rises in a region. Vertical land motion can be a result of geologic processes (e.g. the movement of tectonic plates); human activity, such as removing groundwater or fossil fuels from underground, which can cause the land to sink; or naturally-occurring sediment compaction and settling over time (e.g., subsidence in the Mississippi River delta).

Global mean sea level, or the average height of the ocean surface, has risen 6 - 8 inches (15 - 20 centimeters) since 1920. In the continental U.S., relative sea level has risen about 10 - 12 inches (25 - 30 centimeters) over the same period. Observational data from tide gauges and satellites also show that sea level rise, both globally and along the continental U.S., is accelerating, with more than a third of that rise having occurred in the past two and a half decades (see NOAA and NASA portals for altimeter-based global rates and NOAA for local tide gauge rates).

In the 2017 sea level rise technical report, scenarios were related to representative concentration pathways. The 2022 report and data employ the underlying methods and output from the Sixth Assessment Report and their dependency on shared socioeconomic pathways, but focus more on how these scenarios relate directly to different amounts of end-of-century surface warming associated with the pathways (see Question 3).

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.

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.

The 2100 projections for each global scenario stayed the same, since science suggests this range of futures remains possible. However, the timing for different rates of rise for the different scenarios was updated based on new modeling and more realistic assumptions of Greenland and Antarctic ice sheet behavior based upon the Intergovernmental Panel on Climate Change Sixth Climate Assessment. A result is that there is less acceleration in the higher scenarios until about 2050 and greater acceleration toward the end of this century. This has two primary implications. First, despite maintaining the same target values and having the same range between scenarios in 2100, the range covered by the scenarios is smaller in the near term than in the 2017 report. Second, the likely (17th-83rd percentile) ranges of projections for each scenario before and after the 2100 time point used to define the scenarios are wider than in the 2017 report.

With each passing year, improved observations and modeling help us get a clearer picture of how and when sea level is changing both globally and regionally (see Questions 3 and 4). The scenarios in the 2022 technical report are lower in the near-term decades than they were in the 2017 technical report because there is improved understanding of Antarctic and Greenland ice sheet dynamics (see Question 10). This improved understanding comes from additional observations, research, modeling, and expert elicitation efforts that indicate sea level rise will be slower in the next few decades than previously projected. The 2022 technical report removes the Extreme (2.5 meter) scenario because the probability of this scenario is now thought to be too low to merit inclusion.

Sea level scenarios will continue to be refined as scientists increasingly observe and learn more about the details of dynamic earth system processes (e.g., Antarctic ice sheet response to temperature increases.) Additional data will help to reduce uncertainty. U.S. federal agencies monitor and assess key sea level rise source contributions globally and along U.S. coastlines, and this work can provide early indications of change in the trajectory of sea level rise, which can inform shifts in adaptation planning.

The 2022 technical report further refines and narrows the possible range of scenarios from the 2017 report. Assessment reports like this are the best resource for staying up-to-date on the latest changes to the sea level rise scenarios and why those changes have occurred. These reports are anticipated about every five years.

Included in the 2022 technical report for the first time, observation-based extrapolations are provided for global sea level and eight coastal regions (the Northeast, Southeast, Eastern Gulf, Western Gulf, Southwest, Northwest, Hawaiian Islands, and the Caribbean). Separate extrapolations are also provided for the southern and northern coasts of Alaska and the Pacific islands but caveated with greater uncertainty due to variations in land elevation and underlying regional sea level rise processes.

These observation-based extrapolations 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. Or, put another way, with continued global heating that is expected, there is strong reason to suspect that the current acceleration in sea level rise will continue, and this response is similar in both the observation trends and the modeled scenarios.

The extreme water levels as defined in this report have probabilities, or likelihoods, of occurring in a given year that are based on statistical analysis of regional sets of historical tide gauge measurements, called a regional frequency analysis. Results based on regional frequency analysis typically suggest that higher water levels are more probable than results based upon a single tide gauge data record. This is because regional sets of data observation better capture spatially the overall probability of a high-water event occurring from a passing storm. The perspective from a single gauge can be rather limited due to storm track variability (storm missed the tide gauge) or short data records (important storms occurred prior to the gauge installation).

The report explores how the annual frequencies of high tide flooding are expected to change by 2050 considering the local sea level rise scenario that closely aligns with the rise associated with the regional observation-based extrapolations. The concept of a flood regime shift is used to describe how the annual flood frequency associated with a particular coastal flood type (i.e., NOAA minor, moderate, and major high tide flooding) changes to that of another because of sea level rise. For example, by 2050 the annual frequency of NOAA moderate high tide flooding is expected to occur on average along the U.S coastline at a frequency greater than the NOAA minor high tide flooding events occur today. Or put another way, after about 1 foot (0.3 meters) of sea level rise that is expected to occur on average along the U.S coastline, tides and storm surges that today cause minor and moderate high tide flooding will cause moderate and major high tide flooding.

A technical report (also scientific report) is a document that describes the process, progress, or results of technical or scientific research or the state of a technical or scientific research problem.[1][2] It might also include recommendations and conclusions of the research. Unlike other scientific literature, such as scientific journals and the proceedings of some academic conferences, technical reports rarely undergo comprehensive independent peer review before publication. They may be considered as grey literature. Where there is a review process, it is often limited to within the originating organization. Similarly, there are no formal publishing procedures for such reports, except where established locally.

Technical reports are today a major source of scientific and technical information. They are prepared for internal or wider distribution by many organizations, most of which lack the extensive editing and printing facilities of commercial publishers.

Technical reports are often prepared for sponsors of research projects. Another case where a technical report may be produced is when more information is produced for an academic paper than is acceptable or feasible to publish in a peer-reviewed publication; examples of this include in-depth experimental details, additional results, or the architecture of a computer model. Researchers may also publish work in early form as a technical report to establish novelty, without having to wait for the often long production schedules of academic journals. Technical reports are considered "non-archival" publications, and so are free to be published elsewhere in peer-reviewed venues with or without modification.

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