Abstract

AbstractThe ocean has absorbed the equivalent of 39% of industrial‐age fossil carbon emissions, significantly modulating the growth rate of atmospheric CO2 and its associated impacts on climate. Despite the importance of the ocean carbon sink to climate, our understanding of the causes of its interannual‐to‐decadal variability remains limited. This hinders our ability to attribute its past behavior and project its future. A key period of interest is the 1990s, when the ocean carbon sink did not grow as expected. Previous explanations of this behavior have focused on variability internal to the ocean or associated with coupled atmosphere/ocean modes. Here, we use an idealized upper ocean box model to illustrate that two external forcings are sufficient to explain the pattern and magnitude of sink variability since the mid‐1980s. First, the global‐scale reduction in the decadal‐average ocean carbon sink in the 1990s is attributable to the slowed growth rate of atmospheric pCO2. The acceleration of atmospheric pCO2 growth after 2001 drove recovery of the sink. Second, the global sea surface temperature response to the 1991 eruption of Mt Pinatubo explains the timing of the global sink within the 1990s. These results are consistent with previous experiments using ocean hindcast models with variable atmospheric pCO2 and with and without climate variability. The fact that variability in the growth rate of atmospheric pCO2 directly imprints on the ocean sink implies that there will be an immediate reduction in ocean carbon uptake as atmospheric pCO2 responds to cuts in anthropogenic emissions.

Highlights

  • The ocean has absorbed the equivalent of 39% of fossil carbon emissions since 1750, significantly modulating the growth of atmospheric CO2 and the associated climate change (Friedlingstein et al 2019; Le Quéré et al, 2018a,b; McKinley et al, 2017; Ciais et al, 2013)

  • We have shown that externally forced variability is sufficient to explain a significant portion of current model and observationally-based best-estimates of the recent decadal variability of the global ocean carbon sink (Fig 1a)

  • The reduced ocean carbon sink in the decade of the 1990s was driven by a slowed growth rate of pCO2atmosphere

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Summary

Introduction

The ocean has absorbed the equivalent of 39% of fossil carbon emissions since 1750, significantly modulating the growth of atmospheric CO2 and the associated climate change (Friedlingstein et al 2019; Le Quéré et al, 2018a,b; McKinley et al, 2017; Ciais et al, 2013) This sink is expected to grow and substantially mitigate atmospheric carbon accumulation for the several centuries (Randerson et al, 2015). Recent studies have concluded that ocean hindcast models, which have long been used to assess the ocean sink, may significantly underestimate its variability (Gruber et al, 2019a; Le Quéré et al, 2018b) These conclusions are based on comparisons to new observation-based gap-filled products that suggest substantially larger interannual to decadal variability (Landschützer et al, 2015, 2016; Rödenbeck et al, 2013, 2015). It is critical that we accurately quantify and diagnose this variability so that we can better project the future ocean carbon sink and, the degree to which the ocean carbon sink will continue to mitigate global climate change

Methods
Models and Products
Flux analysis
Accounting for open ocean areas without observationally-based estimates
Upper Ocean Box Model
Findings
Conclusions
Full Text
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