The Yorkshire and Humber region abuts the North Sea along its eastern margins and the River Humber itself is a major tidally-influenced estuary along its eastern reaches. These sections of the region are therefore exposed to the marine processes of tidally-varying sea levels, surges and waves. 

Climate change has the potential to alter the marine processes in three principal ways of interest to the present study. 

• Firstly, the mean sea level could change through an increase in the volume of water within the oceans and a rising of sea levels relative to the land mass (which in parts of the UK is sinking).

• Secondly, extreme sea levels caused by atmospherically-induced surges or wind set-up could change in magnitude or frequency, or both. 

• Thirdly, the wind-generated wave climate could change should there be any changes to the track, magnitude or frequency of storms across the UK. 

Each of these issues is discussed in turn in following sub-sections. 

 

Mean Sea Levels
Climate change has the potential to affect mean sea levels in three principal ways:

1. Melting of glaciers following the last Ice Age has led to a long-term slow and progressive re-adjustment of the land mass of Great Britain (isostatic rebound);

2. A physical increase in water volumes is occurring globally due to the melting of contemporary ice caps and ice sheets (glacio-eustacy); and

3. Ocean water is thermally expanding on a global scale due to rising temperatures.

Isostatic Component
The first change is the result of a very long-term process that operates over geological timescales. During the last Ice Age, northern and central parts of the land mass of Great Britain were covered with glacial ice. As this ice melted, so the loading on the land mass altered. The result of this is the gradual and long-term re-adjustment of the land mass, with the north of Great Britain uplifting and the south sinking as a consequence.

Rates of Isostatic Rebound in Great Britain (in mm/yr)

The scientific literature suggests that the fulcrum of this re-adjustment is (very approximately) along an imaginary line drawn from just north of Tees Bay in the east to the Dee Estuary in the west (pictured, left).  The mechanisms involved in this isostatic re-adjustment, as this process is called, involve a considerable time-lag between the melting of the glaciers and completion of the land mass movements.  The existing literature suggests that the rate of uplifting of northern England is beginning to demonstrably slow.  This means that the eustatic components of global sea level rise will start to become more pronounced in the north of England and in Scotland.

Eustatic Component
The second and third effects of climate change on sea level combine to cause a change in absolute water elevation due to an increase in volume or mass.  This is known as eustatic sea level rise and such changes are felt on a relatively uniform basis around the UK coast.

Of the above contributors to global sea level rise, it is the thermal expansion of ocean waters in response to rising temperatures that yields the greatest proportion.  The implication of this is that even if greenhouse gas emissions were stabilised or reduced, there would remain an inescapable consequence of the emissions already ‘locked-in’ to the atmospheric system, meaning that further global warming and sea level rise would occur.  This is because there is such a long time-lag inherent within global-scale system responses between global warming and the resulting thermal expansion.

Relative Sea Level Change
The change in sea level that we directly observe at the coast is the result of a combination of isostatic and eustatic changes.  The combined effect is known as the relative sea level change (i.e.  the net effect of eustatic change relative to isostatic adjustment).  This is why sea level rise is a particularly severe problem in southern England: the absolute sea level has been rising and the land mass has been sinking.  In northern Great Britain, whilst the sea level has been rising so has the land mass, meaning that the relative effect is less marked than in southern England, although it obviously remains an important consequence of climate change.  Looking to the future, the relative effect is set to become more pronounced as the isostatic adjustment slows demonstrably.

Rates of Sea Level Rise
In order to consider past relative sea level change in the region, two lines of evidence can be pursued.  Firstly, geological evidence can be examined to identify isostatic changes, and secondly actual tide gauge records can be analysed to reveal relative sea level trends.

Shennan and Horton (2002) compiled data regarding the relative rate of land uplift around Great Britain, including data for two stations within the Yorkshire and Humber region.  Their best estimates for the current rate of land uplift at these stations are presented below.

Late Holocene relative land-/sea-level changes in the North East Region

Station	Relative Change
	Value (mm/year)	Description
Humber Inner Estuary)	-0.86	Land sinking
Humber (Outer Estuary)	-0.78	Land sinking

This isostatic rebound means that as well as sea levels in the Yorkshire and Humber region increasing due to global effects, the local land re-adjustment is exacerbating the vulnerability of the coastline due additionally to local land sinking relative to the sea.

It is widely believe that due to the volumes of CO2 already locked into the atmosphere, future relative sea level rise may accelerate beyond past observed rates.  It is for this reason that (then) MAFF (now part of Defra) initially recommended an allowance be made in the Yorkshire and Humber region in the flood and erosion risk management activities for a future sea level rise of 4.0 to 6.0mm/year (MAFF, 1993).  This recommendation was based on the work of the Intergovernmental Panel on Climate Change (IPCC, 1990), which focused on mean sea level trends, and previous research (Shennan, 1989) into isostatic rebound. 

More recently, UKCIP published its climate change scenarios for the UK (UKCIP, 2002) which contained revised sea level rise predictions for the UK.  Following publication of this document, Defra issued revised guidance in October 2006 which better reflects the dependence of future sea level rise on greenhouse emissions by considering increases in stages over different time epochs.  This most recent guidance for the Yorkshire and Humber region is presented below.

Recommended Sea Level Rise Allowances for the Yorkshire and Humber region (source: Defra, 2006)

Region	Net Sea Level Rise (mm/year)
	1990-2025	2025-2055	2055-2085	2085-2115
North East *	2.5	7.0	10.0	13.0
East **	4.0	8.5	12.0	15.0

* North of Flamborough Head
** South of Flamborough Head

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Extreme Sea Levels
Extreme sea level conditions typically arise when a particularly high or particularly low astronomical tide combines with a storm-related surge.  A surge is a positive or negative deviation of the observed tide from the routine, predictable astronomical tide.  It can be caused by high (or low) atmospheric pressure depressing (or elevating) the level that the tide reaches, or caused by the wind set-up of waters.

Although much scientific and media attention has focused in recent years on century-scale changes in mean sea level (as discussed in the previous section), the extreme events also have the potential to cause great impact on the Yorkshire and Humber coastline.  High extreme sea levels could lead to increased coastal erosion, overtopping or breaching of defences and resultant tidal (sea) flooding into areas where assets are located.

The North Sea has historically been subject to positive surge events that have led to widespread flooding (see Box A).

Box A:  Historic records of surge events in the North Sea In the Netherlands on 18 November 1421, water from the North Sea breached sea defences and swept through 72 villages killing over 10,000 people.  Similar disastrous breaches on the Dutch coast occurred in 1570, 1825, 1894, 1916 and 1953.  During the infamous 1953 event, for example, it was estimated that 1,800 people were drowned in the Netherlands.  These events prompted the Dutch Government to adoption a ‘defend at all costs’ policy to protect their country as over 40% of it lies below mean sea level. The 1953 storm surge also devastated the east coast of England, particularly between the Humber and the Thames estuaries.  The worst affected areas were Suffolk, Essex and Kent, including Canvey Island in the Thames where 58 people died.  During the storm, wind speeds exceeding 80mph were recorded.  The event resulted in the loss of over 300 lives, flooding of 100,000 hectares and caused damage to assets worth over £5 billion (in present value).  In response to this event, a massive programme of maintenance and capital works on flood defences was instigated, including the construction of the Thames Barrier.  In addition, the Storm Tide Forecasting Service was introduction, which is operated on behalf of Defra by the Met Office.  The service provides the Environment Agency (in England and Wales) and the Scottish Environment Protection Agency with regular coastal flooding, surge and wave activity forecasts.

As mentioned above, the Thames Barrier was designed with the purpose of reducing the risk of flooding to London from surge events that build in the North Sea and propagate up the Thames Estuary. Construction of the barrier commenced in 1974 and it became operational in 1983. As can be seen from this graph, the number of closures of the barrier has exhibited an increasing trend.

Whether the above trend can be explained as being entirely attributable to the effects of increasing North Sea surge frequency associated with climate change is questionable, since natural fluctuations in the lunar nodal tidal cycle and isostatic re-adjustment could both be affecting the need for barrier closures.  However, this frequency of barrier closure is far greater than was anticipated during its inception, planning and design in the 1960s and 1970s.

Recent research has investigated whether climate change could alter the occurrence of storm surges around the UK.  Lowe et al.  (2001) used changes in wind and pressure from the Hadley Centre’s second-generation regional climate model HadCM2 (using the SRES IS92a emissions scenario) to drive POL’s 35km resolution dynamic storm surge model of the North-West European continental shelf region.  The results of this modelling exercise indicated that a discernable increase in surge heights could be expected as a result of climate change along many part of the UK coast, although interestingly the increase was smallest in the southern North Sea (where the present surge levels are greatest). 

Modelling work was undertaken to inform the production of the UKCIP02 report using the Hadley Centre’s third-generation regional climate model HadRM3 to predict changes to the 1 in 50 year return period surge height  by the 2080s for the Low, Medium-High and High emissions scenarios.  This modelling took into account the combined effects of global average sea level rise, changes in storminess and vertical land movements.  For the medium high scenario (below), the predicted increase in 1 in 50 year return period surge height was 0.3 to 0.4m.

 

1 in 50 year return period surge height [projected by the 2080s] under the Medium-High emissions scenario

Medium-High Emissions Scenario

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Waves

Waves are generated by wind-stresses acting over a water surface and the maximum wave height generation is physically limited by either fetch (the distance across which the wind blows) or time (the duration for which the wind blows).  Consequently, future changes in the generated wave climate will be governed by any change in (wind) storm track, frequency or intensity that results from climate change. 

Once generated, waves become transformed by a number of processes, such as refraction, reflection, diffraction, shoaling and breaking.  Waves shoal and break as they approach the shore due to the depth-limitation provided by the sea bed.  Under future rising sea levels, the depth-limiting effect will be progressively reduced and therefore waves will break slightly closer to shore, potentially leading to increased erosion of coastal landforms (such as shore platforms, beaches, sand dunes and sea cliffs) and increased overtopping of existing coastal defence structures. 

The common perception is that offshore and nearshore wind and wave conditions will worsen under climate change.  Looking more specifically at the available wind and wave data cited in previous literature, anemometer data from 1977 to 1987 at Spurn Point shows an increase in annual mean wind speed of 4.0cm per year, while wave data measured from 1978 to 1986 off the coast from Humber and Lincolnshire shows an increasein annual mean significant wave height of 1.5cm per year (National Rivers Authority, 1994).  However, these data records cover only relatively limited durations (<15 years) and longer time-series are required before more meaningful results are yielded.  Furthermore, these data relate to annual mean conditions and it is of course extreme events that are most damaging to the coastline.

The projection of winds is very difficult to achieve, particularly when considering long-term future conditions associated with climate change.  Consequently, the projection of future wave climates, which is dependent on the future wind climate, is equally uncertain.  Recent research (Sutherland and Wolf, 2002) did attempt to use a climate model to predicted future wind conditions for 2075 and then use these conditions as input to a wave model to assess the changes in wave height at five locations around the UK.  In general terms, this revealed that that changes in the wave climate are likely to be small (<5% of present wave height values).

Due to the uncertainties with projecting future wind and wave climates, UKCIP has not presented any quantified information on wave height or direction changes associated with climate change (UKCIP, 2002).  Instead qualitative hypotheses have been presented, such as the likely future tendency for the North Atlantic Oscillation (NAO), which influences wave heights, to create higher and more westerly index values in the future. 

Other previous research on changes in future wave climates has mostly focused on ‘what-if’ scenarios, rather than definitive predictions.  For example, the Futurecoast study (Defra, 2002) considered what the effect would be on the rate and direction of littoral sediment transport of different hypothetical future wave climate scenarios (i.e.  existing predominant wave direction ±1º or 2º, existing significant wave height ±10%).  This assessment revealed, in general terms, that small changes in wave direction had little effect on energy or sediment transport rates at the shoreline, increased water levels associated with sea level rise had a modest effect, but increased or decreased wave heights by a hypothetical ±10% had the greatest effect, increasing or decreasing wave energy at the shoreline by up to 10%.

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