1 of 11

Exploring the Potential of Ocean-Based Carbon Dioxide Removal Approaches in Achieving Multiple Mitigation Targets

Giang Tran, David Keller, Andreas Oschlies

2 of 11

  • Combine knowledge on tipping elements with new information about model uncertainties for determining multiple mitigation targets and related metrics for limits of damage.

​

  • Generate a baseline perturbed parameter ensemble using UVic ESCM constrained by observations.

​

  • Explore two ocean carbon dioxide removal options:
    • Ocean alkalinity enhancement
    • Macroalgae farming and sinking

​

  • Use the ensembles to assess the potential of the CDR methods in achieving multiple mitigation targets.

​

WP5 – Ocean mitigation options

3 of 11

  • Adding alkaline substance to the ocean to increase the capacity to absorb CO2 from the atmosphere.

​

  • In our simulations:
    • Deploy on a global scale (Keller et al., 2014)
    • From 2025 to 2100
    • Adding 5 Pg Ca(OH)2 or 0.135 Pmol of alkalinity per year

OceanNETS

Ocean Carbon Dioxide Removal options

4 of 11

Ocean Carbon Dioxide Removal options

  • Growing macroalgae on floating infrastructure and sinking biomass to the ocean floor (Wu et al., 2023)

​

  • Seaweed grows quickly and absorbs carbon dioxide through photosynthesis.

​

  • In our simulations:
    • Deploy on a global scale
    • From 2025 to 2100
    • Biomass is sunk annually

​

5 of 11

Mitigation scenarios

Four illustrative Shared Socio-economics pathways:

    • Very high emission scenario

​

    • Intermediate emission scenario

​

    • Low emission scenario

​

    • Overshoot scenario

6 of 11

Projected changes

ΔSAT (◦C) Alk ΔSAT (◦C) MOS ΔSAT (◦C)

SSP1-2.6 1.69 ± 0.19 1.56 (-0.13)

SSP2-4.5 2.65 ± 0.26 2.52 (-0.13)

SSP5-3.4 1.99 ± 0.23 1.86 (-0.13) 1.70 (-0.29)

SSP5-8.5 4.33 ± 0.35 4.23 (-0.10)

​

The weighted mean and 1 standard deviation are listed for each ensemble.

CO2 ccn (ppm) Oceanic uptake (PgC) Land uptake (PgC)

SSP 466 ± 16 271 ± 25 238 ± 63

ALK 442 ± 15 (-24) 331 ± 23 (+60) 221 ± 64 (-17)

MOS 413 ± 25 (-53) 480 ± 76 (+209) 160 ± 58 (-78)

7 of 11

Projected changes

8 of 11

Projected changes

  • A reduction in surface warming leads to more oxygen in subsurface layers for the scenario without and with alkalinization.
  • Deep ocean deoxygenation signal carries on beyond the end of this century.
  • MOS creates new deoxygenation zones at the sea floor through remineralisation of sunken biomass
  • Shrink thermocline minimum oxygen zones through reduction of export of planktonic organic matter.

mol m-3

SSP

ALK

MOS

9 of 11

  • Additional CDRs can push back the timing of crossing a level by a few years.

​

  • They can alleviate some negative impacts due to acidification at surface levels.

​

  • However, macroalgae farming and sinking could cause more deoxygenation in the deep ocean.

​

  • Macroalgae farming also severely impacts NPP at the surface.

​

Impacts of Carbon dioxide removal techniques

10 of 11

Impacts of Carbon dioxide removal techniques

11 of 11

Key takeaways

  • Additional CDRs have limited potential in limiting warming to 1.5oC and 2oC thresholds.
  • Even with stabilized emission and decreasing SAT, many trends continue due to committed warming.
  • Additional CDR can help combating acidification at surface layer but has a small overall potential despite their massive scale of deployment.
  • Macroalgae farming and sinking has a large impact on the distribution of oxygen over the entire ocean.

Further work:

  • Comparing different models.
  • Need to explore a portfolio of CDR techniques.

​