External Forcing Explains Recent Decadal Variability of the Ocean Carbon Sink

Abstract

The ocean has absorbed the equivalent of 39% of industrial-age fossil carbon emissions, significantly modulating the growth rate of atmospheric CO 2 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 pCO 2. The acceleration of atmospheric pCO 2 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 pCO 2 and with and without climate variability. The fact that variability in the growth rate of atmospheric pCO 2 directly imprints on the ocean sink implies that there will be an immediate reduction in ocean carbon uptake as atmospheric pCO 2 responds to cuts in anthropogenic emissions. Plain Language Summary Humans have added 440 Pg of fossil fuel carbon to the atmosphere since 1750, driving up the atmospheric CO 2 concentration. But not all of this carbon remains in the atmosphere. The ocean has absorbed 39%, substantially mitigating anthropogenic climate change. Though this "ocean carbon sink" is a critical climate process, our understanding of its mechanisms remains limited. Of great interest is the unexplained slowdown of the ocean carbon sink in the 1990s and a subsequent recovery. In this work, we use a simple globally-averaged model to show that two processes external to the ocean are sufficient to explain the slowing of the ocean carbon sink in the 1990s. First, a reduced rate of accumulation of carbon in the atmosphere after 1989 reduced the atmosphere-ocean gradient that drives the ocean sink. Second, the eruption of Mt Pinatubo led to changes in ocean temperature that modified the timing of the sink from 1991 to 2001. We illustrate that the most important control on the decade-averaged magnitude of the ocean sink is variability in the growth rate of atmospheric CO 2. This implies that as future fossil fuel emission cuts drive reduced growth of atmospheric CO 2 , the ocean sink will immediately slow down.

Consistency and Challenges in the Ocean Carbon Sink Estimate for the Global Carbon Budget

“…There is growing evidence for a multi-year variability of the ocean carbon sink with remarkable consistency among dataproducts and GOBMs on a global stagnation of the ocean carbon sink in the period 1992-2001 and an extra-tropical strengthening between 2001 and 2011 ( Figure 5, Rödenbeck et al, 2015;Landschützer et al, 2016;DeVries et al, 2019;McKinley et al, 2020). Explanations for this multi-year variability range from the ocean's response to changes in atmospheric circulation (Le Quéré et al, 2007;Landschützer et al, 2015;Keppler and Landschützer, 2019), especially the variations in the upper ocean overturning (DeVries et al, 2017) to external forcing through surface cooling associated with volcanic eruptions and variations in atmospheric CO 2 growth rate (McKinley et al, 2020). The mapping methods and an ocean inverse model suggest that the GOBMs underestimate the magnitude of the multiyear variability (DeVries et al, 2019).…”

“…The area correction is dominated by the coastal ocean, which has a similar flux density as the open ocean (0.39 mol C m −2 yr −1 coastal south of 60 • N vs. 0.35 mol C mr −2 yr −1 globally Wanninkhof et al, 2013;Roobaert et al, 2019). The simplistic area-scaling approach to fill data gaps is hence considered conservative, also in comparison to the 60% higher area correction from a time-resolved gap-filling using ocean models (McKinley et al, 2020).…”

“…An ensemble of GOBMs is a robust tool to estimate the global ocean mean carbon sink and anthropogenic trends, with their spread likely being driven by differences in the strength of the simulated overturning circulation (e.g., Doney et al, 2004;Goris et al, 2018). Large-scale multi-year variability is driven by the interplay of external forcing and ocean circulation (McKinley et al, 2020). The smaller the spatial and temporal scale of interest, the more important it becomes that the GOBMs simulate the delicate interplay between physical and biological processes appropriately.…”

Ocean Carbon Uptake Under Aggressive Emission Mitigation

“…Slowing of the emissions growth rate, and thus the pCO atm 2 growth rate, reduces the efficiency of ocean C ant uptake (McKinley et al, 2020;Raupach et al, 2014); this response is related to the timescales of C ant transfer to the ocean interior (Raupach et al 2014). A reduced pCO atm 2 growth rate is inevitable, due either to climate policy, or by the eventual exhaustion of fossil fuel reservoirs.…”

“…In the past, k M has been influenced by a slowing of the CO 2 emissions growth rate (McKinley et al, 2020), volcanic aerosol induced cooling of the surface ocean (McKinley et al, 2020), changing ocean chemistry, and changes to physical climate (Friedlingstein et al, 2013).…”