There is a history of strong variability in UK wave climate. Inter-annual variability in the modern wave climate is strongest in the winter and can be related to atmospheric modes of variability, most notably the North Atlantic Oscillation. Rather dramatic increases in wave height occurred between 1960 and 1990, but these are now seen as just one feature within a longer history of variability. There is no clear pattern in results since 1990. Natural variability in wave climate is strong and the role of anthropogenic forcing is uncertain. Previous projections of a strengthening storm track (for example featured in the latest IPCC assessment report) have been contradicted by the projections from UKCP09. These latest projections are for a southward displacement of the storm track, resulting in lower wave heights to the north of the UK and slightly greater wave heights in some southern regions. There is however no consensus on the future storm and wave climate, stemming from diverse projections of future storm track behaviour.

What is already happening: Medium

There is a large amount of data at least for the historical period and we know of no substantial contradictions in that data. Model hindcasts (forced by reanalysis data) are in close agreement to direct observations of waves. In these respects, the confidence is high, but dynamic models are unable to reproduce the past behaviour of the storm track. Modes of variability occur spontaneously in climate models mimicking the general behaviour of observed modes, but the specific time history of observed modes has not been explained (but may simply be random). It is questionable whether the full dynamics of the storm track is adequately represented in current models.

We have accumulated new evidence since 2007/8. This has not contradicted the original evidence, but it does set that evidence in a different light, thus the tone of the Summary has changed.

What could happen: Low

There is now accumulating evidence from more and better modelling and projection. There is some reason to prefer the recent UKCP09/HadCM3 projections, but the fact that these projections are contradictory to both a multi-model ensemble considered by UKCP09 and the similar multi-model "average" featured by IPCC AR4 forces us to assign a low level of agreement/consensus. The evidence base has improved significantly since 2007/8, but we cannot be sure what evidence to trust.

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. The basic dynamics of shifts in the strength and path of the mid-latitude storm track are uncertain, so that it is unclear which if any climate model is capable of satisfactory projections. There is most uncertainty about the storm tracks in the NE Atlantic.
2. While there is a reasonable knowledge of storminess and wave statistics over the last sixty years, a more complete synthesis would be most welcome. Ideally this synthesis should be pushed further back in time, so that the important context offered by both early observations and by palaeo-reconstruction is integrated into our understanding. Combining this with 1, we will have greater confidence in climate models if they can be demonstrated to successfully hindcast this record of the past.
3. Predictions are only useful to coastal managers where they can be localized at least to the scale of the "Charting Progress" regions. A global climate model produces a useful large-scale projection, but some uncertainty remains as to the most appropriate methods for downscaling those projections to a useful scale. UKCP09 has used a RCM and this is the generally favoured method. However, there is also a case for statistical downscaling based on projected general structural changes in the atmosphere (e.g. intensity and position of storm track).
4. Waves at the coast can have a large impact. Calculation of this impact depends on good resolution of near-shore bathymetry and may be quite sensitive to small changes in offshore wave conditions, especially wave direction and period, which may not be well-resolved in the regional scale model, thus the details of future wave impacts at the coast remain uncertain.

Waves affect:

• Marine Operations (e.g. transport, fishing, offshore industry). The highest waves are a danger to both fixed platforms (e.g. oil rigs) and shipping and their estimation is essential to safe design.
• Coastal Communities. Waves on their own or (more often) in combination with strong winds, high tides and/or sea surges can cause coastal erosion and damage to infrastructure. Breaches of defences can lead to major flooding incidents.
• Marine Renewable Energy. Waves are of direct interest as a potential source of sustainable energy, but also large winds and high winds are a significant risk factor in the development of offshore wind and tidal energy.
• Marine Ecology. Waves influence stratification and thus the distribution of nutrients, plankton and pelagic ecology. Long waves can affect the sea bed even in shelf water of 200m, inducing currents and stirring up sediment, thus influencing near-shore and benthic habitats.

"Concerned groups" include:

1. Those interested in coastal protection, particularly regarding the combined effect of sea level rise and changes in storm surges and waves on the threat of inundation. An example would be the Western Isles of Scotland where a January 2005 storm caused loss of life, extensive inundation and damage. There, residents, the local council (CnES), Scottish Natural Heritage and other bodies are considering the appropriate response. One of the authors is peripherally involved through "Coast Adapt", http://www.hebridesnews.co.uk/coast_adapt_cnes_sept09.html. The east coast of UK, notably the "soft cliffs" of East Anglia and Yorkshire is considered to be particularly vulnerable. The coastline of East Anglia has been relatively well studied (Leake et al., 2009).
2. Offshore Industry: As evidenced by a recent OGP/JCOMM/WCRP Workshop on "Climate Change and the Offshore Industry", 27-29 May 2008.WMO Headquarters, Geneva, Switzerland. Useful projections for the North Sea would be valued.
3. Marine Renewable Industry. A recent PhD project at University of Edinburgh (student: Lucy Cradden; Supervisor: Gareth Harrison) has looked at projections with respect to onshore wind and there are clearly implications also for marine renewables.
4. There is an awareness among intertidal and benthic ecologists that changing wave exposure is a factor in changing species distributions, but in practice it may be very difficult to disentangle this influence from the effects of changing temperature and sea level.

Woolf, D., and J. Wolf (2010) Storms and Waves in MCCIP Annual Report Card 2010-11, MCCIP Science Review, 15pp. www.mccip.org.uk/arc