Scott Kulp

Evolving Understanding of Antarctic Ice-Sheet Physics and Ambiguity in Probabilistic Sea-Level Projections

Mechanisms such as ice-shelf hydrofracturing and ice-cliff collapse may rapidly increase discharge from marine-based ice sheets. Here, we link a probabilistic framework for sea-level projections to a small ensemble of Antarctic ice-sheet (AIS) simulations incorporating these physical processes to explore their influence on global-mean sea-level (GMSL) and relative sea-level (RSL). We compare the new projections to past results using expert assessment and structured expert elicitation about AIS changes. Under high greenhouse gas emissions (Representative Concentration Pathway [RCP] 8.5), median projected 21st century GMSL rise increases from 79 to 146 cm. Without protective measures, revised median RSL projections would by 2100 submerge land currently home to 153 million people, an increase of 44 million. The use of a physical model, rather than simple parameterizations assuming constant acceleration of ice loss, increases forcing sensitivity: overlap between the central 90% of simulations for 2100 for RCP 8.5 (93–243 cm) and RCP 2.6 (26–98 cm) is minimal. By 2300, the gap between median GMSL estimates for RCP 8.5 and RCP 2.6 reaches >10 m, with median RSL projections for RCP 8.5 jeopardizing land now occupied by 950 million people (versus 167 million for RCP 2.6). The minimal correlation between the contribution of AIS to GMSL by 2050 and that in 2100 and beyond implies current sea-level observations cannot exclude future extreme outcomes. The sensitivity of post-2050 projections to deeply uncertain physics highlights the need for robust decision and adaptive management frameworks.

Global DEM Errors Underpredict Coastal Vulnerability to Sea Level Rise and Flooding

Elevation data based on NASA’s Shuttle Radar Topography Mission (SRTM) have been widely used to evaluate threats from global sea level rise, storm surge, and coastal floods. However, SRTM data are known to include large vertical errors in densely urban or densely vegetated areas. The errors may propagate to derived land and population exposure assessments. We compare assessments based on SRTM data against references employing high-accuracy bare-earth elevation data generated from lidar data available for coastal areas of the United States. We find that both 1-arcsecond and 3-arcsecond horizontal resolution SRTM data systemically underestimate exposure across all assessed spatial scales and up to at least 10m above the high tide line. At 3m, 1-arcsecond SRTM underestimates U.S. population exposure by more than 60%, and under-predicts population exposure in 90% of coastal states, 87% of counties, and 83% of municipalities. These fractions increase with elevation, but error medians and variability fall to lower levels, with national exposure underestimated by just 24% at 10m. Results using 3-arcsecond SRTM are extremely similar. Coastal analyses based on SRTM data thus appear to greatly underestimate sea level and flood threats, especially at lower elevations. However, SRTM-based estimates may usefully be regarded as providing lower bounds to actual threats. We additionally assess the performance of NOAA’s Global Land One-km Base Elevation Project (GLOBE), another publicly-available global DEM, but do not reach any definitive conclusion because of the spatial heterogeneity in its quality.

Rapid escalation of coastal flood exposure in US municipalities from sea level rise

Rising sea levels are increasing the exposure of populations and infrastructure to coastal flooding. While earlier studies estimate magnitudes of future exposure or project rates of sea level rise, here, we estimate growth rates of exposure, likely to be a key factor in how effectively coastal communities can adapt. These rates may not correlate well with sea level rise rates due to varying patterns of topography and development. We integrate exposure assessments based on LiDAR elevation data with extreme flood event distributions and sea level rise projections to compute the expected annual exposure of population, housing, roads, and property value in 327 medium-to-large coastal municipalities circumscribing the contiguous USA, and identify those localities that could experience rapid exposure growth sometime this century. We define a rate threshold of 0.25% additive increase in expected annual exposure per year, based on its rarity of present-day exceedance. With unchecked carbon emissions under Representative Concentration Pathway (RCP) 8.5, the number of cities exceeding the threshold reaches 33 (18–59, 90% CI) by 2050 and 90 (22–196) by 2100, including the cities of Boston and Miami. Sharp cuts under RCP 2.6 limit the end-of-century total to 28 (12–105), versus a baseline of 7 cities in 2000. The methods and results presented here offer a new way to illustrate the consequences of different emission scenarios or mitigation efforts, and locally assess the urgency of coastal adaptation measures.

Extreme sea level implications of 1.5 °C, 2.0 °C, and 2.5 °C temperature stabilization targets in the 21st and 22nd centuries

Sea-level rise (SLR) is magnifying the frequency and severity of coastal flooding. The rate and amount of global mean sea-level (GMSL) rise is a function of the trajectory of global mean surface temperature (GMST). Therefore, temperature stabilization targets (e.g., 1.5 {deg}C and 2.0 {deg}C of warming above pre-industrial levels, as from the Paris Agreement) have important implications for coastal flood risk. Here, we assess differences in the return periods of coastal floods at a global network of tide gauges between scenarios that stabilize GMST warming at 1.5 {deg}C, 2.0 {deg}C, and 2.5 {deg}C above pre-industrial levels. We employ probabilistic, localized SLR projections and long-term hourly tide gauge records to construct estimates of the return levels of current and future flood heights for the 21st and 22nd centuries. By 2100, under 1.5 {deg}C, 2.0 {deg}C, and 2.5 {deg}C GMST stabilization, median GMSL is projected to rise 47 cm with a very likely range of 28-82 cm (90% probability), 55 cm (very likely 30-94 cm), and 58 cm (very likely 36-93 cm), respectively. As an independent comparison, a semi-empirical sea level model calibrated to temperature and GMSL over the past two millennia estimates median GMSL will rise within < 13% of these projections. By 2150, relative to the 2.0 {deg}C scenario, GMST stabilization of 1.5 {deg}C inundates roughly 5 million fewer inhabitants that currently occupy lands, including 40,000 fewer individuals currently residing in Small Island Developing States. Relative to a 2.0 {deg}C scenario, the reduction in the amplification of the frequency of the 100-yr flood arising from a 1.5 {deg}C GMST stabilization is greatest in the eastern United States and in Europe, with flood frequency amplification being reduced by about half.