CoastalDEM also cuts RMSE roughly in half compared to SRTM 26. CoastalDEM covers the same near-global latitudes as SRTM while reducing vertical bias to decimeter scale (0.01 m and 0.11 m as measured versus airborne lidar in the coastal US and Australia −0.29 m as tested versus spaceborne lidar globally). This model incorporates 23 variables, including population and vegetation indices, and was trained using lidar-derived elevation data in the US as ground truth. In this article, we present ECWL exposure assessments that address this problem by employing CoastalDEM, a new DEM developed using a neural network to perform nonlinear, nonparametric regression analysis of SRTM error.
This degree of error leads to large underestimates of ECWL exposure 28, and exceeds projected sea-level rise this century under almost any scenario 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. Spaceborne lidar from NASA’s ICESat satellite 27, a sparser, noisier and less reliable source of ground truth than airborne lidar, indicates SRTM has a global mean bias of 1.9 m in the same band 26. Mean error in SRTM’s 1–20 m elevation band is 3.7 m in the US and 2.5 m in Australia when using DEMs from airborne lidar as ground truth 26. This is especially true in densely vegetated and in densely populated areas 22, 23, 24, 25. It thus suffers from large error with a positive bias when used to represent terrain elevations. SRTM models the elevation of upper surfaces and not bare earth terrain. It is the standard choice for extreme coastal water level (ECWL) exposure analysis covering areas where high-quality elevation data are unavailable or prohibitively expensive 14, 15, 16, 17, 18, 19, 20, 21. By contrast, SRTM is a near-global satellite-based DEM covering latitudes from 56 south to 60 north and thereby land home to 99.7% of world population (based on 2010 Landscan data 13). High-accuracy DEMs derived from airborne lidar are freely available for the coastal United States, much of coastal Australia, and parts of Europe, but are lacking or unavailable in most of the rest of the world. Land topography and elevation, as represented by DEMs, lie at the foundation of such translation. Translating sea-level projections into potential exposure of population is critical for coastal planning and for assessing the benefits of climate mitigation, as well as the costs of failure to act. Under higher emissions scenarios, twenty-first century rise may approach or in the extremes exceed 2 m in the case of early-onset Antarctic ice sheet instability 4, 8. Even with sharp, immediate cuts to carbon emissions, it could rise another 0.5 m this century 3, 4, 5, 6, 7, 8, 9, 10, 11, 12. Driven by climate change, global mean sea level rose 11–16 cm in the twentieth century 1, 2.
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