Abstract
The satellite altimeter record has provided an unprecedented database for understanding sea-level rise and has recently reached a major milestone at 25 years in length. A challenge now exists in understanding its broader significance and its consequences for sea-level rise in the coming decades and beyond. A key question is whether the pattern of altimeter-era change is representative of longer-term trends driven by anthropogenic forcing. In this work, two multimember climate ensembles, the Community Earth System Model (CESM) and the Earth System Model Version 2M (ESM2M), are used to estimate patterns of forced change [also known as the forced response (FR)] and their magnitudes relative to internal variability. It is found that the spatial patterns of 1993-2018 trends in the ensembles correlate significantly with the contemporaneous FRs (0.55 ± 0.10 in the CESM and 0.61 ± 0.09 in the ESM2M) and the 1950-2100 FRs (0.43 ± 0.10 in the CESM and 0.51 ± 0.11 in the ESM2M). Unforced runs for each model show such correlations to be extremely unlikely to have arisen by chance, indicating an emergence of both the altimeter-era and long-term FRs and suggesting a similar emergence in nature. Projected patterns of the FR over the coming decades resemble those simulated during the altimeter era, suggesting a continuation of the forced pattern of change in nature in the coming decades. Notably, elevated rates of rise are projected to continue in regions that are susceptible to tropical cyclones, exacerbating associated impacts in a warming climate.
Highlights
The satellite altimeter record has provided an unprecedented database for understanding sea-level rise and has recently reached a major milestone at 25 years in length
Observed sea-level trends are known to arise from the combined influences of internal variability due to natural variations arising within the climate system and the forced response (FR), defined as the climate response driven by forcings external to the climate system, such as from natural or anthropogenic aerosols, or from increases in greenhouse gas concentrations [14]
As with all finite records and those from satellites, it is unable on its own to definitively address various questions related to the attribution of long-term change
Summary
Observed sea-level trends are known to arise from the combined influences of internal variability due to natural variations arising within the climate system and the forced response (FR), defined as the climate response driven by forcings external to the climate system, such as from natural (i.e., volcanic) or anthropogenic aerosols, or from increases in greenhouse gas concentrations [14]. Studies looking at regional time of emergence based on multimodel archives show strong dependence on region [2, 6] and a benefit in removing patterns of internal variability [11], but they have yet to identify a clear emergence of major features Considerable uncertainty surrounding these finding persists, as contrasts within multimodel archives include both internal variability and structural contrasts between models, with few means of separating the two [6, 7]. A challenge for diagnostic assessments arises, if the spatial pattern of internal modes resembles the pattern of long-term change, as disentangling them during altimeter era remains a challenge [21] To address this challenge, techniques for assessing the time of emergence of climate signals from internal variability have been explored for a range of variables, including temperature [22], rainfall extremes [23], and sea level [6, 7].
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