The oceans are an enormous store of carbon, substantially greater than on land or in the atmosphere and hence play a key role in the global carbon cycle, especially in helping regulate the amount of CO₂ in the atmosphere. The oceans are important because they have taken up around 28-34% of the CO₂ produced by humankind through the burning of fossil fuels, cement manufacturing and land use changes since the industrial revolution. Whilst this has somewhat limited the historical rise of CO₂ in the atmosphere, thereby reducing the extent of greenhouse warming and climate change caused by human activities, this has come at the price of a substantial change to ocean chemistry. In particular, and of great concern, is the 30% decrease in ocean pH and 16% decrease in carbonate ion concentration between pre-industrial days and today. These changes are known as 'ocean acidification'. Our understanding of the impact of CO₂ on the carbonate chemistry is such that we know with very high certainty that ocean acidification will continue, tracking future CO₂ emissions to the atmosphere.

Evidence from experiments and observations are raising concern that future ocean acidification may be a threat to many marine organisms. This is mainly due to the rate at which ocean carbonate chemistry is changing which may exert pressure on marine organisms to respond. Responses can come in many forms. Marine organisms may migrate to alternate habitats, while others may simply be able to acclimatize to changes in their environment. Those that cannot may not survive these rapid changes to ocean chemistry. These different scenarios will have wider implications on food webs and ecosystem function. Moreover, the scale and direction of impacts due to ocean acidification on biogeochemical cycling of carbon, nutrients and climate reactive gases and other potential feedbacks to climate is largely unknown. Here, ocean acidification events in the Earth's past may help interpret the future of our oceans in a world of unabated CO₂ emissions. No exact geological analogue exists for society's activities today as the rate of acidification today is faster than at any time in the last 65 Ma years and the oceans chemistry is different today than during large parts of Earth's history. For example, past oceans contained higher calcium ion concentration, which helped stabilize calcium carbonate minerals in marine organisms' skeletons. The most comparable example is the ocean acidification and warming during the Palaeocene-Eocene Thermal Maximum (PETM) 55 Ma ago. Based on the observed biotic reaction at the PETM recorded in the geological record, the extent of future acidification, particularly in the deep ocean, raises the possibility of similar extinctions. After the PETM, recovery of the deep sea benthic marine ecosystems from this event took more than a hundred thousand years. The much faster rate of future changes in the surface ocean may challenge the ability of some key calcifying organisms to adapt or migrate.

At present, the socio-economic impacts of ocean acidification are difficult to predict. However, the goods and services provided by the marine environment to the UK are important, for example, multi million pound fisheries, fish meal and aquaculture industries employing tens of thousands of people and if impacted by ocean acidification could have a direct economic impact. Globally coral reefs have been valued at $30 billion providing food, tourism and shore protection so any threat to them will be important for the economics of some of UK's overseas territories.

Ocean acidification is a global scale threat but impacts will be felt at the local and regional scale. It is therefore likely that UK coastal waters, ecosystems and habitats will be impacted this century as CO₂ emissions continue to rise. The only way of reducing the impact of ocean acidification is the urgent and substantial reduction of these emissions.

What is already happening: High (Chemistry) / Low (Biology)

The chemical changes (white X) are measurable and happening now, the chemistry is known so certainty is high. There is little evidence for current biological impacts (black X) of ocean acidification up to today (and therefore little agreement) but this may be because there are no long term data bases that look at this.

Our overall assessment of confidence in the summary report card 'for what is already happening' is reported as 'high' where our need to understand what has happened is currently focussed upon changes in ocean chemistry. 

What could happen: High (Chemistry) / Medium (Biology)

Our assessment of what could happen in the future to ocean chemistry (white X) is based on good understanding of ocean carbonate chemistry, general circulation models and dependant the rate of future CO₂ emissions to the atmosphere.

Our assessment of what could happen to biology in the future (black X) is based on recent models projections indicating vulnerability of the Arctic and coastal upwelling to carbonate undersaturation, further work highlighting the vulnerability of corals to decreasing saturation and other experimental evidence increasingly revealing that juveniles, reproduction, physiology etc. of different organisms may change as a result of ocean acidification, even if adults are not. Species specific reactions to ocean acidification experiments show the complexity of the biotic reaction and tradeoffs that might occur between different physiological processes. The great majority of the growing evidence from field work, experiments, modelling and geological record indicates that the consequences for the future could be very serious but there are still substantial knowledge gaps together supporting the continued Medium rating.

Our overall assessment of confidence in the summary card for 'what could happen in the future' is reported as 'medium' where our need to understand what could happen in the future requires a broader view of the potential ecosystem level impacts as well as the changes in ocean chemistry.

The top priority knowledge gaps that need to be addressed in the short term to provide better advice to be given to policy makers are:

1. Impact of future ocean acidification on global and UK coastal organisms and ecosystems taking into account the full life cycle and physiology of individuals and potential "adaptability".
2. Biogeochemical feedbacks between ocean acidification and climate change and the impact of these global scale changes to local and regional scales
3. Impact of changing ocean chemistry on the goods and services provided by the marine environment.

The UK's coastal and shelf sea environments and the biodiversity within them provide a wide range of goods and services that are essential for the maintenance of social and economic well being (Beaumont et al., 2008). These goods and services can be defined as provisioning, regulating, cultural and supporting services (MEA, 2003), providing benefits at several levels (local, regional and global) and to different groups (individuals and public bodies).

Provisioning services (food provision, raw materials)

Globally over 1 billion people rely on fish as their main animal protein source, especially in developing nations (Pauly et al., 2005). The UK population traditionally enjoys fish and although its own national industry has declined over recent decades, it is still an important industry. In 2008, the UK fishing industry had 6,573 fishing vessels utilising a mixture of gears and techniques to catch a broad variety of fish, such as mackerel, cod, scallops and mussels. Landings by UK vessels amounted to 588 thousand tonnes of sea fish with a total value of £629 million (Irwin and Padia, 2009). Shellfish (nephrops, crabs, mussels etc) comprised 42% of total harvest. The fleet comprised 12,761 fishermen with 80% of these being full time fishermen.

Analysis of the catch landings based on sea area showed that the Northern North Sea was the most productive region contributing 62.3% of the value of finfish and 31.3 % of shellfish (Figure 9). Due to its rocky coastline, the Northern North Sea region is valuable for pot fisheries for edible crabs and lobsters, and offshore fisheries for edible crab off the Yorkshire coast.

Figure 9. The value of (a) finfish, and (b) shellfish landed
at the major ports by UK vessels in 2008 arranged into seven
sea areas. The total value for the finfish landed was £260.4
million while the value of shellfish was £257.3 million.
© Crown copyright 2010 Reproduced by permission of
Cefas, Lowestoft.

Mounting evidence indicates that ocean acidification will likely impair calcification in animals with calcium carbonate shells and skeletons (e.g. Section 3, and reviews by Kleypas et al., 2006; Gazeau et al., 2007; Fabry et al., 2008; Hoegh-Guldberg et al., 2008). This includes commercially valuable molluscs, crustaceans and echinoderms. Further, most other commercially harvested species, such as finfish, prey on shellfish, echinoderms, crustaceans or their predators. Ocean acidification could therefore lead to degradation of marine resources which would result in a reduction in fish harvest and protein provision, and loss of revenue and jobs. For instance, using the Defra Sea Fisheries Statistics 2008 (Irwin and Padia, 2009), and assuming a 10-25% reduction in growth/calcification (with a doubling in atmospheric CO₂, see section 3) results in 10-25% loss of shellfish landings that is equivalent to £26.4 - 66 million per year loss in value and around 1,200-3,100 potential job losses.

Two significant raw materials extracted from the UK marine environment are fishmeal and fish oil, and seaweed. Fishmeal and fish oil are key constituents of pelleted diets for the intensive production of carnivorous fish species. In 2004, 192,000 tonnes of fishmeal were consumed in the UK of which 50,000 were produced locally with the remainder imported. The total value of the fishmeal UK market in 2004 was £81 million (European Parliament Report, 2004). Reduction in availability of these raw materials could therefore impact the extent and/or market cost of UK finfish aquaculture. In 2004, England, Scotland and Wales had 613 fish and shellfish farming businesses operating on 1329 sites, employing 3,412 people. The main finfish species farmed are salmon (139 000 tonnes mainly in Scotland) and rainbow trout (16 - 17,000 tonnes) (Defra, 2008). There is also a limited production of other species, such as carp and brown trout, and relatively new species to aquaculture such as turbot, halibut, cod and Arctic char have produced encouraging results. Thus should acidification have significant impact on the production of these raw materials there could be socio-economic consequences on dependent industries.

Globally, warm water coral reefs are valuable marine ecosystems. They are important for nature and represent a very high value for humankind, supporting millions of people through provision of food and income. Cesar et al., (2003) estimate that coral reefs provide nearly US$ 30 billion each year in net benefits in goods and services to world economies, including tourism, fisheries and coastal protection. Increased stress on food production systems such as coral reefs, driven by climate change or ocean acidification, could thus have significant repercussion on food provision and/or security; particularly in developing countries where fish provide the major protein source (Pauly et al., 2005).

Pearls are created naturally by shellfish through the secretion of aragonite but can also be cultured artificially in oysters. Currently, the global pearl farming industry is worth $1.5 billion each year and is expected to grow into a $3 billion per year industry by 2010 (International Pearl Convention 2007). A reduction in aragonite saturation may impact the rate of production and quality of both natural and cultured pearls and therefore the future pearl market. Less expensive "Mother of Pearl" used frequently in costume jewellery, button making and the arts and craft industry may also be impacted.

Regulating services (e.g. gas and climate regulation)

In addition to physical processes such as ocean mixing, tides, current and air-sea exchange, the chemical composition of the atmosphere and ocean is maintained through a series of biogeochemical processes regulated by marine organisms. Their ability to fix CO₂ through photosynthesis and transfer a proportion of this to the deep sea via this biological pump is a key part of the global carbon cycle - essentially the oceans are buffering the effects of climate change through the removal of a large proportion of the anthropogenic CO₂ (see sections 2 and 5). A recent study to value the role of marine biodiversity in gas and climate regulation found that the Isles of Scilly marine environment was fixing 136,495 tC y^-1 with a mean net present value of £47 million (Davis et al., 2008), implying that on a UK scale the value would be £ billions. Any stressor reducing ocean productivity and the biological pump would have substantial environmental and economical impacts. The economic cost of replacing these natural processes with industrial processes would be exorbitant.

Sediments play a crucial role in a number of key ecosystem processes, in particular the microbial cycling of carbon and nitrogen (section 3, Figure 4). In addition, the behavioural characteristics of species that live in or/and on the sediment are important determinants of sediment biogeochemistry and element cycling, as their activities result in a release of dissolved and particulate nutrients from the sediment to the water column where they support primary productivity. Changes to this coupling, of sediment and pelagic biogeochemistry, through ocean acidification could have worrying knock-on consequences to shelf sea productivity and the food webs it supports, but this is highly uncertain.

Cultural services (e.g. leisure and recreation)

A significant component of leisure and recreation in the UK depends upon coastal marine biodiversity (e.g. bird watching, sea angling, rock pooling and diving) which in turn supports employment and small businesses. The rapid growth of sea angling based on sustainable practices is recognized as a significant opportunity for the UK economy. If UK marine biodiversity declines as a result of ocean acidification and other drivers, the value of this sector will decrease, with a potential loss of revenue. In addition, the occurrence of harmful or unpleasant algal bloom can reduce the aesthetics of beach recreation, as has been experienced in the Adriatic over the last decade.

The enormous biodiversity supported by coral reefs underpins substantial tourist industries for many tropical countries, including UK entities, and often provide their main revenue. Countries with coral reefs attract millions of SCUBA divers every year, yielding significant economic benefits to the host country. Globally, tourism is estimated to provide US$ 9.6 billion in annual net benefits (Cesar et al., 2003) and a multiple of this amount in tourism spending. Coral reef biodiversity also has a high research and conservation value, as well as a non-use value, estimated together at US$ 5.5 billion annually (Cesar et al., 2003). Loss of coral reefs and their diversity would impact global tourism to these areas and their enjoyment by tourists, including those from the UK.
Supporting services (e.g. biologically mediated habitat)

Maerl beds, mussel patches and cold-water corals are among the most important biologically mediated habitats in UK waters supporting a large number of species. This includes the provision of refuge and food for juvenile life stages of commercially important shellfish such as the queen scallop, Atlantic cod, saithe and pollack (Hall-Spencer et al., 2003). Cold water corals grow in deep, CO₂ rich waters and may be even more vulnerable to ocean acidification through shoaling of the ASH than their tropical coral reefs (Roberts et al., 2006; Guinotte et al., 2006; Turley et al., 2007). Around 70% of known locations of these old, slow growing corals may be in undersaturated, corrosive waters by the end of this century, with some affected even earlier, thereby impacting economically valuable species that take refuge and feed there (Guinotte et al., 2006; Roberts et al., 2006).

Globally, coral reefs and mangroves play an important role in shore protection and enhance local productivity and biodiversity. It is estimated that tropical coral reef calcification rates will decrease with decreasing CaCO₃ saturation so that reef erosion will be greater than reef accretion in the next few decades depending on the location of the reef (Kleypas et al., 2006). This protective function of reefs was valued at US$ 9.0 billion per year by Cesar et al., (2003) so any decline in this function will have a socio-economic impact either through loss of low lying land habitat and infra structure or through a need for investment in shore protection.
In addition to these quantified values, reefs have drawn a mass of medical and pharmaceutical research interest in the pursuit of finding cures for human diseases. Any loss in these roles will have significant socio-economic impacts on the people that depend on these services.

Turley, C., C. Brownlee, H.S. Findlay, S. Mangi, A. Ridgwell, D.N. Schmidt, and D.C. Schroeder (2010) Ocean Acidification in MCCIP Annual Report Card 2010-11, MCCIP Science Review, 27pp. www.mccip.org.uk/arc