Time of Emergence and Large Ensemble Intercomparison for Ocean Biogeochemical Trends.

Sarah Schlunegger,Tatiana Ilyina,Jorge L Sarmiento,Keith B Rodgers,John P Dunne,James R Christian,Flavio Lehner,Thomas L Frölicher,Yohei Takano,Matthew C Long,Richard Slater

doi:10.1029/2019gb006453

Abstract

Anthropogenically forced changes in ocean biogeochemistry are underway and critical for the ocean carbon sink and marine habitat. Detecting such changes in ocean biogeochemistry will require quantification of the magnitude of the change (anthropogenic signal) and the natural variability inherent to the climate system (noise). Here we use Large Ensemble (LE) experiments from four Earth system models (ESMs) with multiple emissions scenarios to estimate Time of Emergence (ToE) and partition projection uncertainty for anthropogenic signals in five biogeochemically important upper‐ocean variables. We find ToEs are robust across ESMs for sea surface temperature and the invasion of anthropogenic carbon; emergence time scales are 20–30 yr. For the biological carbon pump, and sea surface chlorophyll and salinity, emergence time scales are longer (50+ yr), less robust across the ESMs, and more sensitive to the forcing scenario considered. We find internal variability uncertainty, and model differences in the internal variability uncertainty, can be consequential sources of uncertainty for projecting regional changes in ocean biogeochemistry over the coming decades. In combining structural, scenario, and internal variability uncertainty, this study represents the most comprehensive characterization of biogeochemical emergence time scales and uncertainty to date. Our findings delineate critical spatial and duration requirements for marine observing systems to robustly detect anthropogenic change.

Highlights

The ocean's physical and biogeochemical state determine its habitability and capacity for sequestration of anthropogenic carbon
We focus the first part of our analysis on the Time of Emergence (ToE) diagnostic, which represents the time scale over which an anthropogenic or forced trend in the climate system emerges above the uncertainty induced by natural internal variability
We have conducted a comprehensive multi‐Large Ensemble (LE) analysis of ToE for a range of biogeochemically pertinent variables in the upper ocean, enabling us to provide a multifaceted and quantitative view of uncertainties associated with climate change projections with Earth system models

Summary

Introduction

The ocean's physical and biogeochemical state determine its habitability and capacity for sequestration of anthropogenic carbon. Rising temperatures, changing circulation, and acidification impact marine ecosystems and alter the cycling of carbon within the ocean (Bopp et al, 2013; Frölicher et al, 2016; Lovenduski et al, 2016; Riebesell et al, 2009; Sarmiento et al, 1998); the timing and magnitude of these potential impacts is uncertain due to uncertainty in the projections of Earth System Models (ESMs), global climate models that include an interactive representation of the global carbon cycle.

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Conclusion