Stochastic secular trends in sea level rise

Abstract

Global mean sea level (GMSL) has been rising since (at least) the nineteenth century and the rate of rise may be increasing. Several studies that attempt to explain the long‐term trend of GMSL during the instrumental record share the common assumption that this trend is deterministic in nature and different from natural variations. Here we show that the trend can alternatively be explained, at least in part, as being caused by random variations within the coupled ocean‐atmosphere‐cryosphere system, and hence not having a deterministic origin. These random trends, which add to externally forced changes (e.g., through anthropogenic climate change), are a consequence of the integrated character of GMSL, which is the cumulative addition of temporal contributions that exhibit random character, and whose integration results in GMSL variations with persistence on decadal‐centennial time scales. The generation of trends by integration of random stationary noise (i.e., even in a constant climate) is a robust and fundamental feature of stochastically forced systems with memory. The integrated character of GMSL results in an intrinsic difficulty in distinguishing internal from externally forced trends.

Reassessment of 20th century global mean sea level rise

“…Although it has been widely accepted that an autoregressive process of the order 1 is suitable for this purpose (e.g., ref. 9), recent studies demonstrate that the use of long-memory processes provides a physically more consistent description of the noise (24,28,(47)(48)(49)(50). Ref.…”

Process-based estimate of global-mean sea-level changes in the Common Era

“…We suggest that a similar in- fluence of past variability can also be expected for barystatic sea level owing to its long response time scales, so that the recent GMSL change might be linked to variability in the past. Climate forcing integration can manifest as a lower frequency change in the ocean, which can partly be misinterpreted as trends associated with deterministic forcing, as reported earlier in Ocaña et al (2016). However, with the current simulations and analyses, it is hard to make firm conclusions on these aspects.…”

“…The remaining heat also has an impact on GMSL via altering the mass balance of continen-tal ice (Antarctic and Greenland ice sheets and glaciers) and changing the global hydrological cycle (Rignot et al, 2011;Shepherd et al, 2012;Church et al, 2013). In addition to those responses to anthropogenic forcing, the internal variability in the coupled climate system is also suggested to explain a part of the recent GMSL rise owing to the long memory of the oceans (Ocaña et al, 2016;Gebbie and Huybers, 2019). Hence, the GMSL is integral to changes in the climate system in response to both forced and internal variability.…”

Towards determining uncertainties in global oceanic mean values of heat, salt, and surface elevation

“…in exchange of heat between ocean and atmosphere as a part of the global warming signal and part of the surface boundary conditions, might be regarded as deterministic. But, as has been noted in numerous publications (Ocaña et al, 2016), with a 20-year record, the duration is far too short to distinguish a true deterministic trend from the longterm stochastic shifts characteristic of red-noise processes. Thus, any trend present is treated as though arising from a stochastic process.…”