Editing IPCC:AR6/SROCC/Chapter-5 (section)

==== 5.5.1.3 Climate Mitigation in the Open Ocean ====

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Recent reviews of the scope for using natural processes in the open ocean for climate mitigation are provided by Keller (2019a) and GESAMP (2019). The summary assessment given here is limited to direct and indirect biologically-based approaches, consistent with the scoping of this report and the major governance constraints on the large-scale application of open ocean interventions.

Current NPP by marine phytoplankton is estimated to be 58 ± 7 GtC yr –1 (Legendre et al., 2015), similar to terrestrial primary production and around 6 times greater than anthropogenic emissions (Le Quere et al. (2016). However, over 99% of the biologically-fixed carbon returns to the atmosphere over a range of timescales (Cartapanis et al., 2018).

The direct method of increasing marine productivity involves adding land-derived nutrients that may currently limit primary production, particularly iron. This approach has been investigated experimentally, by modelling and by observations of natural system behaviour (Keller et al., 2014a; Bowie et al., 2015; Tagliabue et al., 2017). The 13 experimental studies to date (seven in the Southern Ocean, five in the Pacific, and one in the sub-tropical Atlantic) have shown that primary production can be, but is not always, enhanced by the addition of iron (Boyd et al., 2007; Yoon et al., 2016; GESAMP, 2019).

The difficulties arise in demonstrating the time-scale of additional carbon removal, and in obtaining information on the consequences of the fertilisation for other marine ecosystem components, including ocean acidification and other potential side-effects (Williamson and Turley, 2012). Modelling studies (Aumont and Bopp, 2006) indicate that the climatic benefits could be relatively short-lived. Furthermore, public and political acceptability for ocean fertilisation is low (Williamson et al., 2012; Boyd and Bressac, 2016; Williamson and Bodle, 2016; Fuentes-George, 2017; McGee et al., 2018). Ocean iron fertilisation is regulated by the London Protocol, with amendments prohibiting such action unless constituting legitimate scientific research authorised under permit (see Section 5.5.4.1). There are additional governance constraints for the Southern Ocean where ocean iron fertilisation is theoretically considered to be most effective (Robinson et al., 2014).

Open ocean fertilisation by macro-nutrients (e.g., nitrate) has also been proposed, with modelled potential for gigaton-scale carbon removal (Harrison, 2017). Similar technical and governance considerations apply with regard to the quantification of mitigation benefits, the monitoring of potential adverse impacts, and the political acceptability of large-scale deployment. This approach would also involve higher costs, because of the much greater quantities of nutrients required (Williamson and Turley, 2012).

The indirect method of enhancing marine productivity uses physical devices to increase upwelling, thereby increasing the supply of a wide range of naturally-occurring nutrients from deeper water. This technique risks releasing additional CO 2 to the atmosphere, reducing its potential for climate mitigation (Bauman et al., 2014). There may also be other undesirable climatic consequences, including disruption of regional weather patterns and long-term warming rather than cooling, if enhanced upwelling is deployed at large scale (Kwiatkowski et al., 2015).

Because of the many technical, environmental and governance issues relating to marine productivity enhancement, by either direct fertilisation or upwelling, there is ''low confidence'' that such open ocean manipulations provide a viable mitigation measure.

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