Sea level
Defra; MOHC;
POL
- Global average sea level has risen during the 20th century by
between 1 and 2 mm per year. The latest published satellite
measurements suggest the rise was around 3 mm per year
between 1993 and 2003.
- Smaller rates of rise are seen in Scotland compared to
the south of England.
- Extreme water levels have also increased in the UK, most
likely as a consequence of mean sea-level rise.
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- During the 21st century a global average sea-level rise of
between 9 and 88 cm relative to 1990 has been predicted but there is uncertainty in ice-melt and its effect on the
upper limit of sea-level rise; thermal expansion will
account for the majority of the overall rise.
- The anticipated range of relative sea-level rise by the 2080s
(relative to the 1961–1990 mean) is 20–80 cm in south-west
England and 0–60 cm in Scotland.
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What is already happening
Global-average sea level rose during the 20th century at an average rate of 1-2 mm/year, with some consensus on the larger value by the research community. The rate was larger (approximately 3 mm/year) during the 1990s. UK sea level records are consistent with these values but with smaller trends observed in Scotland (where the land is uplifting) than in the south of the UK. Extreme sea levels are also known to have increased, both on global-average and UK bases, following to some extent the rise in mean levels but also subject to long term changes in meteorological forcings.
What could happen
Tide gauge measurements suggest that global average sea level has risen during the 20th century by between 1 and 2mm/yr. Recent satellite altimeterInstruments designed to measure the distance between the platform where the instrument is housed and the surface beneath, by timing the reflection of emitted radio waves. measurements, which have better spatial coverage than the tide gaugesA variety of instruments designed to measure changes in sea level. but span a much shorter time period, indicate that between 1993 and 2003 the rate of rise was around 3mm/yr. At present it is unclear whether this apparent increase in rate is due to natural variability of the climate or is part of a long-term acceleration in the rate of rise.
During the 21st century we predict, with high confidence, that the global average sea level will rise. However, we have only a low to moderate confidence in the magnitude of the rise. The third assessment of the IPCC presented a range of projected sea-level rise between 1990 and 2100 of 9-88cm, with the largest contribution coming from thermal expansionThe increase in volume of a liquid (in this case sea water) as a response to warming.. This range aims to account for uncertainty in the emissions scenario and in the modelling techniques. The IPCC fourth assessment has recently been released. However, Defra’s most recent supplementary policy guidance on climate change impacts for appraisal and decision making in flood and coastal erosion risk management, which preceded the IPCC fourth assessment, will await findings from the UKCIP08UKIP08 is the next climate change information package for the UK, consisting of five reports and an interactive website. The project brings together climate science from the Met Office to provide information to decision makers, academics and others, on the current climate, and possible future changes. Its interactive website will provide customisable climate change projections when it is launched in late 2008. programme before receiving a further policy update. For consistency, we continue to report the third assessment results here but will revise them in the future when the Defra guidance is also updated.
We do not expect the pattern of time average sea-level rise to be spatially uniform. We have a low to moderate confidence in the prediction that some locations will experience future increases of around 50% more than the global average, but the confidence in where this will occur is very low. We cannot yet rule out the regional enhancement occurring in the North Atlantic, where it might affect UK waters. When considering flooding the vertical movement of the land must also be included, giving a relative sea level changeThe change in sea level that would actually be seen in a given location given local changes in land height.. The vertical land movement varies considerably with location.
The most recent information for the UK from UKCIPThe UK Climate Impacts Programme (UKCIP) provides scenarios that show how our climate might change and co-ordinates research on dealing with our future climate. forecasts a range of relative sea level rise by the 2080s (relative to the 1961-1990 mean) of between 20 and 80cm in south-west England and 0 and 60cm in Scotland. (http://www.ukcip.org.uk/resources/publications/documents/124.pdf)
What is already happening - High
We have high confidence that sea level has risen globally and around the UK during the 20th century. There is high confidence that sea level extremes have also increased in most locations. There is low confidence that extremes have changed in a significantly different way to mean sea levelsIs the level water surface (or "still water level"), measured with respect to some datum, when oscillatory motions such as waves and tides have been averaged out over a suitable period (e.g. monthly or annually)..
What could happen - Low
Overall confidence is Low (although we have high confidence that sea level rise our confidence isn’t as high on the magnitude of rise and regional variations in sea level rise).
High confidence that the global average sea level will rise.
Low to moderate confidence in the magnitude of the rise.
Low to moderate confidence in the prediction that some locations will experience future increases of around 50% more than the global average, but the confidence in where this will occur is very low.
Philip Woodworth and Kevin Horsburgh
Proudman Oceanographic Laboratory, 6 Brownlow Street, Liverpool, L3 5DA
Jason Lowe
Hadley Centre for Climate Prediction and Research (Reading Unit), Meteorology Building, University of Reading, Earley Gate, Reading RG6 6BB
Karl Hardy
Flood Management Division, Defra, Quantock House, Paul Street, Taunton, TA1 3NX
Executive summary