anthropogenic carbon :: Ocean Carbon & Biogeochemistry

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Archive for anthropogenic carbon – Page 2

Will global change “stress out” ocean DOC cycling?

Posted by mmaheigan

· Tuesday, September 29th, 2020

The dissolved organic carbon (DOC) pool is vital for the functioning of marine ecosystems. DOC fuels marine food webs and is a cornerstone of the earth’s carbon cycle. As one of the largest pools of organic matter on the planet, disruptions to marine DOC cycling driven by climate and environmental global changes can impact air-sea CO2 exchange, with the added potential for feedbacks on Earth’s climate system.

Figure 1. Simplified view of major dissolved organic carbon (DOC) sources (black text) and sinks (yellow text) in the ocean.

Since DOC cycling involves multiple processes acting concurrently over a range of time and space scales, it is especially challenging to characterize and quantify the influence of global change. In a recent review paper published in Frontiers in Marine Science, the authors synthesize impacts of global change-related stressors on DOC cycling such as ocean warming, stratification, acidification, deoxygenation, glacial and sea ice melting, inflow from rivers, ocean circulation and upwelling, and atmospheric deposition. While ocean warming and acidification are projected to stimulate DOC production and degradation, in most regions, the outcomes for other key climate stressors are less clear, with much more regional variation. This synthesis helps advance our understanding of how global change will affect the DOC pool in the future ocean, but also highlights important research gaps that need to be explored. These gaps include for example a need for studies that allow to understand the adaptation of degradation/production pathways to global change stressors, and their cumulative impacts (e.g. temperature with acidification).

Authors:
C. Lønborg (Aarhus University)
C. Carreira (CESAM, Universidade de Aveiro)
Tim Jickells (University of East Anglia)
X.A. Álvarez-Salgado (CSIC, Instituto de Investigacións Mariñas)

Nitrate enrichment may threaten coastal wetland carbon storage

Posted by mmaheigan

· Thursday, February 27th, 2020

With their high primary productivity and slow decomposition in anoxic soils, salt marshes and other coastal wetlands can store carbon more efficiently than terrestrial uplands. These wetlands also provide critical ecosystem services such as interception of land-derived nutrients before they can enter the coastal ocean. Therefore, it is important to understand how anthropogenic supplies of nitrate (NO₃^–) affect marsh sustainability and carbon storage.

In marsh sediment studies, the most common form of experimental nitrogen enrichment uses pelletized fertilizer composed of ammonium, urea, or other organic based fertilizers. Authors of a recent study published in Global Change Biology hypothesized that when nutrients were instead added in the form of nitrate (NO₃^–), the most common form of nitrogen enrichment in coastal waters, it would stimulate microbial decomposition of organic matter by serving as an electron acceptor for microbial respiration in anoxic salt marsh sediments. Furthermore, decomposition would vary with sediment depth, with decreased decomposition at greater depths, where less biologically available organic matter accumulated over time.

Figure 1: DIC production as a proxy for microbial respiration in salt marsh sediments from three distinct depth horizons (shallow 0-5cm, mid 10-15cm, deep 20-25cm) that span a range of biological availability. The addition of NO3- (green) stimulated DIC production relative to unenriched sediments, regardless of sediment depth. All samples were run under anoxic conditions (without the presence of oxygen), closely matching that of normal salt marsh sediments.

Surprisingly, NO₃^–addition stimulated decomposition of organic matter at all depths, with the highest decomposition rates in the surface sediments. This suggests that there is a pool of “NO₃^–-labile” organic matter in marsh sediments that microbes can decompose under high-NO₃^– conditions that would otherwise remain stable. As human activities continue to enrich our coastal waters with NO₃^–through agricultural runoff, septic systems, and other pathways, it could inadvertently decrease coastal wetlands’ carbon storage capacity, with negative consequences for both blue carbon offsets and marsh sustainability in the face of sea level rise.

Authors:
Jennifer Bowen (Northeastern University)
Ashley Bulseco (MBL/WHOI)
Anne Giblin (MBL)

Krillin’ it with poop: Highlighting the importance of Antarctic krill in ocean carbon and nutrient cycling

Posted by mmaheigan

· Tuesday, February 4th, 2020

Scientists have long known the role of Antarctic krill (Euphausia superba) in Southern Ocean ecosystems. Evidence is gathering about krill’s biogeochemical importance through releasing millions of faecal pellets in swarms and stimulating primary production through nutrient excretion. Here, we explore and synthesise the known impacts that this highly abundant and rather large species has on the environment. Krill exemplify how metazoa can play a dominant role in shaping ocean biogeochemistry, thus providing additional motivation for protecting certain harvested species.

Figure 1: The ecological roles of krill in Southern Ocean biogeochemical cycles, including releasing faecal pellets, excreting nutrients whilst grazing, and larval krill migrating throughout the water column, shedding exoskeletons, and feeding on the seabed.

A review published in Nature Communications uncovers at least 13 possible pathways by which Antarctic krill either influence the carbon sink or release fertilizing nutrients (Figure 1). Their large size (up to 7 cm) and swarming nature (millions of krill aggregate) enable krill to strongly impact ocean biogeochemistry. Swarms release large numbers of faecal pellets, overwhelming detritivores and resulting in a large sink of faecal carbon. Krill may physically mix nutrients from the deep ocean and become a decades-long carbon store in whale biomass. Antarctic krill larvae, which live near the sea-ice, undergo deeper diel vertical migrations compared to adult Antarctic krill (400 m vs. 200 m), so any carbon respired or faecal pellets released by larvae could remain in the deep ocean longer than those released by adult krill at a shallower depth; the larval krill contribution to carbon export has not been quantified. Furthermore, it is currently unknown how many krill larvae are removed from the Antarctic krill fishery as by-catch. Perhaps the biggest challenge in constraining the role of krill (adult and larvae) in biogeochemical cycles is our limited capacity to quantify the abundance and biomass of Antarctic krill, since shipboard sampling methods (nets or acoustics) have limited spatial and temporal coverage. Ultimately, the Southern Ocean is an important physical AND biological sink of carbon, and we must consider the role krill and other animals have in this cycle.

Figure 2: Processes in the biological carbon pump including the sinking of dead phytoplankton aggregates, zooplankton, krill and fish faecal pellets and dead animals. Microbial remineralisation is depicted through the return of particulate organic carbon to dissolved organic carbon (DOC) and eventually carbon dioxide.

Authors:
Emma Cavan (Imperial College London and University of Tasmania)
Anna Belcher (British Antarctic Survey)
Angus Atkinson (Plymouth Marine Laboratory)
Simeon Hill (British Antarctic Survey)
So Kawaguchi (Australian Antarctic Division)
Stacey McCormack (University of Tasmania)
Bettina Meyer (Alfred Wegener Institute for Polar and Marine Research and University of Oldenburg)
Stephen Nicol (University of Tasmania)
Lavenia Ratnarajah (University of Liverpool)
Katrin Schmidt (University of Plymouth)
Deborah Steinberg (Virginia Institute of Marine Science)
Geraint Tarling (British Antarctic Survey)
Philip Boyd (University of Tasmania and Antarctic Climate and Ecosystems Cooperative Research Centre)

The past, present, and future of the ocean carbon cycle: A global data product with regional insights

Posted by mmaheigan

· Tuesday, January 21st, 2020

A new study published in Scientific Reports debuts a global data product of ocean acidification (OA) and buffer capacity from the beginning of the Industrial Revolution to the end of the century (1750-2100 C.E.). To develop this product, the authors linked one of the richest observational carbon dioxide (CO₂) data products (6^th version of the Surface Ocean CO₂ Atlas, 1991-2018, ~23 million observations) with temporal trends modeled at individual locations in the global ocean. By linking the modeled pH trends to the observed modern pH distribution, the climatology benefits from recent improvements in both model design and observational data coverage, and is likely to provide more accurate regional OA trajectories than the model output alone. The authors also show that air-sea CO₂ disequilibrium is the dominant mode of spatial variability for surface pH, and discuss why pH and calcium carbonate mineral saturation states (Omega), two important metrics for OA, show contrasting spatial variability. They discover that sea surface temperature (SST) imposes two large but cancelling effects on surface ocean pH and Omega, i.e., the effects of SST on (a) chemical speciation of the carbonic system; and (b) air-sea exchange of CO₂ and the subsequent DIC/TA ratio of the seawater. These two processes act in concert for Omega but oppose each other for pH. As a result, while Omega is markedly lower in the colder polar regions than in the warmer subtropical and tropical regions, surface ocean pH shows little latitudinal variation.

Figure 1. Spatial distribution of global surface ocean pH_T (total hydrogen scale, annually averaged) in past (1770), present (2000) and future (2100) under the IPCC RCP8.5 scenario.

This data product, which extends from the pre-Industrial era (1750 C.E.) to the end of this century under historical atmospheric CO₂ concentrations (pre-2005) and the Representative Concentrations Pathways (post-2005) of the Intergovernmental Panel on Climate Change (IPCC)’s 5^th Assessment Report, may be helpful to policy-makers and managers who are developing regional adaptation strategies for ocean acidification.

The published paper is available here: https://www.nature.com/articles/s41598-019-55039-4

The data product is available here: https://www.nodc.noaa.gov/oads/data/0206289.xml

Authors:
Li-Qing Jiang (University of Maryland and NOAA NCEI)
Brendan Carter (NOAA PMEL and University of Washington JISAO)
Richard Feely (NOAA PMEL)
Siv Lauvset, Are Olsen (University of Bergen and Bjerknes Centre for Climate Research, Norway)

A new roadmap of climate change driven ocean changes

Posted by mmaheigan

· Wednesday, October 2nd, 2019

When will we see significant changes in the ocean due to climate change? A new study in Nature Climate Change confirms that outcomes tied directly to the escalation of atmospheric carbon dioxide have already emerged in the existing 30-year observational record. These include sea surface warming, acidification, and increases in the rate at which the ocean removes carbon dioxide from the atmosphere.

In contrast, processes tied indirectly to the ramp-up of atmospheric carbon dioxide through the gradual modification of climate and ocean circulation will take longer, from three decades to more than a century. These include changes in upper-ocean mixing, nutrient supply, and the cycling of carbon through marine plants and animals.

The researchers performed model simulations of potential future climate states that could result from a combination of human-made climate change and random chance (figure 1). These experiments were performed with an Earth System Model, a climate model that has an interactive carbon cycle such that changes in the climate and carbon cycle can be considered in tandem.

Figure 1: Percentage of ocean with emergent anthropogenic trends in ocean biogeochemical and physical variables. A time series of the percentage of the global ocean area with locally emergent anthropogenic trends illustrates the disparity of emergence timescales for anthropogenic changes in the ocean carbon cycle. Emergence is defined as the point in time when the LE’s signal-to-noise ratio for a linear trend referenced to the year 1990 first exceeds a magnitude of two, which represents a 95% confidence in the identification of an anthropogenic trend in the LE Ω applies to the saturation state of both the aragonite and calcite forms of calcium carbonate (CaCO₃), for which the emergence times are approximately equivalent. The CaCO₃ and soft-tissue pumps were calculated as the export flux at 100 m depth of CaCO₃ and particulate organic carbon, respectively. The heat content was calculated as an integral over 0–700 m, whereas the oxygen (O₂) inventories consider the integral 200–600 m, and chlorophyll inventories were considered over 0–500 m. NPP represents an integral over 0–100 m. All the other variables represent sea surface properties.

The finding of a 30- to 100-year delay in the emergence of effects suggests that ocean observation programs should be maintained for many decades into the future to effectively monitor the changes occurring in the ocean. The study also indicates that the detectability of some changes in the ocean would benefit from improvements to the current observational sampling strategy. These include looking deeper into the ocean for changes in phytoplankton and capturing changes in both summer and winter ocean-atmosphere exchange of carbon dioxide rather than just the annual mean.

Figure 2. Venn Diagram schematic of sources of uncertainty in simulation (using Earth-System Modeling approach) and observation of changes in the Earth system. For emergence, detection or attribution of an observed or simulated signal to occur, the signal must overcome the sources of uncertainty in their respective brackets.

Many types of observational efforts, including time-series or permanent locations of continuous measurement, as well as regional sampling programs and global remote sensing platforms are critical for understanding our changing planet and improving our capacity to detect change.

Authors:
Sarah Schlunegger (Princeton University)
Keith B. Rodgers (Institute for Basic Science and Busan National University, South Korea)
Jorge L. Sarmiento (Princeton University)
Thomas L. Frölicher (University of Bern)
John P. Dunne (NOAA Geophysical Fluid Dynamics Laboratory)
Masao Ishii (Japan Meteorological Agency)
Richard Slater (Princeton University)

Regional circulation changes and a growing atmospheric CO2 concentration drive accelerated anthropogenic carbon uptake in the South Pacific

Posted by mmaheigan

· Tuesday, August 6th, 2019

About one tenth of human CO₂ emissions are currently being taken up by the Pacific Ocean, which makes the seawater more corrosive to the calcium carbonate shells and skeletons of the plants and animals that live there. Now, thanks to hard work by international teams of scientists from the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP), there are decades of data, enough to test how much this anthropogenic CO₂ accumulation varies throughout the Pacific Ocean and regionally on the timescales of decades.

Figure caption: Map of the concentration of human-emitted CO₂ along the sections where data were available from more than one decade, estimated for the year 2015.

Using a new take on an old technique, along with a wide variety of repeat biogeochemical measurements, a study in Biogeochemical Cycles revealed that Pacific anthropogenic CO₂ accumulation increased from the 1995-2005 decade to the 2005-2015 decade. While the magnitude of the decadal increase was consistent with increases in human CO₂ emissions over this period for most of the Pacific, the rate of change was greater than expected in the South Pacific subtropical gyre. The authors suggest that recent increases in circulation in the gyre region could have delivered an unexpectedly large amount of anthropogenic CO₂-laden seawater from the surface to the ocean interior. Programs like GO-SHIP will continue to be critical for tracking the fate of human CO₂emissions and associated feedbacks on climate and marine ecosystems.

Authors:
B. R. Carter (Univ. Washington and PMEL)
R. A. Feely, G. C. Johnson, J. L. Bullister (PMEL)
R. Wanninkhof (NOAA AOML)
S. Kouketsu, A. Murata (JAMSTEC
R. E. Sonnerup, S. Mecking (Univ. Washington)
P. C. Pardo (Univ. Tasmania)
C. L. Sabine (Univ. Hawai‘i, Mānoa)
B. M. Sloyan, B. Tilbrook (CSIRO, Australia)
K. Speer (Florida State University
L. D. Talley (Scripps Institution of Oceanography)
F. J. Millero (Univ. Miami)
S. E. Wijffels (CSIRO and WHOI)
A. M. Macdonald (WHOI)
N. Gruber (ETH Zurich)

A new era of observing the ocean carbonate system

Posted by mmaheigan

· Tuesday, August 6th, 2019

Amidst a backdrop of natural variability, the ocean carbonate system is undergoing a massive anthropogenic change. To capture this anthropogenic signal and differentiate it from natural variability, carbonate observations are needed across a range of spatial and temporal scales (Figure 1), many of which are not captured by traditional oceanographic platforms. A new review of autonomous carbonate observations published in Current Climate Change Reports highlights the development and deployment of pH sensors capable of in situ measurements on autonomous platforms, which represents a major step forward in observing the ocean carbonate system. These sensors have been rigorously field-tested via large-scale deployments on profiling floats in the Southern Ocean (Southern Ocean Carbon and Climate Observations and Modeling, SOCCOM), providing an unprecedented wealth of year-round data that have demonstrated the importance of wintertime outgassing of carbon dioxide in the Southern Ocean.

Figure 1: Observational capabilities and carbonate system processes as a function of time and space. Ocean processes that affect the carbonate system (solid color shapes—labeled in the legend) are depicted as a function of the temporal and spatial scales over which they must be observed to capture important variability and/or long-term change.

Most current autonomous platforms routinely measure only a single carbonate parameter, which then requires an algorithm to estimate a second parameter so that the rest of the carbonate system can be calculated. However, the ongoing development of sensors and systems to measure, rather than estimate, other carbonate parameters may greatly reduce uncertainty in constraining the full carbonate system. It is critical that the community continue to develop and adhere to best practices for calibration and data handling as existing sensors are deployed in increasing numbers and new sensors become available. Expanding autonomous carbonate measurements will increase our understanding of how anthropogenic change impacts natural variability and will provide a means to monitor carbon uptake by the ocean in real-time at high spatial and temporal resolution. This will not only help to understand the mechanisms driving changes in the ocean carbonate system, but will allow new insights in the role of mesoscale processes in regional and global biogeochemical cycles.

Authors:
Seth M. Bushinsky (Princeton University/University of Hawai’i Mānoa)
Yuichiro Takeshita (Monterey Bay Aquarium Research Institute)
Nancy L. Williams (Pacific Marine Environmental Laboratory – NOAA / University of South Florida)

Efficient carbon drawdown allows for a high future carbon uptake in the North Atlantic

Posted by mmaheigan

· Wednesday, November 7th, 2018

As one of the major carbon sinks in the global ocean, the North Atlantic is a key player in mediating and ameliorating the ongoing global warming. Current projections of the North Atlantic carbon sink in a high-CO₂ future vary greatly among models, with some showing that a slowdown in carbon uptake has already begun and others predicting that this slowdown will not occur until nearly 2100. To ensure the credibility of future projections as needed for finding adequate mitigation strategies, it is important to address and reduce this uncertainty.

Percentage of anthropogenically altered carbon stored in the deep North Atlantic (left panel) and North Atlantic uptake of anthropogenically altered carbon (right panel) as simulated by 11 different Earth System Models for a high CO2-future. Black x and line in left panel mark the observational estimate and its uncertainties, while blue and red shading reflect model spread, including models that simulate deep ocean storage within (blue) and outside (red) these observational uncertainties.

A new study in the Journal of Climate identified some of the mechanisms behind this ambiguity by analyzing the output of 11 Earth System Models for a high-CO₂ future. The authors show that discrepancies among models largely originate around high-latitude gateways from the surface to the deep ocean. Through biological production, deep convection and subsequent transport via the deep western boundary current, these gateways remove carbon from the upper ocean. When enough carbon is removed to maintain a lower oceanic pCO₂ relative to atmospheric pCO₂, the ocean continues to take up carbon. The study reveals that the fraction of anthropogenic carbon that is stored below 1000 m depth is not only an indicator of current carbon removal from the upper ocean but also a predictor of future ocean carbon uptake. When models that lie outside the range of observational uncertainties in deep carbon storage (red shading, left panel of figure) were excluded, revised projections showed higher North Atlantic carbon uptake in the future with lower associated uncertainties (blue shading, right panel of figure). This result highlights the need to depart from the concept of more or less randomly chosen models when reporting on future projections and their uncertainties. Results that are more reliable and hence of better use for mitigation strategies can be gained by focusing solely on models that simulate key mechanisms within observational uncertainties.

Authors:
N. Goris (Bjerknes Centre for Climate Research, Norway)
J.F. Tjiputra (Bjerknes Centre for Climate Research, Norway)
A. Olsen (University of Bergen and Bjerknes Centre for Climate Research, Norway)
J. Schwinger (Bjerknes Centre for Climate Research, Norway)
S.K. Lauvset (Bjerknes Centre for Climate Research, Norway)
E. Jeansson (Bjerknes Centre for Climate Research, Norway)

Funding for the Ocean Carbon & Biogeochemistry Project Office is provided by the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA). The OCB Project Office is housed at the Woods Hole Oceanographic Institution.

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