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Executive Summary

This report and accompanying datasets from the U.S. Sea Level Rise and Coastal Flood Hazard Scenarios 

and Tools Interagency Task Force provide 1) sea level rise scenarios to 2150 by decade that include estimates of vertical land motion and 2) a set of extreme water level probabilities for various heights along the 

U.S. coastline. These data are available at 1-degree grids along the U.S. coastline and downscaled specifically at NOAA tide-gauge locations. Estimates of flood exposure are assessed using contemporary U.S. coastal 

flood-severity thresholds for current conditions (e.g., sea levels and infrastructure footprint) and for the next 

30 years (out to year 2050), assuming no additional risk reduction measures are enacted. 

This effort builds upon the 2017 Task Force report (Sweet et al., 2017). In particular, the set of global mean 

sea level rise scenarios from that report are updated and downscaled with output directly from the United 

Nations Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6; IPCC, 2021a), 

through the efforts of the NASA Sea Level Change Team; updates include adjustments to the temporal trajectories and exceedance probabilities of these scenarios based upon end-of-century global temperatures. 

As with the 2017 report, these global mean sea level rise scenarios are regionalized for the U.S. coastline. 

In addition, methodology supporting the U.S. Department of Defense Regional Sea Level (DRSL) database1

(Hall et al., 2016) is adapted for the extreme water level dataset newly developed for this report.

This report will be a key technical input for the Fifth National Climate Assessment (NCA5). These data and 

information are being incorporated into current and planned agency tools and services, such as NOAA’s Sea 

Level Rise Viewer and Inundation Dashboard,2 NASA’s Sea Level Change Portal,3

 and others. Although the 

intent of this report is not to provide authoritative guidance or design specifications for a specific project, it is 

intended to help inform Federal agencies, state and local governments, and stakeholders in coastal communities about current and future sea level rise to help contextualize its effects for decision-making purposes. 

Key Message #1:

Multiple lines of evidence provide increased confidence, regardless of the emissions pathway, in a narrower 

range of projected global, national, and regional sea level rise at 2050 than previously reported 

(Sweet et al., 2017).

• Both trajectories assessed by extrapolating rates and accelerations estimated from historical tide

gauge observations, and model projections, fall within the same range in all cases, giving higher

confidence in these relative sea level (RSL; land and ocean height changes) rise amounts by 2050.

• Relative sea level along the contiguous U.S. (CONUS) coastline is expected to rise on average

as much over the next 30 years (0.25–0.30 m over 2020–2050) as it has over the last 100 years

(1920–2020).

• Due to processes driving regional changes in sea level, there are similar regional differences in

both the modeled scenarios and observation-based extrapolations, with higher RSL rise along the

East (0–5 cm higher on average than CONUS) and Gulf Coasts (10–15 cm higher) as compared to

the West (10–15 cm lower) and Hawaiian/Caribbean (5–10 cm lower) Coasts.

• The projections do not include natural year-to-year sea level variability that occurs along U.S.

coastlines in response to climatic modes such as the El Niño–Southern Oscillation.

1

 https://drsl.serdp-estcp.org/

2 https://coast.noaa.gov/digitalcoast/tools/slr.html

3

 https://sealevel.nasa.gov/
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Key Message #2

By 2050, the expected relative sea level (RSL) will cause tide and storm surge heights to increase and will 

lead to a shift in U.S. coastal flood regimes, with major and moderate high tide flood events occurring as 

frequently as moderate and minor high tide flood events occur today. Without additional risk-reduction measures, U.S. coastal infrastructure, communities, and ecosystems will face significant consequences.

• Minor/disruptive high tide flooding (HTF; about 0.55 m above mean higher high water [MHHW]4) is

projected to increase from a U.S average frequency of about 3 events/year in 2020 to >10 events/

year5 by 2050.

• Moderate/typically damaging HTF (about 0.85 m above MHHW) is projected to increase from a

U.S. average frequency of 0.3 events/year in 2020 to about 4 events/year in 2050.

• Major/often destructive HTF (about 1.20 m above MHHW) is projected to increase from a U.S. average frequency of 0.04 events/year in 2020 to 0.2 events/year by 2050.

• Across all severities (minor, moderate, major), HTF along the U.S. East and Gulf Coasts will largely

continue to occur at or above the national average frequency.

Key Message #3:

Higher global temperatures increase the chances of higher sea level by the end of the century and beyond. 

The scenario projections of relative sea level along the contiguous U.S. (CONUS) coastline are about 

0.6–2.2 m in 2100 and 0.8–3.9 m in 2150 (relative to sea level in 2000); these ranges are driven by uncertainty in future emissions pathways and the response of the underlying physical processes. 

• With an increase in average global temperature of 2°C above preindustrial levels, and not considering the potential contributions from ice-sheet processes with limited agreement (low confidence)

among modeling approaches, the probability of exceeding 0.5 m rise globally (0.7 m along the

CONUS coastline) by 2100 is about 50%. With 3°–5°C of warming under high emissions pathways,

this probability rises to >80% to >99%. The probability of exceeding 1 m globally (1.2 m CONUS) by

2100 rises from <5% with 3°C warming to almost 25% with 5°C warming.

• Considering low-confidence ice-sheet processes and high emissions pathways with warming

approaching 5°C, probabilities rise to about 50%, 20%, and 10% of exceeding 1.0 m, 1.5 m, or 2.0 m

of global rise by 2100, respectively. These processes are unlikely to make significant contributions

with 2°C of warming, but how much warming might be required to trigger them is currently unknown.

• As a result of improved understanding of the timing of possible large future contributions from

ice-sheet loss, the “Extreme” scenario from the 2017 report (2.5 m global mean sea level rise by

2100) is now viewed as less plausible and has been removed. Nevertheless, the potential for increased acceleration in the late 21st century and beyond means that the other high-end scenarios

provide pathways that could reach this threshold in the decades immediately following 2100 (and

continue rising).

• Regionally, the projections are near or higher than the global average in 2100 and 2150 for almost

all U.S. coastlines due to the effects from vertical land motion (VLM); gravitational, rotational, and

deformational effects due to land ice loss; and ocean circulation changes. Largely due to VLM,

RSL projections are lower than the global amounts along the southern Alaska coast and are higher

along the Eastern and Western Gulf coastlines.

4

 Mean higher high water (MHHW) level is estimated over the 1983–2001 tidal epoch period and, in this case, is considered a fixed elevation 

that does not change with sea level rise.

5 The extreme value statistical methods in this report do not directly resolve frequencies >10 events/year.
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Key Message #4

Monitoring the sources of ongoing sea level rise and the processes driving changes in sea level is critical for 

assessing scenario divergence and tracking the trajectory of observed sea level rise, particularly during the 

time period when future emissions pathways lead to increased ranges in projected sea level rise. 

• Efforts are under way to narrow the uncertainties in ice-sheet dynamics and future sea level rise

amounts in response to increasing greenhouse gas forcing and associated global warming.

• Early indicators of changes in sea level rise trajectories can serve to trigger adaptive management plans and are identified through continuous monitoring and assessment of changes in sea

level (on global and local scales) and of the key drivers of sea level change that most affect U.S.

coastlines, such as ocean heat content, ice-mass loss from Greenland and Antarctica, vertical land

motion, and Gulf Stream system changes.