Global sea levels are rising. After two thousand years of stability, the transition to continuous coastal change will be jarring (although this is what our shoreline ancestors experienced more than 6,000 years ago). By the end of this century, millions of people will need to relocate. An estimated two trillion dollars of assets lies within the first meter above extreme high tide.
“Future sea levels” is one of seven “Grand Challenges” of the World Climate Research Programme (WCRP). Through the week of November 11, leading experts from around the world met in Orléans at the headquarters of the Bureau de Recherches Géologiques et Minières (BRGM), the French geological survey.
Sea level data combines local, regional and global factors.
Regional sea levels are affected by prevailing weather systems, by postglacial rebound and tectonics, and at high latitudes by changes in the gravitational pull of the ice sheets. However, it is the “eustatic” global component of sea level that has the attention, driven by changes in the thermal structure of the oceans and by the melting of the ice sheets.
When modelers develop distributions around future sea level rise, they are combining results from four classes of model:
- a model of human activity releasing (and limiting) greenhouse gas emissions
- a model of how greenhouse gas emissions contribute to warming of the atmosphere and oceans
- a model of the evolving thermal structure of the oceans
- a model for the controls of ice sheet (and glacier) melting
There is, inevitably, a lot of uncertainty in the projections, and those who have been bold enough to forecast future sea levels mix physical modeling with informed judgement.
Sunny day high tide nuisance flooding in Brickell, downtown Miami, Florida on October 17, 2016. Image credit: Wikimedia
RMS was part of the WCRP sea level workshop, because of our long-running work to convert future sea level hazard into risk cost information. In 2007 we collaborated on a worldwide study ranking coastal flood risk at the end of twenty-first century to port cities – in a report and science paper that together received more than a thousand citations.
In the 2014 Risky Business study on the economic costs of climate change in the U.S. through to 2100, we calculated the probabilistic hurricane wind and flood risk costs of sea level rise and shifts in storm intensities, incorporating the uncertainties in both sea level and hurricane intensity projections.
While in the Risky Business study probabilistic loss outputs could be fed into economic models, the audience for sea level forecasts are generally looking for something more practical. Everyone with an interest in coastal zones are potential users of sea level forecasts. This could include both residents, and businesses (from nuclear power stations to local fishermen), administrators, insurers, banks, real-estate agents, architects, planners and engineers tasked with action in response, owners of infrastructure, lawyers, politicians. And they will each bring a different perspective as to what constitutes a critical threshold. Many will simply want to be told what to do, through regulation.
So, as important as the question how sea level data will be developed is, of equal importance is how the information will be delivered.
As we convert the distribution on hazard into a distribution on risk, we can expect some extreme nonlinearities. If sea level rise by 2050 is lower than the mean, then spending plans on defenses and relocations can be relaxed. If the rate of rise is faster, huge costs may result.
What can we anticipate about the exposure at risk from sea level rise? Will there be a continual retreat of those on the beachfront inland or will relocation principally occur in the aftermath of some catastrophic storm. For instance, after the 2005 hurricane season, I witnessed beachfront houses being loaded onto barges and moved inland from Dauphin Island, Alabama.
Beachfront houses being loaded onto barges and moved inland from Dauphin Island, Alabama after the 2005 hurricane season. Image credit: Robert Muir-Wood
Should we rotate the data and deliver information around the uncertainty in the date of reaching some threshold? For many users that might be a more useful output. When will FEMA release its best estimate of the sea level for 2050, to help with siting new construction? Sea level rise will be a defining issue for the twenty-first century. Delivering the appropriate risk information to all the affected parties will be critical.
 Nicholls RJ, Hanson S, Herweijer C, Patmore N, Hallegatte S, Corfee-Morlot J, Chateau J, Muir-Wood R (2007) Ranking port cities with high exposure and vulnerability to climate extremes—exposure estimates. Environmental Working Paper No 1, Organisation for Economic Co-operation and Development (OECD), Paris