What is already happening

Air

Marine air temperature measurements made by Voluntary Observing ShipsThe World Meteorological Organisation’s Voluntary Observing Ships programme is a scheme recruiting ocean-going vessels to collect and report meteorological observations. At present there are approximately 4000 ships involved in the programme. have been used in a new dataset of daily air temperatures and other marine meteorological variables currently under development at the National Oceanography Centre, Southampton. This dataset shows that the air temperature over the seas near the UK (7W:3E and 50N:60N) has risen over the period 1970 to 2006 at a similar rate to the The Central England Temperature is a time series of average monthly temperatures representative of  an approximately triangular region of the United Kingdom enclosed by Bristol, Lancashire and London.  Beginning in 1659, it is the world’s longest continuous time series of observed temperatures.Central England Temperature (CET, Parker et al. 1992). However, there are strong regional variations in the linear warming trend over UK territorial waters. Marine air temperatures have risen faster than the CET in the Eastern English Channel and across the majority of the North Sea. The Scottish Continental Shelf, North-West Approaches and Northern North Sea have seen a slower rise than CET and the Irish Sea, South-West Approaches and the Western Channel have seen marine air temperature increasing at a comparable rate to CET. Marine air temperature spatial gradients are thus increasing in the Northern North Sea. As for sea surface temperature (Holliday et al. 2007), air temperature in the winter of 2005/6 was colder than recent years, but the second part of 2006 saw some of the warmest average monthly temperatures in the record.

Due to a decline in the number of reports from Voluntary Observing Ships our confidence in the estimates of marine air temperature has decreased over the last decade, both in UK waters and globally.

Sea surface temperature linear trends within UK Coastal Waters are broadly similar to marine air temperature in both magnitude and spatial pattern.

Sea

Sea surface temperatures (SST) in the north east Atlantic and UK coastal waters have been rising since the 1980s, most rapidly in the southern North Sea and the English Channel. Despite a relatively cold winter in UK waters in 2005/2006 anomalously rapid warming in the spring and early summer meant that 2006 became the second warmest year in UK coastal waters since 1870.

The temperature of the upper ocean (0-800m) to the west and north of the UK has been generally increasing since the 1970s.  A significant period of warming occurred from 1995 to 2003. The decadal-scale pattern of temperature around the UK reflects the mean conditions of the North Atlantic which has evolved from a maximum in the early 1960s and a minimum in the 1980s and 1990s.

West of the UK the water of the deep oceanThe part of the ocean that does not cover the continental shelf margins (the shallower water adjacent to land masses). (>1000m) comes from the Labrador SeaA region of the North Atlantic Ocean located between southwest Greenland and northeast Canada. It is one of two main locations where cold, dense surface water sinks to produce south-flowing North Atlantic Deep Water, the other location being the Greenland Sea. and has cooled since 1975.  North of the UK, the deep water (800 m) flows from the Nordic Seas and shows no long-term trend since 1950.

In the northern North Sea the temperature is most strongly influenced by inflowing North Atlantic water, showing similar decadal variations and a general warming since the mid 1980s.  In the southern North Sea, atmospheric forcing is the dominant influence, with ocean temperatures being generally cool from 1970 to 1987 when a "switch" to warm conditions occurred.

The upper 1500 m of the North Atlantic has warmed since 1999 and remains anomalously warm up to the end of 2006, especially in the zone between 50-70°N.

What could happen

The UK Climate Impacts Programme (UKCIP02, Hulme et al., 2002) shows potential sea surface temperature (SST) rises around the UK coast under different scenarios of greenhouse gas emissions, with more pronounced warming in the southeast than the northwest. The range of future increase in SST in the southern North Sea is 1.5 – 4 ºC by the 2080s whilst that at Rockall is only 0.5 – 2 ºC. New scenarios for the UK are currently being developed UKCIP and are due to be launched in autumn 2008.

What is already happening

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What could happen

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What is already happening - High

Air

Agreement with independent measurements of SST and the CET give a high level of confidence in the marine air temperature data.

Sea

Confidence in the global increase in SST is high (e.g. IPCC 2007), and UK waters are relatively well sampled.

Time series of measurements of water temperature with depth in UK waters have moderate confidence due to relatively sparse observations and a fairly poor sampling of the seasonal cycle. However the observations themselves are typically of good quality and show a consistent overall picture which is well understood, giving a medium confidence overall.

Temperature  profile  information  in  the  North  Atlantic  is  now  much  better  sampled  than  in  the past  due  to  the  deployment  of  many Argo  profiling floatsFree-drifting profiling floats that measure the temperature and salinity of the upper 2000m of the ocean. Argo floats are able to surface and relay observations directly to receiving stations. Globally, there are ~3000 total floats deployed across all of the world’s major oceans..  However  questions  of  biases  in recent batches  of  floats  (Willis et  al.  2007)  and in   homogeneity  between Argo and eXpendable BathyThermographs (A non-recoverable device deployed from moving ships used to record temperature as a function of ocean depth. Information is relayed to the source ship via a copper wire which eventually separates from the device.XBT's) data (Gouretski & Koltermann, 2007) mean the overall confidence probably remains moderate, pending further research.

What could happen - High

High confidence that sea-surface temperature will increase around all UK coasts.

What is already happening

Air

As noted in supporting evidence, uncertainty in marine air temperature fields has increased in recent years due to reduced sampling.

Sea

More stations and improved sampling of the seasonal cycle would be desirable for subsurface ocean temperatures. The available sampling is not sufficient for a full understanding of variability and hence reduces confidence in the representativeness of measurements made.

The deep oceanThe part of the ocean that does not cover the continental shelf margins (the shallower water adjacent to land masses). (below ca. 2 km depth) is poorly sampled.

For the surface to mid-depth ocean questions of the homogeneity of data from Argo floatsFree-drifting profiling floats that measure the temperature and salinity of the upper 2000m of the ocean. Argo floats are able to surface and relay observations directly to receiving stations. Globally, there are ~3000 total floats deployed across all of the world’s major oceans. and between Argo and other sampling technologies (e.g. XBT's) remain.

SST is reasonably well measured, but requires continuity of satellite missions and availability of adequate in situ data for validation and bias adjustment. Recent rapid changes in the in situ observing system means that the homogeneity of the current observing system, and its consistency with earlier observations, needs urgent assessment.

Not stated.

N Penny Holliday, Robert Marsh, David Berry and Elizabeth Kent
National Oceanography Centre, Southampton, European Way, Southampton, SO14 3ZH

John Kennedy
Met Office Hadley Centre, FitzRoy Road, Exeter, EX1 3PB.

Sarah Hughes
Fisheries Research Services, FRS Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen, AB11 9DB.

Toby Sherwin
The Scottish Association for Marine Science (SAMS), Dunstaffnage Marine Laboratory, Oban, Argyll, PA37 1QA

UK Climate Impacts Programme (UKCIP)
www.ukcip.org.uk