4 Changes in extreme surges

In this chapter we describe model projections of changes to storm surges and extreme water levels around the UK. We use a new eleven member perturbed-physics ensemble (PPE) climate model developed at the Met Office Hadley Centre to drive a Proudman Oceanographic Laboratory (POL) storm surge model. In particular we examine changes in return levels for return periods of 2, 10, 20 and 50 yr. A return level can be loosely described as the level expected to be exceeded on average once during the return period, and is useful in planning for extreme conditions.

First we show, by comparison with observations of 50-yr return levels, that the new models are generally better at simulating present day surge than those used in UKCIP02. Then we look for future trends in the model projections of storm surges using a sophisticated statistical technique. The trends that we find are physically small everywhere around the UK, and in many places can be accounted for by natural variability. We find that the surge level we expect to be exceeded on average once in 2, 10, 20 or 50 yr is not projected to increase by more than 9 cm by 2100 anywhere around the UK coast (not including the mean sea level change).

The range of uncertainty in surge trends found from the Met Office/POL models only includes that driven by the differences between the eleven members of the PPE climate model ensemble. However, other international climate models give a wider range of change in the strength of storms over the UK and we consider storm surge increases inferred from these too. We don’t have enough information from these models to quantify the probability of these increases but instead we use the models to develop an improbable but plausible high-end range of surge changes, called H++. This range is beyond the Met Office projections, and is unlikely to occur by 2100 but cannot be completely ruled out. This approach uses the non-Met Office model that reproduces the current storm regime over the UK well but has the greatest UK increase in storm intensity in the future. When the H++ surge and mean sea level scenarios are combined, the inferred increases in the 50-yr return period extreme water level are large in places around the UK (Figure 4.10), increasing by as much as 3 m by 2100 at some locations.

Key Findings

• Confidence in the Met Office/POL models to simulate the present day regime of extreme surges has improved.

• Around the UK the size of surge expected to occur on average about once in 50 yr is projected to increase by less than 0.9 mm yr (not including relative mean sea level change) over the 21st century. In most locations this trend cannot be clearly distinguished from natural variability. Thus our assessment suggests that this component of extreme sea level will be much less important than was implied by UKCIP02, where corresponding values exceeded 5 mm yr in places.

• The largest trends are found in the Bristol Channel and Severn Estuary, where the trend is for an increase in the 50-yr skew surge return level of around 0.8 mm yr, not including relative mean sea level change.

• Since the PPE does not sample the full range of known uncertainties, the uncertainty range quoted for surge from the Met Office models should be regarded as a minimum range.

• The international climate model that projects the strongest changes in storms over the UK may project larger increases in storm surge height around the UK than found from the Met Office projections. This is treated as a high end surge H++ scenario.

• Mean sea level rise and changes in storm surges have been combined to produce changes in extreme water levels.

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