9. Considering the Role of Climate Change and Ocean Acidification in Healthy Seas

Climate change is change in the global or regional prevailing weather and oceanographic conditions. These changes can be due to natural causes (for example, variations in incident solar radiation), but since the mid-nineteenth century, human activities have been the main reason for the observed changes in climate. Human-induced climate change stems from increased concentrations of greenhouse gases in the Earth’s atmosphere. This is primarily due to the burning of fossil fuels, but also to changes in land use and deforestation (such as farming livestock and cutting down forests). The UN Framework Convention on Climate Change recognises seven major greenhouse gases (or groups thereof) as drivers of human-induced climate change: carbon dioxide, methane, sulphur hexafluoride, nitrous oxide, perfluorocarbon, chlorofluorocarbon, and nitrogen trifluoride. All these additional greenhouse gases have caused more of the Earth’s background radiation to become trapped inside the atmosphere (the “Greenhouse Effect”), although CO₂ emissions have contributed most. This additional energy has led to global warming, impacting the atmosphere, land, the cryosphere, groundwater, lakes, streams, and coastal and marine waters. In the ocean, climate change has caused seawater warming and heatwaves, decreasing oxygen concentrations, increasing sea-level and changes in stratification and circulation, together with many other related impacts across marine ecosystems and the services they provide to humanity.

These changes in the physical and chemical conditions of the marine environment are affecting marine ecosystems, with regional and local variations in the pressures and the rate of change, such as the higher rates of ocean temperature increase in the Arctic Waters Region. Thus, while the root cause is global, the effects are felt at more local scales. These localised effects can trigger changes in other regions, as for example when losses of Arctic sea ice affect the position and strength of strong winds such as the polar vortex and the jet stream, which may then cause extreme weather at mid-latitudes. This is truly a shared challenge, one that affects all OSPAR Contracting Parties.

Climate change is highlighted as a major driver of decline in many of the biodiversity Thematic Assessments, including on Marine Birds , Pelagic Habitats and Food Webs . In the OSPAR Maritime Area, climate change is expected to affect ecosystem functioning in the Arctic region more than in other more temperate regions. The Food Webs Thematic Assessment , for instance, points to studies showing that climate change has caused substantial increases in air and water temperature in the Barents Sea, resulting in loss of sea ice and habitats for ice-associated species in the Arctic region and shifts in both upper and lower trophic levels of the food web ( Food Webs Thematic Assessment ). Warming waters change the distribution of species, and the QSR 2023 provides evidence of changes in primary production rates and species composition for both phytoplankton and zooplankton, which in turn trigger changes at other trophic levels.

Although the effects of climate change are being felt across marine ecosystems in the OSPAR Maritime Area, a lack of agreed monitoring / indicators has limited regional assessment of climate change effects. Furthermore, the interaction between environmental degradation and climate change undoubtedly occurs, but the extent to which is it happening is not known. Findings from localised research andfrom other parts of the world point to compound negative effects from contaminant pollution, eutrophication, and ocean warming, for example, which collectively contribute to declines in productivity and increased disease spread. Climate change can also be identified as having a compounding effect (e.g. warming and the spread of thermophilic invasive species; increased storminess increasing resuspension of sediments) leading to accentuated negative impacts on both ecosystem functioning and resilience, as well as on the delivery of ecosystem services and benefits to humans.

The excess CO₂ released into the atmosphere by human activities and then drawn down into the ocean also leads to ocean acidification. The uptake of excess CO₂ by ocean surfaces (20-30% of the atmospheric CO₂ emitted by humans) has also caused changes in the ocean’s carbon chemistry, increasing its acidity. Ocean acidification changes the prevailing chemical environment to which marine organisms are exposed, with direct and indirect consequences for marine ecosystems. This environmental change affects many marine organisms, with significant negative impacts for calcareous or shelled organisms and indirect consequences for entire marine ecosystems.

Figure 9.1: Sea temperature trends at key locations in the OSPAR Maritime Area from HadSST sea surface temperature (Rayner et al., 2003), and selected time series from the ICES Report on Ocean Climate (Gonzalez-Pola et al., 2020).

Statistical significance of trends determined by Kendall rank correlation. Stippled areas show trend is not statistically significant. Data provided by Met Office – UK, Hafrannsoknastofnun - Marine Research Institute - Iceland, Institute of Marine Research - Norway, Marine Scotland Science – UK, Alfred-Wegener Institut Helmholtz-Zentrum für Polar- und Meeresforschung – Germany, Aquarium of San Sebastian (Oceanographic Society of Gipuzkoa and Oceanographic Foundation of Gipuzkoa) – Spain, Marine Institute/Met Eireann - Ireland)

Climate change will not only affect marine ecosystems but also have an impact on land. For instance, climate change is expected to alter land-use patterns and agriculture, resulting in changes in nutrient discharges. Episodic events such as flooding are expected to increase suspended sediment and nutrient inputs in coastal areas, although whether this cancels any general decrease in run-off is unclear. Changes in stratification of the water column, mixing and sea temperature will have profound biogeochemical effects and therefore change the sensitivity of the system to eutrophication pressures. The progress made in combating eutrophication could be put at risk by such climate change-driven nutrient releases, with their attendant effects on biodiversity, ecosystem functioning and human health.

Although both climate change and ocean acidification share their primary driver, they will at times need to be given individual consideration in policy making. Many measures aimed at reducing CO₂ emissions to the atmosphere will mitigate climate change and directly alleviate ocean acidification. On the other hand, some measures to mitigate climate change may focus on other greenhouse gas emissions and will therefore not contribute to combating ocean acidification. Policy responses will need to show the utmost care where potential measures to address climate change risk exacerbating ocean acidification. For example, the potential leakage from carbon dioxide storage sites or approaches that aim to increase ocean uptake of atmospheric CO₂, such as iron fertilisation, could increase ocean acidification. Finally, when considering adaptation to climate change and resilience, responses will also need to take into account the cumulative impact of climate change and ocean acidification, which may trigger synergistic stress in marine organisms. The information from the QSR assessments will be important as further mitigation measures are adopted by Contracting Parties, as adaptation policies are developed, as actions to restore the ecosystems in the OSPAR Maritime Area are taken, and as awareness of climate change impacts is raised.