Anthropogenic carbon emissions and ocean acidification

Since the beginning of the 19th century the ocean has absorbed more than 500 billion tons of CO2 from the atmosphere, around 31% of anthropogenic CO2 emissions since the third industrial revolution in the 1970s1. As a major consequence, seawater has experienced an increase in acidity, and changes in its chemistry are occurring worldwide, a global process known as Ocean Acidification (OA)2. As a consequence of OA, biodiversity, ecosystem functioning, and biogeochemical cycles could be profoundly impacted, leading to major concerns for human food security and well-being3,4,5. Minimizing OA is a big challenge for humanity that requires collective action on a global scale6, including the reduction of atmospheric CO2 emissions7, supporting renewable energy, promoting sustainable production practices8, the protection and restoration of oceanic and coastal ecosystems9, raising awareness and education10, as well as supporting scientific research6. During the last decade, the threat of OA has increasingly reached the public and political spheres. OA is now a headline climate indicator for the World Meteorological Organization (WMO)11 and is part of the 2030 Sustainable Development Goals, SDGs (IOC-UNESCO), which incorporated target 14.3 focused on “minimizing and addressing the impacts of OA”12. For such purposes, some actions have been established, including “the monitoring of average marine acidity (pH) measured at agreed suites of representative sampling stations in both coastal and open ocean” (Indicator 14.3.1). Through this indicator, governments, organizations, and individuals are encouraged to work together to protect and restore seawater pH levels, to ensure the well-being of both marine ecosystems and human communities.

Minimize and address the impacts of ocean acidification and the use of an indicator for a global scale process

At present, an SDG 14.3.1 Data Portal has been implemented for the submission, collection, validation, storage, and sharing pH of data (https://oa.iode.org/). An indicator methodology for this SDG has been shared within the scientific community, which addressed multiple logistic concerns. Sampling strategy protocols recommend the use of ‘long-term monitoring of water quality’ in coastal areas as a useful source of historical records for seawater pH and an ideal location for OA monitoring13. While it is critical to sample these areas, data derived from ‘coastal water quality monitoring’ typically show great natural variability in seawater pH and other carbon chemistry parameters caused by factors other than those driven by OA, especially due to freshwater discharges, organic matter and/or nutrient loading14,15 (Fig. 1). Moreover, methods and data quality (QC, quality control) for addressing water quality objectives are distant from those needed to describe the rate of OA trends (i.e., 0.01–0.02 decade−1)16. Therefore, data aiming to evaluate OA progression must be collected using standardized measurement protocols with common reference materials (CRM)11. Besides recognizing that sites designated for water quality monitoring may not be optimal for OA monitoring, it is strongly recommended to also individualize monitoring programs focused on global-scale drivers (i.e., the increasing ocean uptake of CO2 emissions derived from human activities), of those sites focused on water quality monitoring or other local factors influencing changes in seawater acidity. While coastal pH monitoring can support different objectives (e.g., pollution and ecosystem restoration), the use of this indicator complicates the monitoring of the target 14.3 (i.e., addressing the impact of global OA), since multiple factors and actors are driving decadal trends in seawater pH in the coastal ocean.

Fig. 1: Decadal changes in coastal pH and dissolved oxygen in a coastal area.
figure 1

A A 24-year time-series of pH and dissolved oxygen as part of the water quality monitoring assessed by the Florida Department of Agriculture and Consumer Services (FDACS), (Robins & Lisle19) (Data source: (http://www.freshfromflorida.com/Divisions-Offices/Aquaculture), and B a 34-year time-series of pH, dissolved oxygen, and chlorophyll in the framework of the Marine Water Quality Data, from the Environmental Protection Department, Government of Hong Kong (Data source: https://cd.epic.epd.gov.hk/EPICRIVER/marine/).

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Multiple local drivers of decadal changes in seawater pH in the coastal ocean

Despite the high observed temporal variability, a decadal linear trend in seawater pH can be observed in coastal areas, although substantially driven by other local factors than only global OA, such as anthropogenic nutrient loading or changing river alkalinity15,17,18. For instance, chlorophyll concentration, a proxy for phytoplankton biomass, and dissolved oxygen are both commonly used as indicators of high primary production and eutrophication in coastal zones, as they link nutrient enrichment with algal biomass and nutrient-fueled-respiration respectively. Both indicators have been consistently correlated with seawater pH in coastal environments (Fig. 1). Just using two examples from different continents, different ecosystems (estuary and embayment), and with enough temporal longitude (i.e., >20 years data) confirms this statement. For instance, a 24-year’ time-series in a Florida estuary19 evidences the decreasing trends for both pH and oxygen in this coastal area (Fig. 1A), and a 34-year’ time-series of seawater quality monitoring in coastal Hong Kong demonstrates the impacts of eutrophication, resulting in high phytoplankton biomass (i.e., chlorophyll) in the 1980s and 1990s, which then declined gradually stabilizing the chlorophyll and pH levels in this coastal area (Fig. 1B). There are increasing lines of evidence that changing land use in river basins can also drive significant changes in quantity and quality of terrestrial riverine material exported to the adjacent coastal areas, impacting seawater alkalinity, partial pressure of carbon dioxide (pCO2), and pH20,21. Importantly, fluxes of terrestrial carbon have been altered on decadal scales due to the impact of climate change on hydrological cycles22, and the anthropogenic impact on both river flow regimes (i.e., damming and irrigation)23 and changing land uses24,25. In addition, coastal upwelling regimes, which also impact the carbonate chemistry of shallow waters26 are also changing over decadal scales due to climatic forcing 27,28. Finally, ocean warming 29 could also be playing a role in the decreased solubility of CO2 at increased temperatures and in decreasing alkalinity by ice-melting in high latitudes30,31. Accordingly, decadal patterns of pH in coastal areas have been driven by atmospheric CO2 emission associated with fossil fuel use and industrial processes, but also other human activities such as increased waste, nutrient loadings, and changing land uses32. Therefore, monitoring seawater pH can be a key indicator to assess marine acidity and coastal sustainability, although interacting drivers can confound the direct links with OA, an ongoing process caused by the current ocean uptake of human-derived CO2 emissions.

Future scenarios of changing pH in coastal habitats are extremely difficult to predict due to the multiple drivers of coastal pH33,34. Indeed, for some specific coastal regions, increasing river alkalinity over decadal time scales could increase the seawater pH and slow the OA trend (e.g., Gulf of Mexico35). Hence, factors leading to ocean and coastal acidification often occur concurrently and follow different patterns36,37, (Fig. 2), leading to the emergence of average decadal or yearly trends but also deviating from seasonal extremes due to all those pH/pCO2 controlling variables (e.g., temperature, dissolved inorganic carbon, and alkalinity)38. Despite these complexities, we can move forward in meeting the 2030 agenda for sustainable development in relation to pH levels in the ocean, by accounting for linkages among different drivers and their relationship with advancing key sustainable development goals (SDGs), which can jointly allow us to take actions to ensure healthy ocean chemistry in coastal areas.

Fig. 2: Long-term changes in coastal seawater pH can be driven by multiple processes both influenced by inland and shallow marine waters.
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Different processes determine the long-term variability in coastal pH, including decadal changes in the upwelling regime, the long-term impact of ocean warming on ice melting at high-latitude, decadal anthropogenic forces, such as modifications of natural river flow by damming, changing land uses and freshwater alkalinity, and nutrient loading. Global ocean acidification also has a signature at the decadal scale on coastal pH, and therefore, the combination of drivers may have significant implications for carbonate chemistry in the coastal ocean and marine life. Minimizing the impact of acidification in coastal areas implies multiple actions from different SDGs, such as SDG 2, 6, 12, 13, 14, 15, and 17.

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Linking different SDGs with drivers of coastal acidification

Meeting the 2030 Global Agenda for sustainable development will be challenging and will require partnership, innovation, and holistic and harmonized approaches and strategies at multiple scales. An understanding of the social and environmental contexts39 that drive coastal pH is needed in order to realize how to address the issue from multiple perspectives. As a result of the combination of drivers in coastal habitats, a deeper understanding and public discussion about the societal causes and policy actions to tackle changes in relation to seawater pH is needed40. Minimizing and addressing the impact of changing pH requires a dialog between the policy and scientific communities, but also requires establishing linkages among the different drivers. Although there is recognition of the need to address linkages among SDG 14 targets41, and there have been calls to support synergies and minimize tradeoffs between SDGs42,43, there is still little discussion regarding the role that other SDG goals can play in addressing the impacts of acidification on coastal areas. Research has mainly highlighted the co-benefits of reducing OA for other SDG targets (e.g., lead to greater productivity and reduce poverty and hunger)44, but has focused less on exploring how addressing different SDGs can influence coastal acidification, thereby signaling place-based initiatives that can help limit changing coastal pH trends. In this respect, an example is taking place in Washington State (https://oainwa.org/).

The lack of discussion about how we can link different SDGs with changing seawater pH in the coastal ocean could relate to the fact that indicator 14.3.1 is assumed to exclusively measure a specific global target (i.e., minimize and address the impact of OA). However, such indicators, measured in coastal areas, are, in fact, driven by different factors. The assumption that seawater pH in coastal environments is an issue that is exclusively related to anthropogenic CO2 emissions may paralyze action at local scales. On the other hand, recognizing that addressing coastal pH requires actions across many different SDGs, targets, and indicators may improve action at local scales.

Here, we provide some examples of linkages between pH and its direct and indirect association with SDGs 2, 6, 12, 13, 14, 15, and 17 (Table 1). For instance, Target 2.4.1. of SDG 2, which addresses sustainable agriculture and improvement of land and soil quality, might have direct consequences on changing land uses and nutrient loading in watersheds and, therefore, on coastal pH15,18,19,20,21. SDG 6 indicators linked with improving water quality through pollution reduction and protecting and restoring water-related ecosystems (e.g., rivers) also have direct effects on nutrient loading, eutrophication, and coastal pH trends15,17,18 (Table 1A). Achieving the management of chemical and all wastes stated in SDG 12 also has an indirect effect on coastal pH drivers, such as changing land uses and nutrient loading45,46,47 (Table 1B). One of the more evident linkages between SDGs is those related to Climate Action (SDG 13), since Indicators related to Target 13.2 (i.e., greenhouse emissions) have direct influences in almost all processes driving pH changes in the coastal ocean (e.g., river flow, upwelling intensity/duration, ocean warming and ice melting, changing land uses, riverine nutrient loading, and atmospheric CO2) (Table 1C). Even within SDG 14, various SDG indicators are closely connected to the primary drivers of pH changes in the coastal ocean over decades (‘coastal acidification’), rather than solely focusing on global ocean acidification (Table 1D). SDG 15, which considers the management of forest areas and land uses, has shown direct consequences over pH driven by changing land uses, and some evidence on nutrient loading 20,21 (Table 1E). Finally, those indicators related to necessary partnerships for the goals highlighted in SDG 17 can also evidence some linkage with most drivers of changing coastal pH (Table 1F). Importantly, the strength of the interactions between SDGs and pH can vary in space and time, depending on local and changing conditions. Establishing adaptive monitoring programs48, which assess different SDGs and their interactions with pH, can further aid in exploring these linkages and dimensions in a policy-relevant way.

Table 1 Examples of relationships between targets and indicators of some relevant SDGs, and different processes driving decadal changes in seawater pH in the coastal ocean
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Addressing drivers of coastal pH as opportunities for place-based actions

The synergies among various SDGs with the mitigation of coastal acidification enable the linkage of local and national impacts to global phenomena. This offers opportunities for location-specific interventions that facilitate the design and emphasis of coordinated implementation of programs and policies addressing the SDGs. Some specific examples of how addressing SDGs at a local scale could effectively reduce a regional trend of decreasing seawater pH in coastal waters include;

  1. (a)

    Reducing nutrient loading, residential and agricultural runoff has positive effects over pH in coastal ocean, and therefore, mitigates global OA impacts on marine populations49.

  2. (b)

    In the same line, land use regulation through regional planning or zoning can help reduce both drivers of atmospheric CO2 emissions (SDG13), and therefore, ocean acidification (SDG 14.3), and drivers of pollution and water quality (SDG 6) and coastal acidification.

  3. (c)

    While in many nations, it might be easier to measure pH directly, in some circumstances, while capacity is being developed, monitoring other biological indicators could provide important complementary information. For instance, Pacific Island states or other countries with coral reef ecosystems may also use current legislation for developing biological indicators of water quality for acidification in order to assess if a coastal area is impaired based on the negative trend of a specific biological indicator (e.g., coral biodiversity and biomass)50. This approach could also be useful where monitoring ocean acidity has been patchy over large temporal scales and could even provide further support for the implementation of monitoring programs and policies that integrate SDGs51.

  4. (d)

    Mitigation strategies for controlling ocean alkalinity could be also applied in this context of multi-drivers of coastal pH, such as liming riverine waters, by adding neutralizing materials to lower soil acidity, and/or water quality regulations (SDG6) for increasing freshwater alkalinity creating a buffer for OA in the adjacent coastal ocean35.

  5. (e)

    Another potential approach to mitigate the regional trend of decreasing seawater pH relates to the potential of returning crushed shell material to coastal habitats to mitigate localized acidification impacts on shellfish populations52,53.

  6. (f)

    Finally, although education is an indirect action with a long-term quantifiable benefit, it can create awareness and concerns that might result in coalitions of different actors, such as local governments, industry, and other stakeholders, for taking additional actions54, a partnership for the goals (SDG17) (Table 1).

As indicated, making synergies explicit could aid local and regional authorities in implementing new or existing policies to address many drivers of changing pH in coastal areas. Importantly, considering linkages could also help identify possible tradeoffs, which could be managed and accounted for in policy (see43 for suggestions). For instance, actions to meet Goals 2 and 8 (eliminate hunger and increase economic growth) might generate negative impacts when growth in some economic activities might lead to resource exploitation practices that increase nutrient loading and negatively affect pH in watersheds and coastal zones18,20,21,24,46,47. Indeed, different studies have established how nutrient loading or land use changes for agriculture or farming can lead to changes in alkalinity and reductions in pH20,21,24.

Conclusions

Dealing with ocean pH from an integrated perspective is important. In addition, if indicator 14.3.1. is intended as an instrument for assessing the long-term trend in OA forced by the increase in atmospheric CO2, rather than ‘coastal acidification’, monitoring the average marine acidity (pH) requires some specific recommendations for global actions, including the monitoring of coastal sites located away from the influences of freshwater runoff, upwelling areas, and major human activity driving nutrient loading and eutrophication, and more specifically the selection and more intensive monitoring of oceanic sites, which are less affected by anthropogenic coastal signatures. Unfortunately, at present, only two pH time series data sets are long enough to estimate long-term anthropogenic trends in the open ocean (WHOTS and Strattus) and make it possible to separate the anthropogenic signal from natural variability55. Nevertheless, the scientific community may be able to make more headway in addressing Indicator 14.3.2 by using pCO2 time-series data in conjunction with Total alkalinity–salinity relationships to expand our understanding of changing acidity in open oceans. While monitoring pH is important and necessary, deepening our knowledge and understanding of the linkages between pH in coastal areas and the developments associated with different SDGs can provide key opportunities for coastal acidification to be addressed as part of synergistic interactions between SDGs. Engaging with coastal acidification through other SDGs can inform on what capacities, policies, and governance structures could be prioritized guiding different local actors towards necessary action.