Summary of changes and trends

• Global sea surface temperature (SST) warmed from about 1910 to about 1940, remained steady and then began warming again during the 1970s.
• There is no clear trend in summer SST in the eastern North Atlantic since the 1950s, but a warming in winter SST since the early 1990s is indicated.
• SST at the Continental Shelf Edge warmed between 0.12°C and 0.29°C over the past century.
• Annual SST averaged around the UK coastline has increased by about 0.5°C for the period 1871 to 2000, with most coastal sites showing a warming trend.
• Most of the waters around the UK have been warming since the 1980s, with the trend more pronounced in the southern North Sea and the Irish Sea (between 0.5°C and 1.0°C per decade) than elsewhere (between 0.0°C and 0.5°C per decade).
• There is a warming trend in winter and summer SST averaged over the northern North Sea since the early 1980s, with a warming of about 1°C and 0.5°C respectively.
• North Sea winter bottom temperatures increased by about 0.3°C and 0.6°C per decade since a cool period in the late 1970s.
• Irish Sea annual mean SST increased by about 0.7°C over the last 100 years. Winter SST from 1950 to 2002 shows a clear warming since the 1980s. An apparent cooling in summer SST since the 1980s may be due to sparse data.
• The Faroe Shetland Channel has become warmer over the last 40 years, with temperatures rising at a rate of approximately 0.3°C per decade from the late 1960s minimum.
• Temperatures in the Rockall Trough were relatively low in the early 1990s but then increased. The highest temperatures reached in the 1990s were similar to those in the 1960s.

The role of the world’s oceans is critical in the global climate system because the high density and specific heat of water means that it can store and transport large amounts of heat. A meridional (tropics to poles) transport of energy is required for the Earth system to be in global radiative balance, with some 30-50% of the energy carried by ocean currents at mid latitudes and a higher proportion at lower latitudes (Bryden and Imawaki 2001). The ocean circulation is determined primarily by the forcing due to momentum, heat and water fluxes to and from the atmosphere, and by the distributions of temperature (and salinity) in the ocean that set its density structure, and hence density currents. In particular, the density structure affects the ‘meso-scale’ dynamics of fronts and eddies, which are the most energetic motions in the ocean.

Changes in sea temperature cause sea level changes, e.g. a warming causes sea level rise through thermal expansion.

Changes in sea temperature induce shifts in the geographic distribution of marine biota and changes in biodiversity, with direct effects on the species composition, breeding and population plankton and fish. Edwards et al. (2001) showed that there has been a steady increase in phytoplankton biomass in the North Sea Since the mid 1980s, with a peak in 1989 corresponded with anomalous warm sea surface temperatures. Beaugrand et al. (2002) found that the northward extension of more than 10° of latitude of warm-water copepod is related to the increasing trend in sea temperature. Hughes et al. (2003) suggest that there was a correlation between winter temperatures and catches of young cod in the North Sea during the 1990s.

A strongly positive NAO Index is associated with a large cold intermediate water layer and low surface and bottom water temperature. A strongly negative NAO Index is associated with a small cold intermediate water layer and higher surface and bottom water temperature. Changes in the NAO are reflected in sea surface temperature (SST), e.g. accounting for 40-50% of the variability in winter SST in the southern North Sea (Loewe, 1996).

Descriptions of the monitoring networks that regularly measure sea temperature data are given in Chapter 1, including details of how to access near real-time data.

Click here for a list of links to monitoring networks and data sets.

2. Global and North Atlantic sea temperature

The global ocean heat content has increased significantly since the late 1950s, with all oceans, including the Atlantic, undergoing a net warming (Levitus et al., 2000). More than half of the increase in heat content has occurred in the upper 300m of the ocean, equivalent to a rate of warming of about 0.04ºC per decade (IPCC, 2001). Global SST for the past 100 years shows two distinct warming periods, the first from about 1910 to 1940 and the second starting during the 1970s (IPCC, 2001).

Figures 1 and 2 show winter and summer SST data respectively in the eastern North Atlantic (55-60N, 25-15W). There appears to be no clear trend in summer SST since the 1950s, but a warming in winter SST since the early 1990s is indicated (Dooley, 2003).

In 2001, except for southern areas of the Newfoundland and the northern Scotian shelves, North Atlantic sea temperatures were above normal, (ICES, 2002); and in most areas of the North Atlantic during 2002 temperature was higher than the long-term average (ICES, 2003a).

3. UK waters sea temperature

Click here to see a figure of annual sea surface temperature averaged around the UK coastline for the period 1871 to 2000 (figure 15 of the UKCIP report). Sea-surface temperature has increased by about 0.5°C during this period, with a substantial increase over the last 20 years (Hulme et al., 2002).
Link to http://www.ukcip.org.uk/scenarios/pdfs/UKCIP02TechRep/UKCIP02_Ch2.pdf .

Figure 4 shows the annual sea surface temperature (with best fit line) from coastal stations in the UK.

Click here to see Table 1, showing trends in sea surface temperature from stations in UK waters.

The longest continuing records in UK waters (Dover, Eastbourne and Port Erin) show an increase in sea surface temperature of about 0.6°C over the last 75 to 110 years. The long records from the Newarp, Humber and Seven Stones Light Vessels, discontinued in the 1980s, also show a warming trend since the late 19th Century, but the latter has a cooling of about 1.5°C from the early 1970s to 1986 (which invites further investigation).

Shorter records, of between about 25 and 40 years, generally show a warming in annual SST of about 0.2°C per decade since the mid 1960s/1970s, but there is considerable inter-annual variability. Greater rates of warming are at Sizewell (+0.8°C per decade from 1967 to 2002), Shoreham (+0.3°C per decade from 1966 to 1997) and at Swansea (+0.6°C per decade from 1976 to 1997). The measurements at the St. Gowan Light Vessel and E1 show a cooling of about 0.2°C per decade from 1953 to 1987 and about 0.1°C per decade from 1921 to 1987 respectively.

Since the 1980s, Figure 3 (above) shows that most of the waters around the UK have been warming, with the trend more pronounced in the southern North Sea and the Irish Sea (between 0.5°C and 1.0°C per decade) than elsewhere (between 0.0°C and 0.5°C per decade) (FRS, 2003).

Figures 5 to 11 show mean sea temperature data for 2000 – 2002 from selected stations of the Met Office’s Marine Automatic Weather Station Network (MAWS) Network.

3.1 North Sea

The SST of the North Sea is mainly controlled by local and regional weather conditions, i.e. local solar heating and heat exchange with the atmosphere. However, temperature changes also can reflect the influence of the NAO on the movement of Atlantic water into the North Sea and the meteorological forcing of the ocean-atmosphere heat exchange, with a positive Index usually corresponding with warmer temperatures. In particular, changes in winter temperatures are closely linked with changes in the NAO Winter Index - Loewe (1996) states that the NAO accounts for 40-50% of the winter sea surface temperature variability in the southern North Sea.

Figure 12 shows that in the Fair Isle Current, temperature increased during 2002 to reach the highest values since the data set began in 1972. A cyclical variability is evident since 1977 (ICES, 2003a).

In 2001, the area-averaged mean SST of the North Sea in 2001 was 10.4°C (the same as in 2000), making it the sixth warmest year in records dating from 1971 (ICES, 2002). This was surpassed in 2002, when the area-averaged annual mean SST was 11.0°C (ICES 2003a). Overall, the integrated water column temperature in the North Sea is warmer now than in the 1960s (Brown, personal communication).

For coastal locations in the region, figures 22 to 26 display an analysis of the monthly means of SST for the years 2001 and 2002, with long-term minima, maxima and monthly means. Mean temperatures for 2001 and 2002 were above the long-term averages at most stations.

3.2 English Channel and Celtic Sea, including Bristol Channel

For coastal locations in the region, figures 27 to 29 show an analysis of the monthly means of SST for the years 2001 and 2002, with long-term minima, maxima and monthly means. Mean temperatures for 2001 and 2002 were above the long-term averages at most stations.

Figures 30 and 31 show winter and summer SST anomalies in the Irish Sea from 1950 to 2002, averaged over the area 53º-55ºN and 6º-4ºW. The dominance of the warming that has occurred since the 1980s over all UK waters is clear in the winter record. However, the observation coverage in the Irish Sea decreased significantly from the early 1980s onwards, which may explain the significant increase in variability in the mean since then and the apparent downturn in summer SST since the 1980s (Dooley, 2003).

Figure 3 (above) indicates that, together with the North Sea, the Irish Sea has experienced the most pronounced warming trend in UK waters, of between 0.5°C and 1.0°C per decade (FRS, 2003); albeit rising from the minimum values in the early 1980s.

3.4 Minches, west Scotland, Scottish continental shelf and Faroe Shetland Channel

The Scottish and Northern Ireland Forum for Environmental Research (SNIFFER, 2000) produced three regional sea temperature indices, a West Coast Index, a Shelf Edge Index and a North Sea Index, using both data measured in-situ along ocean transects at multiple depths (except for the West Coast Index) and through use of a global 1° resolution long-term sea surface temperature data set. The Shelf Edge Index (the only long-term index) showed warming of between 0.12°C and 0.29°C over the past century (depending on the use of in-situ measurements or the global sea-surface temperature data set).

Figure 34 shows that the trend in monthly mean sea temperature has been increasing since 1953 at Millport, Fair Isle and Peterhead.

With the exception of occasional episodes the water column at the mooring site is well mixed or weakly stratified throughout the year. The mean water temperature was 10.1°C over the 22 years and the dominant mode of variance in the temperature record is the seasonal cycle, with amplitude of 3.2 °C. Whilst the dominant modes of variability are as expected for a shelf location the seasonally adjusted temperature anomalies exhibit patterns similar to those reported in the NE Atlantic by Holliday (2003). This serves to emphasise the important role of Atlantic water in the Scottish Coastal Current. Comparing the monthly temperature anomalies with the NE Atlantic upper layer heat content anomalies shows that both time series had highs in the late 1980s and 1990s and lows in the early 1980s and mid 1990s (Inall and Griffiths, 2003).

3.5 Rockall Trough and Bank and Atlantic north west approaches

Figure 37 shows de-seasoned upper ocean (0-800m) temperature anomalies from the Rockall Trough, from 1975 onwards.

The early part of the 1990s was characterised by relatively low temperatures, reaching a low in May 1994. In contrast, the 1990s were characterised by increasing temperature with the maximum temperature anomaly occurring in 2002; probably caused by an influx of unusually warm water into the region (Holliday, 2003; ICES, 2003a). Holliday (2003) refers to sea surface observations made from 1948 onwards (Ellett and Jones, 1994), showing that the decade is not perhaps as unusual as indicated by the more recent time-series; the highest temperatures reached in the 1990s were similar to the peak reached in the 1960s following a decade of increasing values.

During 2001, the Rockall Trough began to show signs of cooling (and freshening), following a peak in temperature (and salinity) in 1998–2000. However, the temperature (and salinity) remained high compared to the long-term mean, with values similar to previous peaks in the early 1980s (ICES, 2002).

Holliday (2003) points out that the late nineties have seen a warming phase similar to the one observed in the late fifties but that the significance of the present warming episode is unclear. She states that the NAO Winter Index shows no statistically significant correlation with the time-series of subsurface temperature (or salinity); the Rockall Trough lying in a region of low to zero correlation, between the high positive correlation to the south and east and high negative correlation to the west (Rodwell et al., 1999). Therefore she concludes that the conditions in the Rockall Trough do not appear to be directly related to atmospheric conditions, as indicated by the NAO Index; or to variations in local net atmospheric heat fluxes. Instead, she considers that the variations in temperature are caused by varying inputs of the water masses to the south of the region - central North Atlantic Water, Mediterranean outflow Water, Western North Atlantic Water and SubArctic Intermediate water.

4. References

Beaugrand, G., P.C. Reid, F. Ibañez, J.A. Lindley and M. Edwards (2002). Reorganization of North Atlantic Marine Copepod Biodiversity and Climate. Science, 296: 1692-1694.

Bryden, H.L. and Imawaki, S. (2001) Ocean heat transport in: Siedler, G., Church, J. and Gould, J. eds. Ocean Circulation and Climate: Observing and Modelling the Global Ocean. San Diego CA, Academic Press, pages 455-474.

Dooley, H. (2003). Formal communication from Hydrographer of the International Council for the Exploration of the Sea, Denmark, Copenhagen.

Edwards, M., Reid, P.C., and Planque, B. 2001. Long-term and regional variability of
phytoplankton biomass in the Northeast Atlantic (1960-1995). ICES Journal of Marine Science 58: 39-49.

Ellett, D.J. and Jones, S.R. (1994). Surface temperature and salinity time-series from the Rockall Channel. MAFF Directorate of Fisheries Research, Lowestoft. 24 pages.

FRS (2003). Scottish Ocean Climate Status Report 2000 - 2001. Fisheries Research Services Report
05/03, Marine Laboratory, Aberdeen.

Holliday, N.P. (2003). Extremes of temperature and salinity during the 1990s in the northern Rockall Trough: results from the “Ellett line”. ICES Marine Science Symposia, 219: 95-101.

Hughes, S. L., W.R. Turrell, and A. Newton (2003). Hydrobiological variability on the northwest European continental shelf during the 1990s and its relation to changes in fish stocks. ICES Marine Science Symposia 219: 421-425.

Hulme, M., G.J. Jenkins, X. Lu, J.R. Turnpenny, T.D. Mitchell, R.G. Jones, J. Lowe, J.M. Murphy, D. Hassell, P. Boorman, R. McDonald and S. Hill (2002). Climate Change Scenarios for the United Kingdom: The UKCIP02 Scientific Report. Tyndall Centre for Climate Change Research, School of Environmental Sciences, University of East Anglia, Norwich, UK. 120pp.

ICES (2001). The Annual ICES Ocean Climate Status Summary 2000/2001. Prepared by the Working Group on Oceanic Hydrography, ICES, Copenhagen, Denmark. (Eds. Bill Turrell and N. Penny Holliday.) Retrieved 20th May 2003 from the World Wide Web: http://www.ices.dk/status/clim0001/iaocss2000.pdf

ICES (2002). The Annual ICES Ocean Climate Status Summary 2001/2002. Prepared by the Working Group on Oceanic Hydrography, ICES, Copenhagen, Denmark. (Eds. Bill Turrell and N. Penny Holliday.) Retrieved 20th May 2003 from the World Wide Web: http://www.ices.dk/status/clim0102/IOACSS01.PDF

ICES (2003a). The 2002/2003 ICES Annual Ocean Climate Status Summary. Prepared by the Working Group on Oceanic Hydrography, ICES, Copenhagen, Denmark. (Editors: Sarah L. Hughes & Alicia Lavín.) Retrieved 4th October 2003 from the World Wide Web: http://www.ices.dk/status/clim0203/IAOCSS2002.PDF

Inall, M. and C. Griffiths (2003). The Tiree Passage Time Series: 1981 - 2003. SAMS Internal Report No. 238.

IPCC (2001). Climate Change 2001, the scientific basis. J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden and D. Xiaosu, Editors, Cambridge Univ. Press.

Levitus, S., Antonov, J.I., Boyer, T.P. and S. Stephens (2000). Warming of the World Ocean. Science, 287: 2225-2229.

Loewe, P. (1996). Surface temperatures of the North Sea in 1996. Deutsche Hydrographisce Zeitschrift, 48: 175 – 184.

Rodwell, M.J., D.P. Rowell, and C.K. Folland (1999) Oceanic forcing of the wintertime North Atlantic Oscillation and European climate. Nature 398: 320-323.

SNIFFER, (2000). "Development of Temperature Indices for Scotland and Northern Ireland". Report number SR(99)07F. SNIFFER, 11/13 Cumberland Street Edinburgh EH3 6RT.