Scientists: We Lack A ‘Quantitative, Mechanistic Understanding Of How The Ocean Carbon Sink Works’

By Kenneth Richard

Recent research has emphasized that “critical mysteries remain” in our ability to quantify or even understand carbon cycle processes as they relate to Earth’s water bodies.  Observational constraints prevent the detection of an anthropogenic signal in ocean carbon uptake trends on decadal timescales (McKinley et al., 2017).  Many new papers even contradict the IPCC-endorsed conclusion that the oceans are a net sink for CO2 emission rather than a net natural source.  

The “We Had No Idea” Terrestrial Carbon Cycle

Since the mid-1980s, the Earth’s coasts and land area have been expanding (Donchyts et al., 2016), meaning there is more land mass above sea level today than there was three decades ago.  Sea level rise has not been rapid enough to keep pace with the natural shifts in Earth’s geological processes.

Net growth in global land and soil area could significantly affect the Earth’s carbon budget, especially since “Earth’s soil is releasing roughly nine times more carbon dioxide to the atmosphere than all human activities combined” (Carey et al., 2017).

Scientists frequently “discover” terrestrial locations that are new, unaccounted for sources of natural CO2 emission that “we had no idea” about.  They also routinely “discover” terrestrial surfaces that are deemed new CO2 net sinks that they never knew existed (Bastin et al., 2017).

Furthermore, scientists acknowledge that “the heterogeneous and sparsely measured terrestrial biosphere cannot be directly measured” (McKinley et al., 2017).

With routinely discovered carbon sources and sinks as well as the lack of available direct measurements, why should there be any confidence that our land area carbon budget estimates are reliable?

Earth’s Water Bodies: “A Mechanistic Understanding of Carbon Sink Variability Requires Substantial Additional Elucidation”

Scientists have recently acknowledged that “critical mysteries remain” in ocean carbon uptake processes such that we lack a detailed, quantitative, and mechanistic understanding of how the ocean carbon sink works” (McKinley et al., 2017).

Observational constraints do not even allow us to confirm that the alleged ocean carbon sink has been growing in recent decades due to anthropogenic emissions.

McKinley et al., 2017

“That the growth of the partial pressure of CO2 gas in the atmosphere ( pCO2 atm) drives a growing oceanic sink is consistent with our basic understanding that, as the globally averaged atmosphere-to-ocean pCO2 gradient increases, carbon accumulation in the ocean will occur at an increasing rate. This behavior has been illustrated clearly with models forced with only historically observed increases in pCO2 atm and no climate variability or change (Graven et al. 2012, Ciais et al. 2013). Nonetheless, critical mysteries remain and weigh heavily on our ability to quantify relationships between the perturbed global carbon cycle and climate change.”
The current inability to accurately quantify the mean CO2 sink regionally or locally also suggests that present-day observational constraints are inadequate to support a detailed, quantitative, and mechanistic understanding of how the ocean carbon sink works and how it is responding to intensifying climate change. This lack of mechanistic understanding implies that our ability to model (Roy et al. 2011, Ciais et al. 2013, Frolicher et al. 2015, Randerson et al. 2015), and thus to project the future ocean carbon sink, including feedbacks caused by warming and other climate change, is seriously limited.”
“First, substantial uncertainty remains on the mean sink (∼30% of the total flux). Formally, the quantitative estimate of the 1980–1989 sink (−2.0 ± 0.7 Pg C y−1) is not statistically distinguishable from that for 2000–2009 (−2.3 ± 0.7 Pg C y−1). Reducing this uncertainty is absolutely critical to global partitioning of anthropogenic carbon sources and sinks. Each year, the Global Carbon Project ( estimates global sources and sinks of carbon, but because the heterogeneous and sparsely measured terrestrial biosphere cannot be directly measured, its flux is estimated by difference from estimated anthropogenic sources and the ocean sink (Le Quer´ e et al. 2015). In these budgets, land use change uncertainty is at least 50% of the mean flux, and uncertainty is growing for emissions from fossil fuel burning and cement manufacture (Ciais et al. 2013). Reduction in ocean sink uncertainty could therefore help to compensate from a global budgeting perspective.”
“The sum of the available evidence indicates that variability in the ocean carbon sink is significant and is driven primarily by physical processes of upwelling, convection, and advection. Despite evidence for a growing sink when globally integrated (Khatiwala et al. 2009, 2013; Ciais et al. 2013; DeVries 2014), this variability, combined with sparse sampling, means that it is not yet possible to directly confirm from surface observations that long-term growth in the oceanic sink is occurring.”
“Globally integrated variability fluctuates with ENSO. Yet, at regional scales outside the equatorial Pacific, these modes tend to explain less than 20% of the large-scale variance in pCO2 ocean and CO2 flux (McKinley et al. 2004, 2006; Breeden & McKinley 2016), indicating that much variance remains undescribed. Consistent with the limited amount of variance explained, the mechanistic connections of these modes are not well understood, except in the equatorial Pacific with ENSO. In the North Atlantic, a variety of studies have suggested a connection of the NAO and AMO to pCO2 ocean and CO2 fluxes, but whether these changes occur through convection or advection remains an open question. In the Southern Ocean, the SAM has been linked to pCO2 ocean and CO2 fluxes through impacts on wind-driven ventilation and subduction; however, since the mid-2000s, the clear relationship to SAM has substantially weakened (Fay & McKinley 2013, Landschutzer et al. 2015). In the North Pacific, the relative influence of the PDO ¨ as opposed to ENSO requires further study. Particularly as observations in the high latitudes have become more abundant, evidence has grown that climate modes do not adequately explain carbon cycle variability and that mechanistic understanding of carbon sink variability requires substantial additional elucidation.”
“[T]his CESM-LE analysis further illustrates that variability in CO2 flux is large and sufficient to prevent detection of anthropogenic trends in ocean carbon uptake on decadal timescales.”

The Earth’s Water Bodies: Net CO2 Source Or Sink?

Observational analysis has indicated that water bodies release more of their stored CO2 as they warm and retain more of their stored CO2 as they cool.

This has been borne out in Mauna Loa CO2 records as they relate to a “warm water year” versus a “cold water year”.

Flohn (1982).

The recent increase of the CO2-content of air varies distinctly from year to year, rather independent from the irregular annual increase of global CO2-production from fossil fuel and cement, which has since 1973 decreased from about 4.5 percent to 2.25 percent per year (Rotty 1981). … Indeed the cool upwelling water is not only rich in (anorganic) CO2 but also in nutrients and organisms. (algae) which consume much atmospheric CO2 in organic form, thus reducing the increase in atmospehreic CO2. Conversely the warm water of tropical oceans, with SST near 27°C, is barren, thus leading to a reduction of CO2 uptake by the ocean and greater increase of the CO2. … A crude estimate of these differences is demonstrated by the fact that during the period 1958-1974, the average CO2-increase within five selective years with prevailing cool water only 0.57 ppm/a, while during five years with prevailing warm water it was 1.11 ppm/a.  Thus in a a warm water year, more than one Gt (1015 g) carbon is additionally injected into the atmosphere, in comparison to a cold water year.”

The Intergovernmental Panel on Climate Change (IPCC) has nonetheless claimed the oceans are a net carbon sink  rather than a net source.

Recent research analysis has challenged this conclusion, including several new (2018) published papers.

Astor et al. (2013), for example, found that 72% of the attribution for the increase in CO2 emission for the studied region arose from warming sea temperatures, and thus they concluded “the ocean is primarily a source of CO2 to the atmosphere”.

A Partial List Of Papers Indicating Earth’s Water Bodies Are A Net Source Of CO2

Below is a very non-comprehensive compilation of 12 recently-published papers that challenge the IPCC conclusion that the oceans function as a net sink for CO2.

This list would appear to support the conclusion that “critical mysteries remain” in our ability to quantify or even understand carbon cycle processes as they relate to Earth’s water bodies.

Astor et al., 2013

“Based on these observations, 72% of the increase in fCO2 sea in Cariaco Basin between 1996 and 2008 can be attributed to an increasing temperature trend of surface waters, making this the primary factor controlling fugacity at this location. … An increase/decrease of 1°C is usually followed by an increase/decrease of 16–20 matm of fCO2sea. Thus, the SST increase of 1.3°C between 1996 and 2008 accounted for 16 matm increase in fCO2sea explaining around 72% of the fCO2sea observed variation. This suggests that the changes measured in fCO2 sea were primarily the result of surface-ocean warming in Cariaco Basin. … These observations confirm that this area is a consistent source of CO2 to the atmosphere. The main process controlling the long-term changes in surface fCO2sea at CARIACO was temperature, with net community production playing a secondary role. … At the CARIACO site, the ocean is primarily a source of CO2 to the atmosphere, except during strong upwelling events.”

Ikawa et al., 2013

We estimated that the coastal area off Bodega Bay was likely an overall source of CO2 to the atmosphere based on the following conclusions: (1) the overall CO2 flux estimated from both eddy covariance and pCO2 measurements showed a source of CO2; (2) although the relaxation period during the 2008 measurements were favorable to CO2 uptake, CO2 flux during this period was still a slight source; (3) salinity and SST were found to be good predictors of the CO2 flux for both eddy covariance and pCO2 measurements, and 99% of the historical SST and salinity data available between 1988 and 2011 fell within the range of our observations in May–June 2007, August–September 2008 and November 2010–July~2011, which indicates that our data set was representative of the annual variations in the sea state. Based on the developed relationship between pCO2, SST and salinity, the study area between 1988 and 2011 was estimated to be an annual source of CO2 of ~ 35 mol C m−2 yr−1. The peak monthly CO2 flux of ~ 7 mol C m−2 month−1 accounted for almost 30% of the dissolved inorganic carbon in the surface mixed layer.”

Levy et al., 2013

“Although they are key components of the surface ocean carbon budget, physical processes inducing carbon fluxes across the mixed-layer base, i.e. subduction and obduction, have received much less attention than biological processes. Using a global model analysis of the pre-industrial ocean, physical carbon fluxes are quantified and compared to the other carbon fluxes through the surface mixed-layer, i.e. air-sea CO2 gas exchange and sedimentation of biogenic material. Model-based carbon obduction and subduction are evaluated against independent data-based estimates to the extent that was possible. We find that physical fluxes of DIC [Dissolved Inorganic Carbon] are two orders of magnitude larger than the other carbon fluxes and vary over the globe at smaller spatial scale. At temperate latitudes, the subduction of DIC and to a much lesser extent (<10%) the sinking of particles maintain CO2 undersaturation, whereas DIC is obducted back to the surface in the tropical band (75%) and Southern Ocean (25%). At the global scale, these two large counterbalancing fluxes of DIC [Dissolved Inorganic Carbon] amount to +275.5 PgC y−1 for the supply by obduction and -264.5 PgC y−1 for the removal by subduction [net +11.0 PgC y−1] which is ∼ 3 to 5 times larger than previous estimates.”

Reimer et al., 2013

“The study of air-sea CO2 fluxes (FCO2) in the coastal region is needed to better understand the processes which influence the direction and magnitude of FCO2 and to constrain the global carbon budget. The near-shore region was a weak annual net source of CO2 to the atmosphere (0.043 mol CO2 m-2 y-1); where 91% of the outgassed FCO2 was contributed during the upwelling season.”

Rutherford et al., 2016

“Continental shelves account for a large proportion of global primary production, and potentially a disproportionate fraction of the carbon dioxide (CO2) flux between atmosphere and ocean. The continental shelf pump hypothesis proposes that continental shelves at high latitudes act as net sinks of atmospheric CO2. However, direct measurements on the Scotian Shelf, off eastern Canada, indicate that this shelf region acts as a net source of CO2 to the atmosphere.”

Brown et al., 2015

“Complex oceanic circulation and air–sea interaction make the eastern tropical Pacific Ocean (ETPO) a highly variable source of CO2 to the atmosphere. … Inter-annual variability was observed within the region, with the location of the western extent of the freshpool moving westwards considerably between 2010 and 2014. Previous work within this region suggest that changes in thermocline depth related to ENSO are likely to influence pCO2 within this region. The region is a net contributor to atmospheric CO2, with average sea to air fluxes (over the four years of observations) of 1.6 mmolm−2d−1, with all regions of the ETPO outgassing year-round, except the rainfall diluted Gulf of Panama/Freshpool region.”

Xue et al., 2012

“Air–sea CO2 flux computations indicated that the NYS acted as a net CO2 source with respect to the atmosphere in each season, annually releasing 0.63 ± 0.10 mol C m− 2 to the atmosphere. In combination with the CO2 efflux rate (1.68 ± 0.33 mol C m− 2 yr− 1) reported in the southern Yellow Sea (SYS), we estimate that the entire Yellow Sea, including both the NYS and the SYS, was a net CO2 source at a rate of ~ 1.49 mol C m− 2 yr− 1, annually releasing ~ 6.78 Tg C to the atmosphere (1 Tg = 1012 g).”

Sisma-Ventura et al., 2017

Seasonal pCO2 variability was studied in the Southeast Levantine (SE-Levantine) during 2009–2015 with the aim of quantifying air–sea CO2 fluxes in this ultra-oligotrophic, warm and highly evaporative marginal sea. Mixed layer pCO2 varied significantly between 560 ± 9.0 μatm in August (summer) and 350 ± 8.7 μatm in March (winter). Comparison of pCO2 to Sea Surface Temperature (SST) yielded a strong positive correlation (n = 135, r 2 = 0.94), suggesting that the seasonal variations are the result of a thermodynamic effect on the carbonate system in seawater. Using the coupling between pCO2 and SST, we calculated the mean monthly values and the air-sea fluxes in this region. These calculations indicated that this region is a net source of CO2 to the atmosphere over an annual cycle, with an average flux of 845 ± 270 mmol C m2 y−1 (~0.98 Tg C y−1 ).”

Biswas et al., 2018

“The era of global warming and increased emission of greenhouse gases can be marked by the beginning of the industrial age. It is also true that under several conditions, natural ecosystems can be equally responsible for CO2 emission like any other anthropogenic activities which continuously release heat-trapping gases in the process of development. … East Kolkata Wetland (EKW) is an urban or peri-urban wetland located on the outskirts of the Kolkata City which performs multi-facet activities, carbon sink being one of them. The raw waste from the city is naturally treated in this wetland system, however, the aquaculture ponds situated in these wetlands which make use of this waste water for fishery is rarely studied. The present study aims to see whether the aquaculture ponds of EKW complex are acting as a source or a sink. Airwater carbon dioxide (CO2) flux was estimated for three consecutive seasons in a year and it was found that the system is acting as a CO2 source in all the three seasons.”

Wang et al., 2018

“We conducted a free‐water mass balance‐based study to address the rate of metabolism and net carbon exchange for the tidal wetland and estuarine portion of the coastal ocean and the uncertainties associated with this approach were assessed. We measured open water diurnal O2 and dissolved inorganic carbon (DIC) dynamics seasonally in a salt marsh‐estuary in Georgia, U.S.A. with a focus on the marsh‐estuary linkage associated with tidal flooding. We observed that the overall estuarine system was a net source of CO2 to the atmosphere and coastal ocean and a net sink for oceanic and atmospheric O2.”

Li et al., 2018

“Our calculated CO2 areal fluxes were in the upper-level magnitude of published data, demonstrating the importance of mountainous rivers and streams as a global greenhouse gas source, and urgency for more detailed studies on CO2 degassing, to address a global data gap for these environments. …  Rivers have been widely reported to be supersaturated in carbon dioxide (CO2) with respect to the atmosphere, and are a net source of atmospheric CO2 (Butman and Raymond, 2011; Raymond et al., 2013).”

Rosentreter et al., 2018

“Although the overall status of mangroves [creeks] is net autotrophic (Alongi, 2002), mangrove sediments and waters have been shown to be a large source of CO2 to the atmosphere due to large organic matter inputs from diverse sources such as the mangrove biomass itself, other terrestrial detritus, nutrients from land, microphytobenthos, phytoplankton and the exchange of organic matter with the open ocean (Lekphet et al., 2005; Borges et al., 2005; Bouillon and Boschker, 2006; Kristensen et al., 2008). … The vast majority of mangrove CO2 gas exchange studies found surrounding waters were supersaturated in CO2 with respect to the atmosphere, hence, a net source of CO2.”
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