Abstracts - Symposium on Marine Carbon Sequestration

Implementing the Paris Agreement is a formidable challenge. In pathways limiting global warming to 1.5°C with no or limited overshoot as well as in pathways with a higher overshoot, CO2 emissions are reduced to net zero globally around 2050. This would require far-reaching and unprecedented transitions in all sectors, and also large-scale use of negative emissions technologies (NETs), that is removal of greenhouse gases from the atmosphere by deliberate human activities. In that context, it is timely to assess the opportunities offered by the ocean to reduce the causes and also the consequences of climate change, globally and locally.

The effectiveness, feasibility, duration of effects, co-benefits, disbenefits, cost effectiveness and governability of four ocean-based negative emissions technologies (NETs) have been assessed together with other ocean-based measures. Their role in revising UNFCCC Parties’ future Nationally Determined Contributions will be discussed in the broad context of ocean-based actions for both mitigation and ecological adaptation.

All measures are clustered in three policy-relevant categories (Decisive, Low Regret, Concept Stage). None of the ocean-based NETs assessed are identified as Decisive at this stage. Negative emissions by Restoring and increasing coastal vegetation are uncertain, unreliable and unlikely to be climatically cost-effective but qualify as Low Regret due to their few disbenefits and many benefits for climate adaptation, food provision and biodiversity. Concept stage measures have potential but their practicality and cost-effectiveness for climatic benefits have yet to be demonstrated. Three NETs are at Concept Stage, one with low to moderate potential disbenefits (Marine bioenergy with carbon capture and storage) and two with potentially high disbenefits (Enhancing open-ocean productivity and Enhancing weathering and alkalinization). Ocean-based, Concept stage NETs are uncertain but potentially highly effective. They have high priority for research and development.

The ocean’s biological capacity to take up CO2 from the atmosphere and store it in the deep ocean and the seafloor is governed by the biological carbon pump (BCP). The BCP is a natural mechanism that sequesters carbon away from the atmosphere for decades to millennia and exerts an important control on global climate. Without the BCP, current atmospheric CO2 levels would be almost twice as high. 

The BCP is composed of two component pumps associated with the production, transformation, and downward transfer of two different types of biogenic carbon: (1) the organic carbon pump driven by photosynthetic fixation of CO2 into particulate organic carbon (POC) by phytoplankton, and (2) the carbonate pump driven by the production of particulate inorganic carbon (PIC) by calcifying plankton. The relative strength of these component pumps of the BCP determines the biological capacity of the ocean to sequester atmospheric CO2. 

In this talk, I will present the state-of-the-art on our mechanistic and quantitative understanding of the open ocean BCP and its components pumps. I will present current global estimates of the basic metrics of the BCP: how much biogenic carbon (POC and PIC) is synthesized in the sunlit surface ocean (0 – 100 m depth) each year, how much of this biogenic carbon gets exported from the surface to the underlying twilight zone (100 – 1000 m depth), and how little of this carbon survives its passage to the bottom of the twilight zone, thereby leading to carbon storage on century-timescales. 

I will also present our current understanding of the major mechanisms of the BCP: gravitational sinking of particles, active vertical transfer by migrating organisms, and physical mixing processes, each with best estimates of their carbon export and sequestration timescale. 

The BCP has traditionally been observed using particle traps with too limited spatial and temporal coverage. Now, technological developments in autonomous sampling platforms and biogeochemical sensors are rapidly changing the way we observe the BCP. I will present some of the technological highlights and discuss remaining observational gaps.

The impasse in international efforts to curb greenhouse gas emissions has led to increased interest in the study and development of carbon dioxide removal (CDR) methods to enhance atmospheric carbon drawdown and sequestration. Ocean iron fertilization (OIF) is arguably one of the best studied ocean-based CDR approaches. However, OIF is not featured in most CDR research agendas and initiatives, being perceived as inefficient and with potential large negative impact on ecosystems. Since the early 90s, when OIF in the Southern Ocean was first proposed as an ocean-based CDR approach, seven OIF experiments and two studies of natural iron fertilization (downstream of islands) were carried out in the Southern Ocean. While in all cases iron fertilization has resulted in significant increases in plankton biomass and productivity, the impacts of iron fertilization on plankton assemblage composition, food webs, carbon export and carbon export efficiency are still poorly known and understood. The potential and uncertainties of this approach both as a tool for basic research and ocean-based CDR approach will be highlighted, based on results from 3 OIFs (EisenEx, EIFEX and LOHAFEX) carried out in closed eddy cores ensuring horizontal containment and vertical coherence from the surface down to 3000 m depth.

Ocean alkalinization is an approach to remove carbon dioxide from the atmosphere by increasing the alkalinity of the coastal seas and the surface ocean to enhance the ocean’s capability to act as a natural carbon sink. Adding alkalinity to the ocean removes carbon dioxide (CO2) from the atmosphere through a series of reactions that convert dissolved CO2 into stable bicarbonate and carbonate molecules, which in turn causes the ocean to absorb more CO2 from the air to restore equilibrium. 

Ocean alkalinization comprises the natural response of earth system to counteract global warming, and the geological record demonstrates that it is capable of substantial CO2 drawdown. Ocean alkalinization occurs naturally through chemical rock weathering. Rising temperatures and higher CO2 concentrations increase the weathering rate of carbonate and silicate rocks due to faster reaction rates (temperature effect) and greater acidity (CO2 effect). This negative feedback counteracts global warming and stabilizes the Earth’s climate over longer timescales. As it happens, most of the anthropogenic CO2 that is currently emitted to the atmosphere will be removed again by natural ocean alkalinization. Yet, this process will take 1000’s of years, as weathering only responds slowly to global warming. 

The idea behind “ocean alkalinization” is to speed up this natural CO2 neutralization process by adding minerals to the ocean that release alkalinity upon chemical weathering. This idea of “enhanced weathering” involves: (1) selectively using minerals with high dissolution rates, such as freshly mined carbonate or silicate rock, but also waste products from mining or industrial byproducts (2) increasing the reactive surface area and dissolution rate by pulverizing the source rock into small particles, and (3) distributing the resulting mineral particles in locations with high weathering rates. 

In theory, ocean alkalinization could remove many billions of tons of CO2 per year, yet the true CO2 sequestration potential remains poorly constrained, as research on ocean alkalinization itself remains in the early stages of research and development. Much research remains to be done to assess the CO2 sequestration efficacy, the cost and economic feasibility as well as the environmental impacts. In this lecture, I will give an overview of our ongoing research on ocean alkalinization. We investigate the potential of enhanced weathering in coastal environments in an internationally unique mesocosm facility (Oostende, Belgium) that enables us to study the enhanced silicate weathering in a controlled aquatic environment closely simulating natural conditions. Through this unique research facility, scientists can gain a better understanding of how certain factors from “the real world” will have to be accounted for in any future projects of the technology.

To limit global average temperature rise to below 2 °C above pre-industrial levels, global emissions of CO2 must be drastically reduced and excess greenhouse gases removed from the atmosphere. An emerging option to achieve both of these goals consists of natural climate solutions, including carbon sequestration through bio-based solutions. Seaweed production, both from wild stocks and from aquaculture, potentially represents an option for CO2 removal from the atmosphere. The high growth attained by seaweed lies at the basis for them being considered as an effective solution to capture CO2, but also the fact that seaweed cultivation does not compete for arable land, does not rely on fertilizers or pesticides makes a seaweed solution attractive. Yet, the potential of seaweed production to mitigate climate change by sequestering CO2 has not yet been fully incorporated into the emergent concept of Blue Carbon. Several options are being considered at present, including large-scale offshore cultivation of large brown seaweed. Here I synthesize data from scientific literature, assess the extent and cost of scaling seaweed aquaculture to provide sufficient CO2 sequestration, discuss potential transfer of farmed seaweeds to the deep-sea to accomplish a longer removal from the carbon cycle and their putative role in reducing greenhouse gas emissions from the overall food system through carbon offset.

This presentation will summarise evolving insights from a running feasibility study on the application of shell material in building materials. Shells form a waste-, or by-product of shellfish aquaculture, and consist almost completely of calciumcarbonate and thus for 12% of pure carbon. When used as building material this carbon can be captured for a long time. There is however debate if carbon sequestration in shell material does have a positive climate impact.

Coastal habitats, especially tidal marshes are extremely valuable habitats delivering a lot of ecosystem services such as coastal protection and carbon sequestration. However due to embankments and changing hydrodynamic conditions many of the marshes are lost and the remaining ones are threatened by sea level rise (coastal squeeze). The last decades, restoration of tidal marshes became an essential element in estuarine management and restoration, mainly driven by environmental legislation. Many different techniques are used varying from breaching a former dike and just let the site develop to more engineered systems where the tidal influence can be manipulated and vegetation is managed. The success of these restoration projects is very variable and depends on many different factors such as the condition of the soil (former agricultural soils can be very compacted) location within the estuary, sediment concentration etc. The role of these marshes for carbon sequestration also depend on different factors, first of all the sedimentation rate which is dependent on suspended sediment concentrations and inundation frequency but of course also on the amount of organic matter. This can come as allochthonous carbon, imported from the catchment or as autochthonous carbon coming from phytoplankton or the marsh vegetation itself. As fresh water tidal marshes have a much higher primary production compared to saltmarshes, there is in general a clear gradient in carbon burial from the freshwater tidal marshes to the saltmarshes. However carbon mineralisation might be higher in the freshwater tidal areas. 

As marshes deliver many different ecosystem services there might be important trade-offs. Where tidal marshes are aimed at storing water during storm tides to improve safety, sedimentation should be minimal to keep the storage volume, hence carbon burial is less. This is the case in restored sites like the marshes with a controlled reduced tide along the Schelde estuary. These are designed to keep sedimentation minimal to keep the storage capacity for storm water. Marshes that should protect the dikes from erosion should be as high as possible and hence have a low inundation frequency and sedimentation.

Although tidal marshes can play an important role in carbon sequestration there is a clear trade-off between the different ecosystem services delivered as well as an important temporal aspect related to the succession of the marsh.

These points will be discussed during the presentation.

The seabed receives about 2 Gt carbon per year from organic matter settling down from the water column, and ~10% of this organic carbon is sequestered in sediments. This seabed carbon sequestration corresponds to a removal of up to 4% of our annual global carbon emissions, locking the carbon away from exchange with the atmosphere for centennial to millennial timescales. This seemingly small fraction is however comparable to the contribution of the aviation industry to global carbon emissions. Especially shelf seas (< 200m water depth) are important sites for carbon sequestration, burying up to 1.5% of our current annual emissions. Yet, shelf sea sediments also experience intense disturbance from various human activities that affect carbon sequestration. Here, we show how seabed carbon sequestration is affected by natural processes and human activities, and discuss what is needed to protect marine sediments in the Exclusive Economic Zone to optimize the potential for carbon sequestration.

A transformation to true CO2 neutrality will bring deep change into our society. One aspect of that change, that we all too easily claim to take into account but rarely really do, is the social side of the transformation. The social consequences of policy measures and climate action initiatives can be widespread and severe and often less visible or easily ignored. My talk is a reflection on that issue with a little focus on fisheries.

This contribution outlines the current international policy approaches and challenges on different marine carbon sequestration pathways. 

Since the adoption of the Paris Agreement (COP 2015, Decision 1/CP.21) and the publication of the IPCC’s Special Report on Global Warming of 1.5 °C (SR 1.5, IPCC 2018), numerous political actors have pledged zero emission targets at a certain point in the future. This balancing of carbon emissions with carbon reductions and removals has triggered a new organizing principle of climate policy at nearly all political levels. Contrary to emission reduction strategies, pathways involving carbon dioxide removal techniques (CDR), including marine carbon sequestration, are only very recently receiving increased political attention. 

Despite a growing political awareness of the oceans’ potential to mitigate climate change, the current policy frameworks of nature-based and geological storage options still leave many unanswered questions, slowing their implementation. Depending on the technique, uncertainties on the management of transboundary effects, ecological and societal risks, the technical feasibility or a general lack of scientific understanding are considered major obstacles.

The key challenge in the development and implementation of fit for purpose policy frameworks on marine carbon sequestration is to reconcile a safe and sustainable carbon sequestration process (i.e. without jeopardizing the ecosystems ability to absorb and contain carbon), while at the same time guaranteeing a sustainable development of the Blue Economy and a healthy living environment for society. Informed decision making aided by scientifically-underpinned carbon accounting mechanisms is considered to be key to meet this challenge.

Global warming is a fact and we need negative emissions to have a chance at keeping global warming at 1,5 degrees. The sea will play a crucial role in this.

However today we have no way of measuring the impact of different possible carbon sequestration ideas on effectiveness, biodiversity and biodensity. The current way of measuring successful carbon sequestration will probably lead to a tragedy of the commons scenario.

When we start measuring the amount of carbon in the ecosystem and rewarding a rise in carbon, then the risks of a negative scenario are a lot less than the current system.

The cost of doing nothing will be much higher since inevitably the current system will be applied and with it a high probability of negative outcome for biodiversity and biodensity.