Climate change is already observable within some parts of the Celtic Seas ecoregion. Long-term datasets from the Malin Shelf indicate an overall rise in historical coastal sea surface temperature, ranging from around 10–10.5°C from 1960 to 1990, to around 11°C since. In southwest Ireland and the western Channel, the trend has averaged around 13°C since 2003. Analysis of the time‑series showed that there have been considerable changes in the physical environment and across multiple trophic levels in the Celtic Sea over the last 50 years, many of which are associated with ocean warming.
Mean annual sea surface temperature in the Celtic Seas ecoregion has shown an overall upward trend of about +0.5°C since 1975, with a steeper rise from 1980–2005 and a broadly flat trend since (Figure 8). Water temperatures are strongly influenced by the strength of the North Atlantic thermohaline circulation, itself influenced by complex ocean-atmospheric coupling resulting in alternating warming and cooling periods. Water temperatures on the shelves are influenced by the degree of cross-shelf transport, as well as local seasonal heating.
Within the Celtic Seas ecoregion, the IPCC Regional Concentration Pathway (RCP) 8.5 scenario projects a 1.5 to 2.5°C warming above mean conditions for years 2050–2099. There is, however, considerable spatial variability in projected warming.
Projections suggest increases in mean sea level, surges and storminess bringing increased risk of coastal flooding, coastal erosion, damage to vulnerable coastal habitats, and socioeconomic risk. If global warming continues, sea levels could rise by between 15 and 95 cm within this century, increasing the risk of flooding and coastal erosion in low-lying coastal areas.
Primary production
Net primary productivity (biomass of carbon produced per m2 per day [NPP]) derived across the whole ecoregion shows a decline from just below 800 mg °C /m2/day in the early 2000s to around 600–650 from 2006.
Phytoplankton
The positive trends in diatoms (see the 'State' section) are not significantly associated with sea surface temperature, while the decreasing trend of dinoflagellates are largely negatively correlated with temperature, particularly in offshore areas.
Zooplankton
The decline in the abundance of larger copepods and gradual change to a warmer water zooplankton community is likely driven at least in part by climate change. The abundance of meroplankton relative to holoplankton shows an increasing trend throughout most of the Celtic Seas offshore areas and adjacent regions. In the neighbouring North Sea, a strong correlation of the meroplankton increase with increasing SST has been taken as strong evidence that it is driven by climatic and oceanographic change. The decline in holoplankton is mainly driven by small copepods and may reflect a change in growing conditions associated with earlier blooms and lower/higher phytoplankton/picoplankton biomasses during summer (i.e. potential climate driven nutrition‑related mismatch). The overall decline in holoplankton suggests a shift from pelagic to benthic productivity in the plankton community.
Fish
Temperature affects the migration, distribution, onset of spawning, and recruitment of commercially important widely distributed species such as, blue whiting, Northeast Atlantic mackerel, western horse mackerel (Trachurus trachurus), boarfish (Capros aper), Atlantic cod, sole, and plaice. Forecasting the consequences of climate change for recruitment and overall productivity of commercially targeted fish stocks remains challenging. To date, there are no examples of where environmental drivers are use to routinely forecast recruitment.
Based on the available information, species in the Celtic Sea are at risk from warming and lower primary production, while species west of Scotland are shifting northwards. Distributional shifts have been reported for most of the commercially important species in the ecoregion: cod, haddock (Melanogrammus aeglefinus), whiting, hake, monkfish (Lophius sp.), plaice, sole, megrim (Lepidorhombus whiffiagonis), blue whiting, herring, mackerel, and horse mackerel. Temperature has been suggested as the main driver of these changes but other processes such as density-dependent habitat selection, geographical attachment, and species interactions are also thought to play a role.
In recent decades several commercial fish species have shown changes in growth that have consequences for stock productivity and may indicate a change in ecosystem structure and functioning. Most notably, there was a sharp reduction in size‑at‑age of Celtic Sea herring from the mid-1970s to the 2000s, which was most strongly and non-linearly associated with temperature.
Socioeconomic impacts
There have been no detailed assessments of the effects of climate change on fleets and fishery-dependent communities in the ecoregion. The main management implication of these distributional shifts is the mismatch between regional abundances and TAC allocation, with hake and mackerel being two examples. This mismatch, in combination with the landing obligation, could result in choke species issues and challenges the relative stability currently used to distribute quotas. The changes in mackerel distribution has complicated international management agreement and sustainability certification for the fishery.
An important development driven by the need to respond to climate change is the rapid increase in offshore renewables. This is especially marked in the North Sea, but there are also development proposals for tidal-stream, for floating wind to the west of Scotland, Irish Sea, and Celtic Sea. The development of large offshore renewables in the ecoregion is likely to affect fisheries because mobile gears are often excluded from such areas, although static gears may be compatible.
Knowledge gaps
Synoptic information on oxygen levels, pH, and nutrients is difficult to source for the Celtic Seas. Detailed knowledge about climate change interactions for marine mammals and seabirds is also a major gap. Biogeographic changes linked to climate change also require work. One initial study showed evidence that Lusitanian species were moving into the Celtic Sea, although boreal species were not diminishing.
Finally, understanding where climate change effects are expected to exacerbate the effects of other human pressures is not understood or sufficiently researched.
