With the Paris Agreement, 197 countries have committed to keep global warming “well below 2°C” and to “continue to strive to limit [medio] to a temperature increase of 1.5°C” (UNFCCC 2015). In October 2018, the Intergovernmental Panel on Climate Change presented a special report stating that the 1.5 degree target is still achievable, but only if anthropogenic CO2 emissions start to decline sharply now and reach net zero by 2050 (IPCC 2018). The report also made it clear that net zero emissions will not be achievable without some form of carbon dioxide removal (CDR), in the order of 100-1000 Gt of CO2, during the 21st century.
To date, research on RDCs focuses on possible land-based solutions, such as afforestation and bioenergy with carbon capture and storage (BECCS). These and other land options often conflict with other societal needs and interests, such as food security, urbanisation and ecosystem conservation. Furthermore, it is already clear that many of these RDCs have side effects, sustainability-related trade-offs or limited CO2 removal potentials (Fuss et al. 2018).
Much less is known about ocean-based CDRs (Gattuso et al. 2018; Keller 2018; Figure 1) although the ocean (1) covers more than 70% of the Earth’s surface – implying that the area available for some CDR is less limiting than on land, (2) contains multiple times the amount of carbon in the atmosphere and terrestrial biosphere – and can store much more, and (3) is the largest long-term sink of anthropogenic CO2 – suggesting that the ocean’s natural carbon sinks would have high potential if they could be enhanced.
These characteristics suggest that the ocean has the potential to play an important role in CDR. In fact, by absorbing 90% of the additional heat generated by global warming and more than 25% of the CO2 released by human activities, the ocean already acts as a powerful buffer in the Earth’s climate system.