• From tide gauge records, global mean sea level rise over the last 55 years is measured at 1.8 (± 0.2) mm per year. Satellite altimetry gives a rate of about 3 mm per year since 1992, but it is unclear if this is a permanent acceleration or natural variability in the rate.
• Mean sea level rise measured at sites around the UK is consistent with the globally averaged figure.
• Projections of 21^st century sea level rise (excluding vertical land movement) for the UK give a range of 12-76cm for the year 2095.
• When vertical land movement is taken into account then larger sea level rises are projected for southern parts of the UK with smaller increases in relative sea level for the north.
• Projected sea level increases (including vertical land movement) for 1990-2095 for London are approximately 21-68cm.
• A low probability sea level range, denoted H++, has been defined for the purposes of contingency planning only. This extreme estimate of sea level rise ranges from 93 cm to 1.9 m by 2100.
• There is no significant evidence for any recent observed trend in storm surge frequency or magnitude. This component of extreme sea level appears to be less important than changes in global mean sea level over the next 100 years.

What is already happening: High

The observational evidence (for "what is already happening") is of the highest quality and has, through the IPCC process and the references herein been subjected to considerable scientific analysis.

What could happen: Medium

The largest challenges for future projections of sea level, or sea level extremes, is the inherent uncertainty in climate model predictions that is due to the treatment of small scale processes, insufficient knowledge of the initial state, and aspects of the physical world whose physics are not completely understood. Ensemble simulations, where several versions of the climate model are run can help quantify this uncertainty. Perturbed parameter models were used in the storm surge projections of UKCP09. Multi-model ensembles use models from several international institutes; this ensures a further level of robustness and was used in the mean sea level projections of UKCP09.

The H++ scenario is synthetic and contains very large uncertainty. Such a large amount of sea level rise is thought to be highly unlikely during the 21^st century but cannot be ruled out completely.

The single largest uncertainty for mean sea level projections is the ice melt contribution in response to increased global mean temperatures. This ranges from global sea level rises of 10-20cm, as considered within IPCC (2007), to values of 1-2m (e.g. Rohling et al., 2008).

The top priority knowledge gaps that need to be addressed in the short term to provide better advice to be given to policy makers are:

1. Better quantification of the contribution of land ice to sea level rise is essential. In particular whether the amount of sea-level rise resulting from ice lost from the Greenland ice sheet and Western Antarctic Ice Sheet is likely to continue increasing.
2. The regional variability of sea level changes is poorly addressed at the current resolution of global general circulation models used for climate projections. Obviously, regional sea level response is what is required for planning and adaptation.
3. There is scope for the use of further advanced statistics in the design and interpretation of probabilistic forecasting. The forthcoming programme at the Isaac Newton Institute, "Mathematical and statistical approaches to climate modelling and prediction", will begin to address this. See: http://www.newton.ac.uk/programmes/CLP/index.html 
4. A more realistic representation of storm track and cyclogenesis (formation of mid-latitude depressions) in climate models. This will, in part, be addressed as the spatial resolution of climate models approaches that of short-range weather models as computer power increases.

There is wide agreement on these key challenges.

In England and Wales there is at least £150 billion worth of property and 430,000ha of agricultural land at risk from coastal flooding and towards 100,000 properties in areas that, without protection, could be eroded. The area at risk of coastal flooding equates to a coastline of 3500km, of which 3200km is defended.

The Environment Agency's Long Term Investment Strategy ( http://www.environment-agency.gov.uk/research/library/publications/108673.aspx) does not provide separate analysis for coastal flooding, but the findings illustrate the increasing investment need required to fully respond to climate change. Modelling of both river and coastal flood risk suggests that to sustain current levels of protection in the face of climate change requires an increase in investment from current levels of £570 million to more than £1 billion a year, plus inflation, by 2035. Conversely, keeping investment in building and maintaining defences at current (2010/2011) levels could increase the number of properties at significant risk by 350,000 over the same period.

There has been considerable progress in quantifying the socio-economic impacts of sea level rise for London, where a significant proportion of UK GDP is focussed. The Thames Estuary project, reported in the UKCP09 report (Chapter 7), combined climate projection information with socio-economic scenarios for the future of London to assess the impact of flooding on the capital city.

For further details regarding the socio-economic impacts of sea level rise readers are directed to the full MCCIP report card submissions on coastal flooding and coastal erosion.

Horsburgh, K. and J. Lowe (2010) Mean Sea Level and Sea Level Extremes in MCCIP Annual Report Card 2010-11, MCCIP Science Review, 9pp. www.mccip.org.uk/arc