Climate change and salinity of the coastal and marine environment around the UK.

What is already happening • Salinity of eastern North Atlantic waters west of the UK has dramatically decreased over the last five years, probably in response to changes in the atmosphere in the western North Atlantic in the first years of the decade. • This dramatic freshening in ocean waters is not evident on the shelf beyond the northern North Sea waters east of Scotland with changes that appear within the bounds of typical interannual variability. • North of the UK, where the deep water (>800 m) flows from the Nordic Seas, the water freshened for five decades up until the late 1990s but has gradually become more saline over the last 20 years. • In the deep Rockall Trough where waters are thought to have originated in the North-West Atlantic, the salinity has remained stable over the last decade without the increases expected to have been passed on to it from changes in the Labrador Sea. • Some sustained observations are no longer available, but new models merged with data are just becoming available that may fill gaps in coverage. • Large interannual to decadal variability makes simple linear trends of salinity in the shelf seas less useful than assessments of identification of periods of fresh and saline anomalies. What could happen in the future? • There is considerable uncertainty regarding future salinity. • Most 21st Century projections suggest UK shelf seas, and the adjacent Atlantic Ocean, will be less saline than present, driven by ocean circulation changes in response to climate change. • Centennial-scale salinity decreases in UK shelf seas are likely to be driven by ocean circulation change. Reduced inflow across the ocean-to-shelf boundary is thought to be the main driver of the salinity decrease. • Greater salinity decreases are projected for the North Sea, than the Irish and Celtic Seas. Readers are referred to previous reports for MCCIP that have described the drivers, evidence base and decadal evolution of salinity in the seas around the UK (Gommenginger, 2006; Holliday et al., 2008, 2010; Dye et al., 2013). Here we update the time–series observations and describe how our knowledge has changed over the last five years.

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