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)
