Sea-level rise – an unavoidable threat
Loss of habitats and cultural treasuresSea-level rise is one of the most serious consequences of global warming. No one can really imagine how the coasts will look if the waters rise by several metres over the course of a few centuries. Coastal areas are among the most densely populated regions of the world and are therefore particularly vulnerable to the impacts of climate change. They include major agricultural zones, conurbations and heritage sites. How will climate change affect their appearance? Researchers around the world are seeking answers to the question of how rapidly, and to what extent, sea level will rise as a consequence of climate change. In doing so, they must take account of the fact that sea level is affected not only by the human-induced greenhouse effect but also by natural processes. Experts make a distinction between:
- eustatic causes: this refers to climate-related global changes due to water mass being added to the oceans. The sea-level rise following the melting of large glaciers at the end of ice ages is an example of eustatic sea-level rise;
- isostatic (generally tectonic) causes: these mainly have regional effects. The ice sheets formed during the ice ages are one example. Due to their great weight, they cause the Earth’s crust in certain regions to sink, so sea level rises relative to the land. If the ice melts, the land mass rises once more. This phenomenon can still be observed on the Scandinavian land mass today.
- 3.1 > Until 6000 years ago, sea level rose at an average rate of approximately 80 cm per century, with occasional sharp increases. There were at least two periods, each lasting around 300 years, when sea level rose by 5 metres per century. This was caused by meltwater pulses.
- Sea level can change by 10 metres or so within the course of a few centuries and can certainly fluctuate by more than 200 metres over millions of years. Over the last 3 million years, the frequency and intensity of these fluctuations increased due to the ice ages: during the colder (glacial) periods, large continental ice sheets formed at higher latitudes, withdrawing water from the oceans, and sea level decreased dramatically all over the world. During the warmer (interglacial) periods, the continental ice caps melted and sea level rose substantially again. The last warmer (interglacial) period comparable with the current climatic period occurred between 130,000 and 118,000 years ago. At that time, sea level was 4 to 6 metres higher than it is today. This was followed by an irregular transition into the last colder (glacial) period, with the Earth experiencing its Last Glacial Maximum (LGM) 26,000 to 20,000 years ago. At that time, sea level was 121 to 125 metres lower than it is today. Then the next warmer period began and sea level rose at a relatively even rate. There were, however, intermittent periods of more rapid rise triggered by meltwater pulses. These were caused by calving of large ice masses in the Antarctic and in the glacial regions of the Northern hemisphere, or, in some cases, by overflow from massive natural reservoirs which had been formed by meltwater from retreating inland glaciers. This relatively strong sea-level rise continued until around 6000 years ago. Since then, sea level has remained largely unchanged, apart from minor fluctuations amounting to a few centimetres per century. >
- 3.2 > Every time there is a storm, the North Sea continues its relentless erosion of the coast near the small English town of Happisburgh. The old coastal defences are largely ineffective. Here, a Second World War bunker has fallen from the eroded cliffs, while elsewhere along the coast, homes have already been lost to the sea.