Evidence to Policy

Extreme sea-level rise: The greatest threat from global warming for the UK

The Paris and Bonn Agreements

With the Paris Agreement, countries within the United Nations aimed to respond to the threat of global warming by keeping temperature rise below 2 degrees Celsius above pre-industrial levels and to pursue efforts to limit the temperature increase further to 1.5 degrees. Each country was allowed to determine its own contribution in order to meet these objectives. As a result, the measurements proposed did not even come close to limit global temperature rise to 2 degrees, but would more likely result in 3 degrees warming.

In the follow-up Bonn climate change conference, held recently, countries agreed on how to measure each country’s contribution to mitigate global warming. However, neither the contributions themselves, nor whether they would be sufficient to meet the Paris’ goal of limiting warming to 2 degrees were discussed. Likewise, it hasn’t been agreed which measures should be taken in case the nations fail to meet this target. This has important consequences for sea-level rise.

Consequences for sea-level rise

Sea-level rise can be seen as one of the most certain and costly impacts of climate change. Furthermore, it will affect a host of small island states, that have never contributed anything to global warming through emissions of carbon dioxide, to the level that they will cease to exist in this, or the coming century. This perspective was a large inspiration for the Paris Agreement.

The small island states formed an allegiance to make their voice heard better, which suggested an important and worrying aspect in the whole discussion, namely, sea-level rise is not seen as a serious threat by the majority of the countries. This misconception probably arises from the rather modest amounts of sea-level rise projected by the Intergovernmental Panel on Climate Change (IPCC) for this century and completely wrong assumption that sea-level rise stops when global warming stops. In fact sea-level rise will continue for many hundreds to thousands of years, even if carbon emissions stopped today.

Sea-level rise should be a big worry for almost every country with a coastline

Our societies have become vulnerable to even small changes in sea level, as our populations and industrial development in coastal zones have dramatically increased. It is estimated that around 600 million people live within 10 m of present day mean sea level.

Sea-level rise has a range of impacts on our coastal areas. First, it will increase coastal flooding and inundation. This is obviously of great concern to coastal communities, particularly the 139 coastal cities that have populations of more than 1 million people. Second, it will lead to more coastal erosion. About 17% of the UK coast is currently experiencing erosion. Third, sea-level rise will lead to wetland loss, which will result in changes to important ecosystems. Coastal wetlands are being lost worldwide at an alarming rate, estimated at 25–50% over the last 150–300 years. This is a result of sea-level rise, but also increased development on the coast. Fourth, it will result in salt intrusion into ground water, which will affect fresh water availability. This is particularly important for small-island states that depend on small fresh ground-water sources.

Committed sea-level rise

Even if the transformation to sustainable energy would occur tomorrow, and after tomorrow global temperature would not rise any further, sea-level will continue to increase, and not by a small amount, but by many meters.

With the Paris Agreement fully implemented it would imply 12 m sea level rise in response to 2 degrees warming. Because this equilibrium response was always thought to occur on timescales of a few thousands years, politicians and journalists were not interested. Nevertheless, the scientific community has sent out warnings, that in a time-span of a few decades we can severely change the surface of the planet for thousands years, making the habitat of 600 million people inhabitable. But this future prospect lies too far ahead to concern the people that live here today.

Ice caps could disintegrate much faster

After the publication of the latest IPCC’s report it became clear that the ocean is quickly melting floating ice-shelves surrounding large ice caps of Greenland and West-Antarctica. Especially around the western part of the Antarctic ice cap the process progresses disturbingly fast. Moreover, ice shelves feature deepening cracks and rifts and crumble into pieces, as has been witnesses for 2 ice shelves of the size of Wales, Larsen A and Larsen B. This year Larsen C lost a large fragment of 300 by 50 km. While melting ice shelves do not raise the sea level (like melting ice cubes in your glass of softdrink), their indirect effect on sea level is large. The large ice caps are unstable and need the ice shelves to prevent them from collapsing under their own weight.

Think of a big cathedral needing buttresses to stay upright. What we see now occurring is that the buttresses start tumbling down (melting away). Iceberg calving and ice flow into the sea from the land ice has been seen to increase with a factor of 5 to 10 when Larsen A and Larsen B disappeared. When this occurs at larger scale we run into severe problems. A recent American study accounting for all these processes suggested that sea level could rise by 6 meters in 200 years from now, with a very fast acceleration in sea-level rise by the end of this century. The outcome of this study is still debated, but cannot be neglected or put aside.

How we in Southampton contribute to better understanding of changes in sea level

We follow several different lines of research to address the huge uncertainty and possibly dramatic impact of the consequences of global warming.

• First, we have installed, configured and tested an ice-sheet model. At present we carry out experiments to better understand the interaction between ocean and ice-shelves. This work is carried out together with groups in the Netherlands and in a later phase we will also collaborate with the British Antarctic Survey.  
• Second, we make new projections of possible extreme sea-level rise, together with the Dutch who are even more vulnerable than the UK to this threat, and work together with scientific advisory groups including the UK Met Office to advise extreme scenarios for which the Thames Barrier should be resilient.
• Third, we carefully monitor the UK network of tide gauges for changes in sea level, tides and flooding and use these data to benchmark the climate models.
• Fourth, we have developed and currently maintain a website full of data related to storm surges, called SurgeWatch, which is public available and contains a wealth of information: www.surgewatch.org.uk

Conclusions

It is really important we reduce our carbon emissions, but we must recognise that even if we do so, sea level will continue to rise for many hundreds to thousands of years by several meters. This will mean, as a society, we will have to significantly upgrade coastal flood defences, seek alternatives ways of adapting to changing sea levels (i.e. floating houses) or possibly even abandon some coastal settlements and move further inland.

Professor Sybren Drijfhout and Dr Ivan Haigh

Ocean and Earth Science, University of Southampton