“We are reaching the ocean’s limits as a climate safeguard”

The goal is clear: to keep glob­al warm­ing “well below +2°C” by 2100 (rel­at­ive to the pre-indus­tri­al era). How­ever, glob­al efforts to mit­ig­ate green­house gas emis­sions are insuf­fi­cient. To achieve the UN’s sus­tain­able devel­op­ment goals, it is now crit­ic­al for decision-makers world­wide to be more ambi­tious in terms of both mit­ig­a­tion as well as eco­sys­tem and soci­et­al adapt­a­tion. This raises the ques­tion as to what oppor­tun­it­ies are provided by the ocean to sup­port inter­na­tion­al cli­mate action.

The ocean – includ­ing land­locked and mar­gin­al seas – is a ‘cli­mate reg­u­lat­or’ for the plan­et (Fig. 1). Since the 1970s, it has absorbed 93% of excess heat, lim­it­ing the warm­ing of the atmo­sphere. It has also sequestered 25–30% of man-made CO2 emis­sions since 1750 and has received almost all the water released by melt­ing gla­ciers and polar ice caps. Without the ocean, cli­mate change would be much more intense than it is today.

How­ever, this has come at the cost of sig­ni­fic­ant con­sequences on the chem­ic­al and phys­ic­al pro­cesses in the ocean, such as warm­ing, acid­i­fic­a­tion, deoxy­gen­a­tion, and rising sea levels. These upheavals have already had detect­able implic­a­tions for eco­sys­tems and their ser­vices, and for soci­et­ies around the world¹². The ocean is how­ever more than just a vic­tim of cli­mate change, it is also a source of poten­tial solu­tions. We have assessed the main ocean-based meas­ures that have been described in the sci­entif­ic lit­er­at­ure³. They cov­er both mit­ig­a­tion and adapt­a­tion⁴⁵ and refer to address­ing the causes of cli­mate change, pro­mot­ing bio­lo­gic­al, eco­lo­gic­al and soci­et­al adapt­a­tion, and man­aging sol­ar radiation.

Accord­ing to dif­fer­ent cri­ter­ia – effect­ive­ness, feas­ib­il­ity, dur­a­tion of effects, co-bene­fits, dis­ad­vant­ages, cost-effect­ive­ness, and gov­ern­ab­il­ity – the ocean-based meas­ures we assessed can be grouped into 4 cat­egor­ies: Decis­ive, Low Regret, Unproven, and Risky. Such cat­egor­isa­tion is inten­ded to guide the devel­op­ment and imple­ment­a­tion of cli­mate policies, com­bin­ing mit­ig­a­tion and adapt­a­tion, at vari­ous stages of action: from the inter­na­tion­al level, under the frame­work of the revi­sion of Nation­ally Determ­ined Con­tri­bu­tions, to the loc­al level, through con­crete and planned action strategies, and passing through the nation­al level when defin­ing Cli­mate Plans.

This cat­egor­isa­tion sug­gests that, more ambi­tious con­tri­bu­tions should stim­u­late action based on ocean-based solu­tions by pri­or­it­ising Decis­ive (e.g. mar­ine renew­able energy) and Low Regret meas­ures (e.g. con­ser­va­tion and res­tor­a­tion of coastal veget­a­tion, involve­ment of loc­al com­munit­ies in adapt­a­tion actions, or revi­sion of risk reduc­tion policies to bet­ter take into account anti­cip­ated cli­mate change).

Unproven meas­ures have very high poten­tial effect­ive­ness but have so far been through little or no test­ing and as some of them, such as improv­ing pro­ductiv­ity in the open sea and alka­lin­isa­tion, may have high poten­tial draw­backs. There is a need to improve know­ledge of these Unproven meas­ures as well as those that are con­sidered Risky due to their poten­tial neg­at­ive col­lat­er­al effects (e.g. sol­ar radi­ation management).

It is also cru­cial to note that the rel­ev­ance of some meas­ures will depend on the con­text in which they are deployed. While infra­struc­ture-based adapt­a­tion (e.g. coastal dykes) may, in some situ­ations, offer a sus­tain­able solu­tion for cli­mate risk reduc­tion (Decis­ive), in oth­er con­texts it will prove coun­ter­pro­duct­ive in the long term (Risky). Sim­il­arly, the relo­ca­tion of people and eco­nom­ic activ­it­ies can be decis­ive in the long term for low-lying coastal areas (Decis­ive), provided that it is sup­por­ted by a long plan­ning and sup­port pro­cess before­hand, and without which there is a high risk of increas­ing the vul­ner­ab­il­ity of relo­cated pop­u­la­tions and activ­it­ies (Risky).

Anoth­er level of com­plex­ity lies in the fact that none of the meas­ures will be suf­fi­cient on their own, and there­fore that the design of any robust “cli­mate solu­tion” neces­sar­ily relies on the iden­ti­fic­a­tion of con­text-spe­cif­ic com­bin­a­tions of responses. While Decis­ive and Low Regret meas­ures are clearly pri­or­it­ies for action, it is import­ant to under­stand that full imple­ment­a­tion of Decis­ive meas­ures will not com­pletely elim­in­ate coastal risks. Also, the long-term effect­ive­ness of Low Regret meas­ures, par­tic­u­larly nature-based solu­tions, will be determ­ined by the future level of glob­al warm­ing. There­fore, sci­entif­ic research must con­tin­ue to explore the field of Unproven solu­tions and to under­stand the con­di­tions of applic­a­tion of Risky solu­tions.

This ele­ment of dia­gnos­is refers to a key prin­ciple of cli­mate action: rather than think­ing in terms of indi­vidu­al, ideal­ised solu­tions, a more prom­ising aven­ue con­sists of think­ing in terms of mit­ig­a­tion and adapt­a­tion “tra­ject­or­ies”. The “tra­ject­ory” lens refers to the sequen­cing of a diversity of responses over time, accord­ing to new know­ledge on cli­mate change and its impacts at both glob­al and loc­al levels.