37 Marine Solutions to Climate Change
Atmospheric carbon dioxide has risen to 420 ppm from pre-industrial levels of 280 ppm, triggering climate change impacts on biodiversity and society (Bolin and Doos, 1989; Houghton, 1996; Bennett et al., 2016; Pecl et al., 2017; IPCC, 2019; Pörtner et al., 2019; Smale et al., 2019; Straub et al., 2019). Whereas reduced consumption, increasing energy efficiency and transitioning towards renewable energy and more sustainable agricultural systems are essential to achieve climate goals, enhancing removal of excess atmospheric CO2 is crucial to limit warming to 1.5 °C (IPCC, 2022).
‘Smoke Stacks Behind Trees Near the Water’ by Игорь Альшин used under a Pexels License
Half of the global CO2 fixation occurs in the oceans (Nellemann et al., 2010). As such, management of blue carbon – the organic carbon that is captured and stored by coastal and marine ecosystems, is of importance as it provides opportunities to increase oceanic sequestration of atmospheric CO2 (Mcleod et al., 2011). Vegetated coastal ecosystems are in the spotlight as their protection and restoration may offer pathways for climate change mitigation, and adaptation in the coastal ocean (Duarte et al., 2013; Macreadie et al., 2021).
Text Attribution
This text is from ” Potential role of seaweeds in climate change mitigation” © 2023 Finnley W.R. Ross, Philip W. Boyd, Karen Filbee-Dexter, Kenta Watanabe, Alejandra Ortega, Dorte Krause-Jensen, Catherine Lovelock, Calvyn F.A. Sondak, Lennart T. Bach, Carlos M. Duarte, Oscar Serrano, John Beardall, Patrick Tarbuck, Peter I. Macreadie and published by Elsevier B.V. which is licensed under CC BY-NC 4.0, except for some small text edits, which has been added by Trish Corbett.
End Text Attribution
Blue Carbon
Blue carbon describes the organic carbon, fixed from the atmosphere by marine ecosystems, that is stored over hundreds to thousands of years in the oceans. Blue carbon contributes to the regulation of the Earth’s climate, and thus it has been proposed that maintaining and enhancing blue carbon storage through maintaining the health of marine ecosystems will contribute to climate change mitigation and adaptation (Duarte et al., 2013; Nellemann & Corcoran, 2009). Coastal blue carbon ecosystems, which include vegetated coastal ecosystems dominated by saltmarsh, mangrove, and seagrass plants, store carbon in their sediments and biomass, estimated at between 10 and 24 Pg. (Duarte et al., 2013). Although they make a relatively small contribution to global climate regulation, (Taillardat et al., 2018) they are important components of the marine carbon cycle, accounting for approximately 50% of the annual organic carbon storage in the coastal oceans. (Duarte et al., 2005) They have high rates of carbon sequestration in their sediments per area of habitat that are up to ten times that of terrestrial ecosystems (Duarte et al., 2013; Mcleod et al., 2011) and have sediment carbon stocks per unit area exceeding those of soils in terrestrial ecosystems (Alongi, 2014). Some of the most impressive examples of blue carbon stocks include the 10-m-thick sediment peat deposits that underlie the mangrove islands of Belize, which are formed largely of dead mangrove roots (McKee et al., 2007), the 60-m-tall Rhizophora mangrove forests of Gabon (Simard et al., 2019), the 3-m-thick seagrass “mattes” of the Mediterranean(Mateo et al., 1997), which are sediments composed of seagrass roots, and the 7 m of saltmarsh peat, composed of saltmarsh roots, and its erosion after conversion to agriculture in the San Joaquin Delta of California (Drexler, 2011).
Global losses in the extent of coastal ecosystem are estimated at between 25% and 50% (Alongi, 2002; Mcowen et al., 2017; Waycott et al., 2009). Losses of mangroves and seagrass have largely occurred since the 1950s (Alongi, 2002; Waycott et al., 2009), while saltmarshes have been degraded and converted for centuries (Mcowen et al., 2017). These losses have negative impacts on populations of marine fauna, coastal protection, nutrient cycling, and delivery of other ecosystem services (Barbier et al., 2011; Duarte et al., 2013; Hochard et al., 2019). The global degradation of coastal ecosystems has led to significant carbon dioxide (CO2) emissions as carbon in biomass and sediments is disturbed and mineralized to CO2 (Pendleton et al. 2012; Murdiyarso et al., 2015; Atwood et al., 2017) with remaining degraded ecosystems having reduced capacity to provide climate change mitigation and other ecosystem functions and services (Howard et al., 2017). Thus, conservation and rehabilitation of coastal ecosystems are proposed as important pathways to maintain and increase carbon storage in the marine environment, thereby contributing to mitigation of climate change and supporting coastal resilience (Duarte et al., 2013; Howard et al., 2017). But conservation and rehabilitation of coastal ecosystems occur against a backdrop of climate change. This results in uncertainty in the effectiveness of protection and rehabilitation of coastal ecosystems and their carbon deposits.
The accumulation of sediment organic carbon in coastal wetlands occurs because a proportion of their high net primary productivity is deposited under anoxic conditions imposed by sediments saturated with saline water. Low oxygen levels in saturated sediments suppress microbial decomposition of organic matter (Chapman et al., 2019) (Figure 1). The organic matter stored is derived mostly from roots (Krauss et al., 2018; Xiong et al., 2018), although woody tissues and leaf litter can also be important components (Krauss et al., 2018; Van de Broek et al., 2018). Accumulation of organic matter in coastal wetland sediments is also enhanced by the trapping of organic material produced from sources other than coastal wetlands (i.e., allochthonous organic carbon from the land or sea), which can accumulate in high proportions in some sites (Breithaupt et al., 2019). Influx of mineral sediments in coastal wetlands from rivers or marine sources contributes to the rapid burial of organic matter (Spivak et al., 2019) and may also offer protection of organic carbon from decomposition through the formation of organic carbon-mineral complexes that are resistant to microbial attack (Spivak et al., 2019).
Image Text Version
Figure 1. Processes Contributing to Accumulation of Blue Carbon Stocks in Coastal Ecosystems Processes include burial of detritus and trapping of sediments and organic carbon delivered in tidal flows (allochthonous carbon from land or sea), which contribute to vertical accretion of sediments.
Figure 1 is entitled ‘Processes Contributing to Accumulation of Blue Carbon Stocks in Coastal Ecosystems Processes include burial of detritus and trapping of sediments and organic carbon delivered in tidal flows (allochthonous carbon from land or sea), which contribute to vertical accretion of sediments’. The diagram is superimposed over a photo of a coastal wetland environment which shows underwater vegetation and the wetland floor.
Inputs into the system encounter the vegetation, where the structure of the vegetation slows water and intercepts particles that sink. Sunken particles form organic carbon-mineral complexes resistant to decomposition, labelled as soil carbon stock. Matter from the soil carbon stock may be exported or re-enter the system via vertical accretion.
Solid carbon stock may enter the system as CO2 which, through photosynthesis, forms biomass carbon stock which may either:
- Result in burial of organic matter (detritus) in anoxic sediment, becoming soil carbon stock, or
- Enter respiration pathways, becoming CO2, which may be used in photosynthesis
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This text, Figure and it’s associated caption are from “Variable Impacts of Climate Change on Blue Carbon” © 2020 Catherine E. Lovelock, Ruth Reef and published by Elsevier Inc., which is licensed under CC BY-NC-ND 4.0
Studies into BC increasingly include a valuation aspect, focussed on coastal sites (Barbier et al., 2011) but more recently also including offshore sites (Barange et al., 2017), showing a range of values for different ecosystems as depicted in Figure 2. Differences in values are driven by differences in BC sequestration and storage capacity and/or potential avoided emissions through conservation and restoration of ecosystems.
Seaweed
Seaweeds are fast-growing marine macrophytes with modelling suggesting a global area and net primary production comparable to the Amazonian forest (Duarte et al., 2022). In addition to wild seaweed forests, seaweed farming contributes about half of the production of marine aquaculture (Duarte et al., 2021). Seaweed has been suggested to contribute to climate change mitigation through four potential pathways (Krause-Jensen et al., 2018; Duarte et al., 2021; Troell et al., 2022): 1) protecting and restoring wild seaweed forests with potential climate change mitigation co-benefits; 2) expanding sustainable nearshore seaweed aquaculture with potential climate change mitigation co-benefits; 3) offsetting industrial CO2 emissions using seaweed products for emission abatement; and 4) sinking seaweed into the deep sea to sequester CO2.
While seaweeds have great potential as a blue carbon system with kelp forests estimated to sequester 4.91 megatons of carbon from the atmosphere each year (by Eger et al., 2023), there remain a number of key research and policy challenges that need to be addressed to assess the viability of seaweed to as a potential solution to mitigate climate change (Figure 2; Ross et al., 2023).
Image Text Version
Figure 2. Diagram detailing how the identified research challenges relate to either seaweed aquaculture and/or wild/naturally occurring seaweeds.
Figure 2 is entitled ‘Diagram detailing how the identified research challenges relate to either seaweed aquaculture and/or wild/naturally occurring seaweeds’. The diagram shows a cross section of open air above an ocean, a seaweed aquaculture structure just below the ocean surface, and a wild seaweed forest anchored to the sea floor. Annotations from 1 to 8 describe the research challenges, these are detailed bellow alongside their location in the diagram.
Challenge 1
Knowledge gaps in the seaweed carbon cycle. Location is in the seaweed aquaculture zone.
Challenge 2
Developing technologies, computational models and tools. Location is in the wild seaweed zone.
Challenge 3
Contribution of wild seaweed to climate mitigation. Location is in the wild seaweed zone, close to the sea bed.
Challenge 4
Carbon sequestration from seaweed farming during farming phase. Location is above the seaweed aquaculture zone, into the open air.
Challenge 5
Emissions abatement from seaweed products. Location is above the seaweed aquaculture zone, into the open air.
Challenge 6
Sinking seaweed into the deep sea to sequester CO2. Location is above the seaweed aquaculture zone, into the open air.
Challenge 7
Risks of accelerating seaweed carbon sequestration. Location is in the seaweed aquaculture zone.
Challenge 8
Policy and governance challenges. Location is in the wild seaweed zone.
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Text and Image Attribution
This text and image are from ” Potential role of seaweeds in climate change mitigation” © 2023 Finnley W.R. Ross, Philip W. Boyd, Karen Filbee-Dexter, Kenta Watanabe, Alejandra Ortega, Dorte Krause-Jensen, Catherine Lovelock, Calvyn F.A. Sondak, Lennart T. Bach, Carlos M. Duarte, Oscar Serrano, John Beardall, Patrick Tarbuck, Peter I. Macreadie and published by Elsevier B.V. which is licensed under CC BY-NC 4.0, except the paragraph before Figure 2. which has been added by Trish Corbett.
End Text and Image Attribution
Reducing greenhouse gas emissions through reduced methane emissions from cows
Watch the video below from ABC News, which shows a Victorian initiative exploring the use of seaweed farms to reduce livestock emissions.
Further reading and viewing
Explore the podcast from 100 Climate Conversations featuring Maria del Mar Palacios on Blue Carbon to expand your thinking.
Activity: Test your Knowledge
Test your knowledge in the quiz below:
Activity Text Version
Activity: Test your Knowledge
This exercise invites readers to answer questions to test their knowledge, based on the marine solutions to climate change information above. Questions and answers are listed below.
Question 1
Select all correct statements about coastal vegetation from:
- Coastal vegetation such as saltmarsh, mangrove and seagrass store huge amounts of ‘Blue Carbon’ because the sediment the live in is highly oxygenated and breaks down detrital material and stores carbon in new vegetation.
- Coastal vegetation such as saltmarsh, mangrove and seagrass store huge amounts of ‘Blue Carbon’ because the sediment they live in is anoxic and is therefore detrital material is stored and not broken down.
- Mangroves have the highest ecosystem service value when compared to saltmarsh, seagrasses and coastal plankton.
- Saltmarsh have the highest ecosystem service value when compared to mangroves, seagrasses and coastal plankton.
Answers to question 1
- Coastal vegetation such as saltmarsh, mangrove and seagrass store huge amounts of ‘Blue Carbon’ because the sediment they live in is anoxic and is therefore detrital material is stored and not broken down.
- Mangroves have the highest ecosystem service value when compared to saltmarsh, seagrasses and coastal plankton.
Question 2
Promising ocean solutions to climate change include:
Select all that apply from:
- Reducing methane emissions in cows by supplementing feed with seaweed
- Blue carbon storage in coastal wetlands (saltmarsh, seagrass, mangrove habitats)
- Blue carbon storage in seaweeds such as kelp
- Blue carbon storage in marine phytoplankton
Answers to question 2
- Blue carbon storage in coastal wetlands (saltmarsh, seagrass, mangrove habitats)
- Blue carbon storage in seaweeds such as kelp
End Activity Text Version
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