“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.