One of the key aspects of the current concern over global warming is the predictions for sea-level rise and the likely impact of this. This aspect of the whole global warming story is poorly understood by most and yet it will have the greatest impact on Ireland, of that there is no doubt. Even though the climate of Ireland will get warmer with more seasonality of rainfall which could lead to increased flooding in winter and water shortages in summer especially in the eastern half of Ireland. This will not have a big impact on the country with the exception of agriculture where some adjustments may have to be made. At the moment however the lengthening of the growing season for grass by a number of weeks over the last 30 years or so is of benefit to farmers in Ireland.

In my opinion the real threat to Ireland comes from ongoing and enhanced sea-level rise and associated with this increased coastal and estuarine flooding, first we need to understand what sea-level change is about, what is the current evidence for it and what are the drivers behind it and why it is such a threat.

Sea-level change is a normal part of the response of the oceans and ice areas to climate change. If we go back to the end of the last ice age around 13,000 years ago sea-level was much lower than at present by as much as 80-100m, this was because huge volumes of freshwater were locked up in much larger ice sheets than exist at present. The current ice sheets and ice caps around the globe are only remnants of what would have existed at this time. The last ice age extended ice as much as 1km deep over most of the northern two-thirds of Ireland and is known as the Midlandian Ice Age in an Irish context. There was also a localised ice cap over the Cork-Kerry Mts. because of their height. So this is our starting point.

As the climate warmed up at the end of the last ice age these huge volumes of ice began to melt and retreat polarward or altitudinally. This released huge volumes of water into the oceans which began to rise and this rise has been occurring since then with some variations due to periods such as the Little Ice Age AD1450-1850 when there is some evidence to suggest a small growth in ice sheets and glaciers. Since then however there has been a consistent rise in sea-level on a global basis at most sites so much so that from 1900 to 2000 the global average was between 10 and 15cm and in many cases the rate of rise is accelerating. The exceptions to this are areas which were under heavy ice sheets including the northern half of Ireland which because of the weight of the ice depressed the earth’s crust, which is still rebounding since the ice melted. This complicates the story a little bit but we can mostly ignore this.

Most of the current sea-level rise which has been going on since the mid-1800’s is due to the thermal expansion of water as it heats up, just in the same way as you would boil a kettle. However, this masks the real concern which is that there is going to be a substantial melting of the world’s ice areas including many glaciers in places such as the Alps, Andes and other great mountain ranges. There is no doubt that most of the world’s mountain glaciers are melting, some have melted entirely even though they were photographed as little as 30 years ago. But even if they all melted they would add very little to global sea-level rise. Of much more critical concern is that the world’s major ice caps would melt in particular in Greenland and Antarctica. If the entire Greenland Icesheet was to melt the release of water would raise global sea-level by around 6 metres. If even a relatively small part of the Antarctic melted it could raise global sea-level by 10’s of metres. The world would be a very different place if any of there events were to happen.

Current measures of sea-level change in Ireland are based on water level data from a number of tide gauges throughout the island of Ireland. Unfortunately nearly all these records do not go back into the 1800’s to give us a good picture of the long term trend.

But it is clear that at the moment in the northern half of Ireland sea-level rise and land level rise are roughly about even so there is no net change, however in the southern half of Ireland as well as sea-level rise being recorded there is also land-level fall so the effect of sea-level rise is being enhanced. The predictions and the evidence suggests that as sea-level rise accelerates then even in the northern half of Ireland the sea-level rise will be faster than the land-level rise and the rate of sea-level rise in the southern half of Ireland will be far greater. Currently, the predictions for global sea-level rise assuming no catastrophic ice sheet melt is from 15cm to 90cm by 2100 far in excess of the rate for the 20th century. This is the background to the threat, lets now look at the threat in Ireland’s context.

Ireland is an island nation, all our major urban areas are either coastal or estuarine including Belfast, Dublin, Waterford, Cork, Limerick, Galway and Derry to name the major ones. In 2002 Dublin experienced significant coastal flooding so much so that the current estimate for urgent flood barriers for the city to prevent a recurrence is 100 million euro out of a current government budget of around 25 million euro for this type of work for the whole country. Cork and Waterford have experienced major flooding in 2004, and all the other coastal cities and towns have all records of coastal or estuarine flooding, so they are already vulnerable to sea-level rise not to mention what is likely to happen in the future. Although all these events are associated with big storms what is happening is that on top of the normal tidal cycle with high and low tides a storm effectively pushes a block of water in front of it and this tops up the usual water level. These storms stop the removal of water from rivers as happened in 2004 when flooding became widespread in the Suir Valley as far inland as Clonmel. As sea-level rise continues the potential for big storms to raise water levels higher and higher well beyond existing record levels increases significantly so its not a question of if but when. This will increase the areas flooded and the severity of flooding. The impact of Hurricane Katrina on New Orleans should be a clear example to everyone of the potential destruction that could occur.

The rest of the coastlines of Ireland are also under threat with the exception of rock cliff coastlines which are very resilient to attack. For low coastlines made up of sand, mud and loose rock, particularly the east and south coasts but also in many parts of the west and north coastlines. As sea-level rise occurs high tides will penetrate further inland and lead to increased coastal erosion especially if storm intensity changes. Currently average erosion rates of between 1 and 2m are occurring in parts of the east coast of Ireland, this figure could rise dramatically. Not only will this reduce the size of our beaches and sand dunes it will also cause the loss of agricultural land, whole farms have disappeared into the sea since they were first recorded in the 1st edition of the Ordnance Survey in the 1840’s. There is also a great potential for damage to coastal aquifers through salinization and once they become salty they are not suitable for use by humans and animals and are it is almost impossible to remove the salt. This could have a big impact on coastal rural areas which rely on local water schemes.

From a government perspective the treatment of our coastlines has been very poor, currently the county councils are responsible for coastal protection on a piecemeal basis and with limited budgets to tackle the problem. The cost of coastal protection is very high involving millions of euro per km protected for rural coastlines whereas high value urban coastal areas will have to be protected even though the cost per km is much higher. We cannot afford to protect all the coastline of Ireland because of its length and complexity. Sadly, although a very extensive integrated national coastal zone management strategy was developed by consultants Brady, Shipman and Martin involving a huge effort by all the stakeholders from government departments right down to local groups and which would have covered the whole country and would be completely integrated, this was shelved by the then government, so no change has taken place. Even a basic idea of not allowing any new development where ground floor entry level is below 5m OD has not been implemented even though it represents a sensible response to the potential threat.

This article presents a very grim picture of sea-level rise and its likely impact on Ireland, but if we get away with no catastrophic ice melt from one of the world’s major ice sheets over the next 100 years then we will have done well. With an integrated central government approach to the issue we should be able to manage the expected sea-level rise up to 2100, but it will cost us billions of Euro in terms of defences and compensation.Tough political decisions will have to be made.