Prevailing Conditions - Marine online assessment tool

The prevailing physical and chemical characteristics of UK seas help to determine the structure and function of our marine ecosystems. While they are not a measure of Good Environmental Status (GES) in themselves, these provide the background context within which GES is or is not achieved.

In the UK Marine Strategy Part One (2019), we were able to provide an update on the prevailing conditions of sea surface temperature, ocean acidification, turbidity (sea-surface suspended sediments), and salinity but assessment of the impacts of climate change on these conditions or on GES indicators was largely out of scope. The prevailing conditions are determined by a range of factors, including natural variability in the Earth’s climate system and change because of human activities.

Since the industrial revolution, the combustion of fossil fuels and changes in land use, deforestation and expansion of agriculture have led to an increase of greenhouse gas concentrations in the Earth’s atmosphere. These gases, especially carbon dioxide, have caused additional heat to remain trapped within the Earth’s atmosphere, causing climate change and ocean acidification. In the period 1971-2020, the ocean is estimated to have taken up approximately 90% of this additional heat. This has caused warming, decreased oxygen concentrations and rising sea-levels, with many further related impacts across marine ecosystems and the services they provide.

In the case of carbon dioxide (CO2), the elevated atmospheric concentrations have also been transferred to the ocean, with 20 – 30% of the CO2 added to the atmosphere by human activities since the 1980s now held in sea water (Pörtner, et al., 2019). This has caused changes in the ocean’s carbonate chemistry, in a process called ocean acidification. A sequence of chemical reactions causes CO2 and water to form carbonic acid, which is subsequently broken up into a bicarbonate ion and hydrogen ion. The result of this is an increase in the quantity of hydrogen ions (an increase in acidity which is measured as lower pH). This change in the prevailing chemical environment that marine organisms are exposed to may have consequences for marine ecosystems.

The Marine Climate Change Impacts Partnership (MCCIP) regularly updates its peer-reviewed assessments around the UK’s coasts, seas and surrounding ocean. These assessments of the prevailing physical and chemical conditions are used here to determine whether any trends observed in UK Marine Strategy Part One have continued, and to provide context for observed changes in assessed indicators. In addition, the recent OSPAR Quality Status Report includes an assessment of climate change (OSPAR, 2023) and ocean acidification in the Northeast Atlantic region, including UK waters. Below, we include an overview of these assessments in three themes: changes in the physical and chemical environment, consequences for the marine environment and impacts to society.

Changes in the physical and chemical environment

Increasingly, changes in the prevailing physical and chemical characteristics of UK waters are being seen, including increased sea temperature, acidification, and reduction in dissolved oxygen. These changes have been documented in several recent MCCIP Science Reviews (see MCCIP Physical Environment).

Sea surface temperatures (SST) around the UK have generally warmed by around 0.3 °C per decade in the last 40 years, although there are regional variations in trends, with warming rate highest in the southern North Sea. Marine heatwaves (short-lived extremes in temperature) have also increased in occurrence with, on average, four events more around the British Isles in 2000-2016, compared to 1982-1998 (Cornes and others 2023). In future, model simulations indicate a continued warming trend. By the end of the century (2079-2098), average annual mean temperature values are predicted to be 3.11 °C warmer than those in 2000-2019, for a high emissions scenario. This warming trend is expected to be stronger in the shallower region of the North Sea, and weaker in the surface waters of the adjacent North Atlantic. For more information on sea temperatures, see MCCIP Science Review on Sea Temperature (Cornes and others 2023).

Continued emissions of carbon dioxide have caused the average concentration of this greenhouse gas to exceed 414 ppm in 2021, with an average rate of increase of approximately 2.4 ppm per year over the last decade (2010-2019) (NOAA, 2022) leading to a reduction in surface ocean pH or Ocean Acidification. The decline in sea surface pH varies depending on location, even in the seas around the UK. On average, ocean pH has reduced by -0.02 pH units per decade (1990-2015). Near-bed estimates are similar for the UK shelf region, but again variable by location. In the central to north-eastern North Sea the rate has been higher (-0.05 pH units per decade). Climate models predict that ocean acidification will continue at the current rate until the second half of the century when it is thought it will progress faster depending on the rate of carbon dioxide emissions. The MCCIP report also noted that “Some species are already showing effects from ocean acidification when exposed to short term fluctuations and could be used as indicator species for long-term impacts on marine ecosystems”. For more information on ocean acidification, see MCCIP Science Review on Ocean Acidification (Finlay and others 2022).

Dissolved oxygen has been added to the set of parameters that MCCIP report on (Mahaffey and others 2023) since the UK Marine Strategy Part One (2019) noted its primary importance to life in the seas. OSPAR included the assessment of dissolved oxygen for the first time in Climate Change Thematic Assessment (OSPAR 2023). As the amount of oxygen that can be dissolved in seawater decreases with warmer temperatures, there is a strong relationship between climate change and dissolved oxygen. Climate change will also impact the strength and duration of stratification (where limited mixing creates a warm surface layer overlaying a cooler deep layer) and therefore will limit the processes whereby stratified deep water takes up oxygen from the atmosphere. MCCIP (Mahaffey and others 2023) reported that oxygen content has decreased globally since the 1960s. Recently, observations of dissolved oxygen in the North Sea and Celtic Sea have shown the onset of oxygen deficiency in late summer. In future, models predict that the shallow seas around the UK will decline most strongly, increasing the risk of oxygen deficiency in summer. The impacts of eutrophication (where nutrient inputs from human activities cause adverse effects on the ecosystem – see Eutrophication thematic assessment) are also a key control on dissolved oxygen concentrations. Additional impacts of climate change could occur due to increased rainfall and runoff could increase the risk of eutrophication and therefore cause local reductions in oxygen concentrations.

Changes in the ocean climate from human causes are not limited to rising temperature, increasing acidity or decreasing oxygen concentrations. For further information, the MCCIP evidence reviews of these other aspects of the physical environment change provide a summary of the latest evidence (www.mccip.org.uk).

Consequences for the marine environment

The changes in the prevailing physical and chemical marine environment due to human-induced climate change are already causing changes for marine organisms and their habitats. Across species, these changes can generally be categorised as: (1) habitat loss; (2) shifts in distribution; (3) changes to species composition and food webs; and (4) changes to life history events.

Climate change and ocean acidification will act as a pressure on all aspects of the food web, as increasing temperatures and declining levels of dissolved oxygen will affect the metabolic demands of organisms. Ocean acidification may make it harder for organisms to secrete calcium and thus may damage shellfish populations and reef structures.

The human-induced climate change effects have most directly been identified for the lowest components of the marine food web. Warming temperatures and increased stratification have almost certainly caused changes to the pelagic habitat, with evidence of declines in key phytoplankton and their zooplankton grazers. Changes include an increased dominance of small cells, which leads to longer food chains and less efficient transfer of energy to higher trophic levels. Biodiversity indicators reveal that the observed widespread declines in phytoplankton abundance and biomass are linked to changes in sea surface temperature from 1960 to 2019. Plankton species are important food sources for several fish species and impacts to fish and fisheries are therefore also likely.

There is a substantial evidence of the impacts of human-induced climate change and ocean acidification on benthic (sea floor) habitats (OSPAR, 2023). The intertidal community index (an indicator of sea surface temperature response on intertidal rock communities as part of the benthic biodiversity component- see Benthic habitats thematic assessment) has shown changes due to warmer temperatures.

Further examples of the identified impacts of climate change on the marine ecosystem include effects on fish and shellfish species and declines in breeding success and survival of some seabird species (OSPAR, 2023). Marine breeding success in most colonies is impacted by other pressures, in addition to climate change, and the marine bird distribution shows expansion in range by those associated with warmer climates. Fish and shellfish tolerance to changes in climate conditions, ecosystem and food web dynamics varies, these changes will affect fisheries activities and quota allocation, although to varying degrees depending on the species (see thematic assessment for Commercially exploited fish and shellfish)

Warming seas and ocean acidification will affect contaminants in the marine environment, although great uncertainty remains on the resulting changes in concentrations due to the complex impacts on bioavailability and degradation rates and differences between different chemicals. In general, warmer temperatures result in higher chemical reaction rates, thus increasing biological uptake (see Contaminants thematic assessment). Changes in the ocean climate will also likely influence the underwater soundscape and the propagation of sound, although the latter will likely be relatively modest (see Underwater Noise thematic assessment).

Impacts to society

Climate change and ocean acidification create risks and opportunities for the marine ecosystem and the human activities that depend on the ecosystem services it provides. Moreover, the ocean plays an important role in regulating the Earth’s climate and a range of natural ocean processes and new uses of the marine space offer opportunities to support reducing the concentrations of greenhouse gases in the atmosphere and therefore reducing impacts of climate change and ocean acidification.

MCCIP have reviewed the latest scientific evidence on the potential impacts to fisheries, aquaculture, coastal flooding, transport, infrastructure, and many other societal impacts. Fisheries impacts include shifts in the distribution and abundance of commercially exploited fish and shellfish, although other species have shown noticeable increases (for example tuna and squid), creating opportunities for new fisheries. Impacts on aquaculture include reduced fish survival at salmon farms when winter temperatures are milder.

Overall, many knowledge gaps remain that limit the possibility to fully assess the impacts of climate change and ocean acidification.

Reducing greenhouse gas concentrations in the atmosphere (also known as climate change mitigation) is a key commitment by the signatories of the Paris agreement. To achieve these ambitions, strong reductions in emissions of carbon dioxide, methane and other greenhouse gases from human activities are necessary. The uptake and storage of greenhouse gases through human or natural processes in the marine environment may also contribute to reaching Net Zero. Our use of the marine space has a role to play in such climate change mitigation: increasing renewable energy developments (wind, floating wind, tide and wave), capture and subsequent storage in geological reservoirs of carbon dioxide emissions and protecting habitats where the uptake and storage of carbon dioxide by natural processes occurs (blue carbon habitats and marine sediments). Such industrialisation of the marine space may increase pressures on species and habitats, for example increased underwater noise relating to marine renewable and offshore wind energy development and Carbon Capture and Storage exploration).

Specific benthic habitats could also provide opportunity to address climate change impacts (climate change adaptation) and reduce greenhouse gas concentrations in the atmosphere (climate change mitigation); the natural carbon uptake and storage capacity of some benthic habitats highlights their role in climate change mitigation, while some habitats such as saltmarsh can also act as protection against coastal flooding and erosion. These therefore generate co-benefits for climate adaptation and supporting the resilience of biodiverse ecosystems.

References

Cornes, R., Tinker, J., Hermanson, L., Oltmanns, M., Hunter, W., LloydHartley, H., . . . Renshaw, a. (2023). Climate change impacts on temperature around the UK and Ireland. MCCIP Science Review, p. 18pp.

Devlin, M., Fernand, L., & Collingridge, K. (2022). Concentrations of Dissolved Oxygen Near the Seafloor in the Greater North Sea, Celtic Seas, Bay of Biscay and Iberian Coast. The 2023 Quality Status Report for the North East Atlantic. Retrieved from https://oap.ospar.org/en/ospar-assessments/quality-status-reports/qsr-2023/indicator-assessments/seafloor-dissolved-oxygen

Findlay, H., Artoli, Y., Birchenough, S., Hartman, S., Leon, P., & and Stiansy, M. (2022). Climate change impacts on ocean acidifcation relevant to the UK and Ireland. MCCIP Science Review, p. 24pp.

Mahaffry, C., Hull, T., Hunter, W., Greenwood, N., Palmer, M., Sharples, J., . . . and Williams, C. (2023). Climate change impacts on dissolved oxygen concentrations in marine and coastal waters around the UK and Ireland. MCCIP Science Review, p. 31pp.

NOAA. (2022). Increase in atmospheric methane set another record during 2021. Retrieved from https://www.noaa.gov/news-release/increase-in-atmospheric-methane-set-another-record-during-2021

OSPAR. (2023). Climatic Change Thematic Assessment. OSPAR Commission. London: OSPAR Quality Status Report. Retrieved from https://oap.ospar.org/en/ospar-assessments/quality-status-reports/qsr-2023/thematic-assessments/climate-change/

Pörtner, H., Roberts, D., Masson-Delmotte, V., Zhai, P., Tignor, M., Poloczanska, E., . . . Weyer, N. (2019). IPCC Special Report on the Ocean and Cryosphere in a Changing Climate. IPCC.

HM Government (2019). Marine Strategy part one: UK udpated assessments and Good Environmental Status. (https://www.gov.uk/government/publications/marine-strategy-part-one-uk-updated-assessment-and-good-environmental-status)