Ocean acidification; what is it and what can we do?
Message 1: Ocean acidification is a global threat with local consequences. Through humanactivities atmospheric carbon dioxide (CO2) levels are increasing and are currently around 425ppm(May 2023), up from 280ppm at the start of the industrial revolution. This increase would have beeneven greater were it not for the ocean, which has absorbed around a third of the estimated 1.5
trillion tons of CO2 we have generated so far. As CO2 enters the ocean it reacts with the seawater tocreate carbonic acid which reduces the pH (a measure of acidity) [Figure 1, top panel]. Many years oftaking up human-derived CO2 has made our ocean around 40% more acidic than pre-industrial levels[Figure 1, bottom panel]. The CO2 also reacts with other chemical elements to alter the availability ofcarbonate, a key component in the shells and skeletons of marine organisms, such as shellfish andcorals. The rate of ocean acidification is accelerating globally but is experienced most strongly incoastal areas, where other processes exacerbate the phenomenon.

Figure 1: Top panel, a simplified description of the chemical reactions which occur when carbon
dioxide (CO2) dissolves in seawater. Collectively these reactions represent the phenomenon known as
Ocean Acidification. Bottom panel, “pH stripes” illustrating the relative change in global pH levels
over the past 35 years with red colours representing higher pH levels (low acidity) and blue colours
representing lower pH levels (higher acidity). SOCAT pCO2 data and ML salinity-derived alkalinity
data from satellites were used to calculate pH. Stripes were created by Luke Gregor, ETHZ. ESA
OceanHealth -Ocean Acidification project. Data: https://doi.org/10.5194/essd-13-777-2021

Message 2: The threat ocean acidification poses to marine ecosystems, biodiversity, andecosystem services is recognised at the highest level. The recent IPCC report on climate change
stated with high confidence that “Ocean warming and ocean acidification have adversely affectedfood production from fisheries and shellfish aquaculture in some oceanic regions” (IPCC, Summaryfor Policymakers, AR6, 2023,
https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_SPM.pdf). Seeking to protectbiodiversity, the UN’s Global Biodiversity Framework (December 2022) committed all member states
to minimizing the impact of ocean acidification on biodiversity and to increasing biodiversity
resilience through mitigation, adaptation, and disaster risk reduction actions. In the North Atlanticand adjacent Arctic waters, the OSPAR Quality Status Report highlighted ocean acidification as apresent and growing threat to marine ecosystems, and highlighted grave concern for importantcommercial species as well as those habitats and species with high conservation status. The impactsof ocean acidification on the marine environment are also at the heart of the 2030 Agenda for
Sustainable Development, with the Intergovernmental Oceanographic Commission of UNESCO
acting as custodian for the SDG indicator 14.3.1 “Average marine acidity (pH) measured at agreed
suite of representative sampling stations” which aims to track progress made towards the SDG target
14.3 Minimize and address the impacts of ocean acidification, including through enhanced scientificcooperation at all levels. In response to the ocean acidification problem the ocean science
community has come together to speed up the collection and sharing of knowledge.
Message 3: Combatting ocean acidification requires rapid and significant decarbonisation. As it isfor climate change, the single most important thing we can do to halt ocean acidification is tosignificantly reduce CO2 emissions as quickly as possible. However, even if we managed to achieve
this overnight, the amount of CO2 already in the system will take hundreds of years to remove andwill continue to drive ocean acidification for many years to come. But it is not too late to make adifference, the quicker we act to reduce emissions, the less severe and long-lasting future
acidification will be, avoiding irreversible regime shifts and leaving marine organisms and coastal
communities time to adapt.
Message 4: Protecting and restoring nature will increase resilience to the impacts of ocean
acidification. Healthy marine ecosystems, with high levels of biodiversity, will be better able to resist
the worst effects of ocean acidification. We must take actions that successfully reduce the myriad ofthose other environmental impacts that threaten our ocean (e.g. overfishing, habitat destruction,
resource extraction and pollution, including chemicals, human sewage, nutrients, light and noise). In
doing so we will give natural systems the best chance to keep functioning, as climate change andocean acidification pressures increase. This is not solely an altruistic pursuit. If we risk ecosystem
collapse then we also risk losing the goods and services we humans derive from our ocean; food,medicines, coastal protection, climate regulation and leisure, not to mention cultural and spiritualwell-being.
Message 5: Nature is our ally in the war against climate change and ocean acidification. The ocean,and the marine ecosystems it contains, is a natural part of the global carbon cycle. ‘Coastal Blue
Carbon’ habitats, such as mangroves, saltmarshes, seagrasses, and kelp beds capture large amountsof CO2, storing that carbon in sediments on the seafloor. However, this should not be seen as anovernight climate solution as these habitats take many years to build up their carbon stocks. Blue
carbon should be seen as our ‘Climate Pension’. In addition to their important role in climate action,

these habitats also support high numbers of species, and their protection is an effective weapon in
the fight against the current global crisis of biodiversity loss and species extinctions.
Message 6: We cannot rely solely on new marine Carbon Dioxide Removal (mCDR) technologies tosolve the climate and ocean acidification crisis. Chemical and biological ocean processes can act toremove CO2 from the atmosphere, storing it either as dissolved inorganic carbon in the watercolumn or as organic carbon that is eventually transported to the deep ocean to be recycled orsequestered into the seafloor. There is currently much interest in the potential to manipulate
natural systems in ways that will increase the scale or speed of these ocean processes as a way of
reducing atmospheric CO2 levels. Whilst these proposed mCDR technologies, if successful, wouldremove CO2 from the atmosphere, some of them would result in even more CO2 being present inocean systems, exacerbating the problem of acidification. For mCDR to be beneficial to ocean life it
must ensure that all the additional CO2 captured from the atmosphere is safely sequestered deep
beneath the ocean floor. Currently, the scientific evidence needed to judge the various risks andbenefits associated with different mCDR techniques, as well as the practicality, scalability andaffordability of proposed projects, is lacking. It should also be noted that, without significant
reductions in CO2 emissions, the application of widescale mCDR will do little to address the climate
or acidification crisis in our ocean, in the short term. Our approach should be to continue to researchpotential mCDR technologies, their risks and benefits, but with the intention of using them to reduce
the residual stock of atmospheric and oceanic CO2, once we have reduced CO2 emissions to as nearto zero as possible.
Message 7: Everyone can join the battle against ocean acidification. The Global Ocean Acidification
Observing Network (GOA-ON, http://www.goa-on.org) connects nearly a thousand scientist fromover 100 countries. Their goals are to observe and predict the rates and impacts of ocean
acidification. But it isn’t just scientists that are required to combat the causes and impacts of ocean
acidification. The ‘Ocean Acidification Research for Sustainability’ (OARS) programme (www.oarsun.org) is part of the United Nations’ Decade of Ocean Science for Sustainability initiative. OARS
provides a roadmap towards delivering 7 key outcomes (Figure 2) and will require engagement froma wide range of different ocean professionals and stakeholders. If you are interested in finding out
more about how you could contribute to OARS, please contact the GOA-ON Secretariat via email,secretariat@goa-on.org.

Figure 2: The 7 desired outcomes from the UN Ocean Decade programme “Ocean Acidification
Research for Sustainability”.