A new study by scientists from the Geological Survey of Denmark and Greenland (GEUS) shows that sea level will continue to rise across Denmark this century but that some cities are more vulnerable than others.
“Our research shows that sea-level projections vary greatly from city to city, even in a relatively small country like Denmark. Looking at sea-level rise projections for Copenhagen are not necessarily suitable for planning elsewhere in the country,” says lead-author William Colgan, Senior Researcher in glaciology and climate at the Geological Survey of Denmark and Greenland (GEUS).
“We cannot rely on national-scale estimates of sea level rise to plan for the future. The sea-level rise associated with different timelines, climate scenarios, and acceptable risk will vary substantially from one location to another,” he says.
The scientists combined geological records of sea-level change over the past 2000 years, with modern observations from tide gauge records to see how future projections of sea-level rise compare with past changes. They then translated global-scale projections of sea-level rise to a local scale for three cities in Denmark: Esbjerg, Skagen, and Copenhagen.
Two scenarios
Colgan and colleagues studied how sea level is expected to rise at each city by the middle of the century and the end of this century, under two different climate change scenarios defined by the UN IPCC (Intergovernmental Panel on Climate Change).
First, they looked at the so-called RCP 4.5 scenario, which most closely resembles a scenario in which greenhouse gas emissions are cut according to the Paris Climate Agreement. And secondly, under the RCP 8.5 scenario, often referred to as the ‘business-as-usual’ scenario, in which greenhouse gas emissions continue to rise.
In both scenarios, sea level will rise at all three cities, even if emissions are cut. Under the RCP 4.5 scenario, Copenhagen can expect 51 cm (+/–21 cm) sea-level rise by the end of this century, Skagen can expect 39 cm (+/–21 cm), and Esbjerg can expect 63 cm (+/–21 cm). If greenhouse gas emissions continue to increase then sea level is expected to rise even more: by 64 cm (+/–28 cm) at Skagen, 77 cm (+/–28 cm) at Copenhagen, and 89 cm (+/–28 cm) at Esbjerg under the RCP 8.5 ‘business-as-usual’ scenario.
“We can now say that Esbjerg will likely experience approximately as much sea-level rise during the remainder of this century, as it has in the last twenty centuries. And by 2100, the difference in relative sea-level rise between Skagen and Esbjerg is about 25 cm. That’s a huge difference in sea-level rise over a relatively short distance of 300 km or so between the two cities,” says co-author Kristian Kjeldsen, Research in glaciology and climate at the Geological Survey of Denmark and Greenland (GEUS).
Why the difference?
The discrepancy in local sea level around Denmark is largely related to events that occurred more than 10 000 years ago at the end of the last ice age – when the vast Scandinavian ice sheet that once pressed down on the land, finally melted.
Land located directly underneath the ice sheet was suddenly relieved of the weight of the ice pressing down on them and the land began to rise in response. Skagen, which was situated underneath the Scandinavian ice sheet during the last ice age, is still rebounding today. As a result, Skagen will experience less sea-level rise than it might otherwise.
Esbjerg was located under the so-called crustal forebulge of the Scandinavian ice sheet – where the Earth’s crust was pushed up on either side of the ice. Since the ice age ended, this land has been subsiding.
Combined with the effects of rising seas due to climate change, the subsiding land at Esbjerg will lead to more sea-level rise at Esbjerg than Skagen. And such strong differences in local sea-level have implications for planners looking to protect coastlines against rising seas, William Colgan points out.
“A recent US study highlighted that a 15 cm sea-level increase means that a 1-in-500-year coastal flooding event becomes a 1-in-10-year event along the US west coast. For comparison, we are projecting a 25 cm discrepancy between Esbjerg and Skagen by 2100. So that extra 25 cm will really, really, increase the frequency of extreme events in Esbjerg,” says William Colgan and continues:
“In other words, what we consider an extreme event today is likely to be a far more normal occurrence in the near future. And by the end of the century, what is considered an extreme event in Skagen will be a far more regular occurrence in Esbjerg.”