As global temperatures are projected to rise, extreme sea levels could further intensify the risk of coastal flooding, according to two new studies released in Nature Communications and Environmental Research Letters. The studies, co-authored by Dr Luke Jackson, Researcher for Climate Econometrics, which is part of the Programme for Economic Modelling at INET Oxford, made projections of future extreme sea levels and their economic impact upon global coastlines under moderate mitigation and business-as-usual climate scenarios through the 21st century.

The first of these papers estimated, for the first time, the global increase in flood risk accounting for the major sources of coastal flooding (sea-level rise, tides, wind-blown waves, storm surges) along the global coastlines over this century. For the 1-in-100 year flood event (a flood event with a 1% chance of occurring each year), they projected a global average increase in the height of flooding of 34 - 76 cm (moderate mitigation) and 58 - 172 cm (no mitigation) by 2100.

Most of this increase is driven by sea-level rise (as previously highlighted here), says Dr Jackson, but there are significant differences from the global average. Under both of these scenarios, large parts of the tropics are very likely to be exposed to the present-day 1-in-100 year flood level on an annual basis by 2050, extending to most of the global coastline by 2100.

Dr Jackson commented: “The findings are important because they show that the risk to coastlines increases because the size of similar flood events rises, while the time between flood events falls. Future coastal protection will thus have to be more resilient to a larger number of potentially damaging coastal flood events, and flood events that are themselves larger than before."

This second of these papers developed previous research on projected sea-levels under warming scenarios satisfying the Paris Climate Agreement and used an advanced coastal-economic damage model to estimate losses due to coastal flooding. The research found that global sea-level rise would be 52 cm and 63 cm if temperatures were limited to 1.5 °C and 2.0 °C by 2100.

“While 11 cm difference doesn't sound like a lot, the estimated global annual flood costs by 2100 (assuming no future adaptation) could be up to US$ 1.4 trillion per year (0.25% of global GDP) more for the 2 °C target than the 1.5 °C target,” said Dr Jackson.

In fact, under the no mitigation scenario, the study finds that global annual flood costs could re"ach 2.8% of global GDP by 2100 (assuming no future adaptation). The authors warn that upper middle-income countries are likely to experience the largest increase in annual flood costs (up to 8% GDP) with China's vulnerability highlighted.

Dr Jackson summarised; "These pieces of research show that coastal adaptation through a range of flood defence schemes are critical. Extreme sea levels pose a long-term threat to coastal communities that is likely to be exacerbated through time. The implementation of a range of adaption measures could potentially reduce coastal flood costs by a factor of 10.

"Failing to achieve the Paris Accord will lead to greater damage and higher levels of coastal flood risk worldwide."

Read more:

• M. I. Vousdoukas, L. Mentaschi, E. Voukouvalas, M. Verlaan, L. Feyen, S. Jevrejeva and L. P. Jackson (2018). Global projections of extreme sea levels in view of climate change, Nature Communications. Doi: 10.1038/s41467-018-04692-w
• S. Jevrejeva, L.P. Jackson, A. Grinstedn, D. Lincke, B. Marzeion (2018). Flood damage costs under the sea level rise with warming of 1.5° C and 2° C, Environmental Research Letters, Doi: 10.1088/1748-9326/aacc76